From 23f50092bf4af21742e834ef7b908102c85af4d8 Mon Sep 17 00:00:00 2001
From: Matthias Koeppe <mkoeppe@math.ucdavis.edu>
Date: Tue, 9 Nov 2021 18:23:35 -0800
Subject: [PATCH 01/29] sage.features.normaliz: New

---
 src/sage/features/normaliz.py                 | 28 +++++++++++++++++++
 .../geometry/polyhedron/backend_normaliz.py   |  4 +--
 src/sage/geometry/polyhedron/base.py          |  6 ++--
 3 files changed, 34 insertions(+), 4 deletions(-)
 create mode 100644 src/sage/features/normaliz.py

diff --git a/src/sage/features/normaliz.py b/src/sage/features/normaliz.py
new file mode 100644
index 00000000000..5135b7254ab
--- /dev/null
+++ b/src/sage/features/normaliz.py
@@ -0,0 +1,28 @@
+r"""
+Check for pynormaliz
+"""
+from . import PythonModule
+from .join_feature import JoinFeature
+
+
+class pynormaliz(JoinFeature):
+    r"""
+    A :class:`sage.features.Feature` describing the presence of the
+    Python package ``igraph``.
+
+    EXAMPLES::
+
+        sage: from sage.features.igraph import pynormaliz
+        sage: pynormaliz().is_present()                    # optional - pynormaliz
+        FeatureTestResult('pynormaliz', True)
+    """
+    def __init__(self):
+        r"""
+        TESTS::
+
+            sage: from sage.features.igraph import pynormaliz
+            sage: isinstance(pynormaliz(), pynormaliz)
+            True
+        """
+        JoinFeature.__init__(self, 'pynormaliz',
+                             [PythonModule('PyNormaliz', spkg="pynormaliz")])
diff --git a/src/sage/geometry/polyhedron/backend_normaliz.py b/src/sage/geometry/polyhedron/backend_normaliz.py
index d1f30e2d31f..7c48b54cd96 100644
--- a/src/sage/geometry/polyhedron/backend_normaliz.py
+++ b/src/sage/geometry/polyhedron/backend_normaliz.py
@@ -26,10 +26,10 @@
 from sage.structure.element import Element
 from sage.misc.cachefunc import cached_method
 from sage.misc.misc_c import prod
-from sage.features import PythonModule
+from sage.features.normaliz import pynormaliz
 from sage.misc.lazy_import import lazy_import
 lazy_import('PyNormaliz', ['NmzResult', 'NmzCompute', 'NmzCone', 'NmzConeCopy'],
-                    feature=PythonModule("PyNormaliz", spkg="pynormaliz"))
+            feature=pynormaliz)
 
 from sage.rings.integer_ring import ZZ
 from sage.rings.rational_field import QQ
diff --git a/src/sage/geometry/polyhedron/base.py b/src/sage/geometry/polyhedron/base.py
index d44c3d508fb..f6969eab1eb 100644
--- a/src/sage/geometry/polyhedron/base.py
+++ b/src/sage/geometry/polyhedron/base.py
@@ -8729,16 +8729,18 @@ def volume(self, measure='ambient', engine='auto', **kwds):
                 Latte().require()
                 engine = 'latte'
             except FeatureNotPresentError:
+                from sage.features.normaliz import pynormaliz
                 try:
-                    PythonModule("PyNormaliz", spkg="pynormaliz").require()
+                    pynormaliz.require()
                     engine = 'normaliz'
                 except FeatureNotPresentError:
                     raise RuntimeError("the induced rational measure can only be computed with the optional packages `latte_int`, or `pynormaliz`")
 
         if engine == 'auto' and measure == 'induced_lattice':
             # Enforce a default choice, change if a better engine is found.
+            from sage.features.normaliz import pynormaliz
             try:
-                PythonModule("PyNormaliz", spkg="pynormaliz").require()
+                pynormaliz.require()
                 engine = 'normaliz'
             except FeatureNotPresentError:
                 try:

From 0b65b799ed5bc66fed7e1e39c1a07980a3a08616 Mon Sep 17 00:00:00 2001
From: Matthias Koeppe <mkoeppe@math.ucdavis.edu>
Date: Tue, 9 Nov 2021 19:58:58 -0800
Subject: [PATCH 02/29] src/sage/features/normaliz.py: Fix up

---
 src/sage/features/normaliz.py | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/sage/features/normaliz.py b/src/sage/features/normaliz.py
index 5135b7254ab..e00077011aa 100644
--- a/src/sage/features/normaliz.py
+++ b/src/sage/features/normaliz.py
@@ -8,11 +8,11 @@
 class pynormaliz(JoinFeature):
     r"""
     A :class:`sage.features.Feature` describing the presence of the
-    Python package ``igraph``.
+    Python package ``PyNormaliz``.
 
     EXAMPLES::
 
-        sage: from sage.features.igraph import pynormaliz
+        sage: from sage.features.normaliz import pynormaliz
         sage: pynormaliz().is_present()                    # optional - pynormaliz
         FeatureTestResult('pynormaliz', True)
     """
@@ -20,7 +20,7 @@ def __init__(self):
         r"""
         TESTS::
 
-            sage: from sage.features.igraph import pynormaliz
+            sage: from sage.features.normaliz import pynormaliz
             sage: isinstance(pynormaliz(), pynormaliz)
             True
         """

From 00cb9c7fe2e9dacb1270d556e51b2a8e08bf92f4 Mon Sep 17 00:00:00 2001
From: Matthias Koeppe <mkoeppe@math.ucdavis.edu>
Date: Wed, 10 Nov 2021 16:13:54 -0800
Subject: [PATCH 03/29] src/sage/features/normaliz.py: Rename class to
 PyNormaliz, fix uses

---
 src/sage/features/normaliz.py                    | 10 +++++-----
 src/sage/geometry/polyhedron/backend_normaliz.py |  4 ++--
 src/sage/geometry/polyhedron/base.py             |  8 ++++----
 3 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/src/sage/features/normaliz.py b/src/sage/features/normaliz.py
index e00077011aa..ceb67875b01 100644
--- a/src/sage/features/normaliz.py
+++ b/src/sage/features/normaliz.py
@@ -5,23 +5,23 @@
 from .join_feature import JoinFeature
 
 
-class pynormaliz(JoinFeature):
+class PyNormaliz(JoinFeature):
     r"""
     A :class:`sage.features.Feature` describing the presence of the
     Python package ``PyNormaliz``.
 
     EXAMPLES::
 
-        sage: from sage.features.normaliz import pynormaliz
-        sage: pynormaliz().is_present()                    # optional - pynormaliz
+        sage: from sage.features.normaliz import PyNormaliz
+        sage: PyNormaliz().is_present()                    # optional - pynormaliz
         FeatureTestResult('pynormaliz', True)
     """
     def __init__(self):
         r"""
         TESTS::
 
-            sage: from sage.features.normaliz import pynormaliz
-            sage: isinstance(pynormaliz(), pynormaliz)
+            sage: from sage.features.normaliz import PyNormaliz
+            sage: isinstance(PyNormaliz(), PyNormaliz)
             True
         """
         JoinFeature.__init__(self, 'pynormaliz',
diff --git a/src/sage/geometry/polyhedron/backend_normaliz.py b/src/sage/geometry/polyhedron/backend_normaliz.py
index 7c48b54cd96..4b413193c26 100644
--- a/src/sage/geometry/polyhedron/backend_normaliz.py
+++ b/src/sage/geometry/polyhedron/backend_normaliz.py
@@ -26,10 +26,10 @@
 from sage.structure.element import Element
 from sage.misc.cachefunc import cached_method
 from sage.misc.misc_c import prod
-from sage.features.normaliz import pynormaliz
 from sage.misc.lazy_import import lazy_import
+import sage.features.normaliz
 lazy_import('PyNormaliz', ['NmzResult', 'NmzCompute', 'NmzCone', 'NmzConeCopy'],
-            feature=pynormaliz)
+            feature=sage.features.normaliz.PyNormaliz())
 
 from sage.rings.integer_ring import ZZ
 from sage.rings.rational_field import QQ
diff --git a/src/sage/geometry/polyhedron/base.py b/src/sage/geometry/polyhedron/base.py
index f6969eab1eb..354672ba01d 100644
--- a/src/sage/geometry/polyhedron/base.py
+++ b/src/sage/geometry/polyhedron/base.py
@@ -8729,18 +8729,18 @@ def volume(self, measure='ambient', engine='auto', **kwds):
                 Latte().require()
                 engine = 'latte'
             except FeatureNotPresentError:
-                from sage.features.normaliz import pynormaliz
+                from sage.features.normaliz import PyNormaliz
                 try:
-                    pynormaliz.require()
+                    PyNormaliz().require()
                     engine = 'normaliz'
                 except FeatureNotPresentError:
                     raise RuntimeError("the induced rational measure can only be computed with the optional packages `latte_int`, or `pynormaliz`")
 
         if engine == 'auto' and measure == 'induced_lattice':
             # Enforce a default choice, change if a better engine is found.
-            from sage.features.normaliz import pynormaliz
+            from sage.features.normaliz import PyNormaliz
             try:
-                pynormaliz.require()
+                PyNormaliz().require()
                 engine = 'normaliz'
             except FeatureNotPresentError:
                 try:

From 7632e539cf7c7bfa15e8be3ed2cb7d6804d4ce27 Mon Sep 17 00:00:00 2001
From: Matthias Koeppe <mkoeppe@math.ucdavis.edu>
Date: Wed, 10 Nov 2021 17:04:07 -0800
Subject: [PATCH 04/29] src/sage/geometry/polyhedron/base.py: Remove unused
 import

---
 src/sage/geometry/polyhedron/base.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/sage/geometry/polyhedron/base.py b/src/sage/geometry/polyhedron/base.py
index 354672ba01d..32a95a8b7a6 100644
--- a/src/sage/geometry/polyhedron/base.py
+++ b/src/sage/geometry/polyhedron/base.py
@@ -8717,7 +8717,7 @@ def volume(self, measure='ambient', engine='auto', **kwds):
             sage: Q.volume.is_in_cache()
             True
         """
-        from sage.features import FeatureNotPresentError, PythonModule
+        from sage.features import FeatureNotPresentError
         if measure == 'induced_rational' and engine not in ['auto', 'latte', 'normaliz']:
             raise RuntimeError("the induced rational measure can only be computed with the engine set to `auto`, `latte`, or `normaliz`")
         if measure == 'induced_lattice' and engine not in ['auto', 'latte', 'normaliz']:

From fa45c36e6009c1bafaed18efb75eaac975ab6aa3 Mon Sep 17 00:00:00 2001
From: Matthias Koeppe <mkoeppe@math.ucdavis.edu>
Date: Wed, 10 Nov 2021 17:58:39 -0800
Subject: [PATCH 05/29] sage.features.sagemath.all_features: New

---
 src/sage/features/sagemath.py | 40 ++++++++++++++++++++++++++++-------
 1 file changed, 32 insertions(+), 8 deletions(-)

diff --git a/src/sage/features/sagemath.py b/src/sage/features/sagemath.py
index 2d7d6e9ab79..4c5ae934015 100644
--- a/src/sage/features/sagemath.py
+++ b/src/sage/features/sagemath.py
@@ -162,7 +162,7 @@ def __init__(self):
                              spkg="sagemath_symbolics")
 
 
-def sage_features(logger=None):
+def all_features():
     """
     Return features corresponding to parts of the Sage library.
 
@@ -177,6 +177,36 @@ def sage_features(logger=None):
     Instead, we associate distribution packages to Python modules in
     :mod:`sage.features.sagemath` via the ``spkg`` parameter of :class:`Feature`.
 
+    EXAMPLES::
+
+        sage: from sage.features.sagemath import all_features
+        sage: list(all_features())
+        [Feature('sage.combinat'), ...]
+    """
+    return [sage__combinat(),
+            sage__geometry__polyhedron(),
+            sage__graphs(),
+            sage__plot(),
+            sage__rings__number_field(),
+            sage__rings__real_double(),
+            sage__symbolic()]
+
+
+def sage_features(logger=None):
+    """
+    Return features corresponding to parts of the Sage library that are present.
+
+    These tags are named after Python packages/modules (e.g., :mod:`~sage.symbolic`),
+    not distribution packages (``sagemath-symbolics``).
+
+    This design is motivated by a separation of concerns: The author of a module that depends
+    on some functionality provided by a Python module usually already knows the
+    name of the Python module, so we do not want to force the author to also
+    know about the distribution package that provides the Python module.
+
+    Instead, we associate distribution packages to Python modules in
+    :mod:`sage.features.sagemath` via the ``spkg`` parameter of :class:`Feature`.
+
     EXAMPLES::
 
         sage: from sage.features.sagemath import sage_features
@@ -186,13 +216,7 @@ def sage_features(logger=None):
          Feature('sage.rings.number_field'),
          Feature('sage.rings.real_double')]
     """
-    for feature in [sage__combinat(),
-                    sage__geometry__polyhedron(),
-                    sage__graphs(),
-                    sage__plot(),
-                    sage__rings__number_field(),
-                    sage__rings__real_double(),
-                    sage__symbolic()]:
+    for feature in all_features():
         result = feature.is_present()
         if logger:
             logger.write(f'{result}, reason: {result.reason}\n')

From 17e30e2660400b7825e0a14c10b09da5458bb59d Mon Sep 17 00:00:00 2001
From: Matthias Koeppe <mkoeppe@math.ucdavis.edu>
Date: Wed, 10 Nov 2021 17:59:04 -0800
Subject: [PATCH 06/29] sage.features.all: New

---
 src/sage/features/all.py | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)
 create mode 100644 src/sage/features/all.py

diff --git a/src/sage/features/all.py b/src/sage/features/all.py
new file mode 100644
index 00000000000..dc13b375959
--- /dev/null
+++ b/src/sage/features/all.py
@@ -0,0 +1,24 @@
+
+def all_features():
+    r"""
+    Return an iterable of all features.
+
+    EXAMPLES::
+
+        sage: from sage.features.all import all_features
+        sage: sorted(all_features(), key=lambda f: f.name)  # random
+        [...Feature('sage.combinat')...]
+    """
+    import pkgutil
+    import importlib
+    import sage.features
+    # Following https://packaging.python.org/guides/creating-and-discovering-plugins/#using-namespace-packages
+    for finder, name, ispkg in pkgutil.iter_modules(sage.features.__path__, sage.features.__name__ + "."):
+        module = importlib.import_module(name)
+        try:
+            af = module.all_features
+        except AttributeError:
+            pass
+        else:
+            if af != all_features:
+                yield from af()

From 8bfebc9c04fc997d2595babb9cc2f78df4184b2c Mon Sep 17 00:00:00 2001
From: Matthias Koeppe <mkoeppe@math.ucdavis.edu>
Date: Wed, 10 Nov 2021 18:06:03 -0800
Subject: [PATCH 07/29] sage.features.bliss.all_features: New

---
 src/sage/features/bliss.py | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/src/sage/features/bliss.py b/src/sage/features/bliss.py
index d870ade5973..6171f9935ce 100644
--- a/src/sage/features/bliss.py
+++ b/src/sage/features/bliss.py
@@ -69,3 +69,8 @@ def __init__(self):
         JoinFeature.__init__(self, "bliss",
                              [PythonModule("sage.graphs.bliss", spkg="bliss",
                                            url="http://www.tcs.hut.fi/Software/bliss/")])
+
+
+def all_features():
+    return [BlissLibrary(),
+            Bliss()]

From 893ec23c44ae4c85c4297f2b06ccb102673e8582 Mon Sep 17 00:00:00 2001
From: Matthias Koeppe <mkoeppe@math.ucdavis.edu>
Date: Wed, 10 Nov 2021 19:09:34 -0800
Subject: [PATCH 08/29] src/sage/features/kenzo.py: Move import from
 sage.libs.ecl into method

---
 src/sage/features/kenzo.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/sage/features/kenzo.py b/src/sage/features/kenzo.py
index 7012dc230ec..f1778d9a379 100644
--- a/src/sage/features/kenzo.py
+++ b/src/sage/features/kenzo.py
@@ -3,7 +3,6 @@
 Check for Kenzo
 """
 
-from sage.libs.ecl import ecl_eval
 from . import Feature, FeatureTestResult
 
 class Kenzo(Feature):
@@ -37,6 +36,7 @@ def _is_present(self):
             sage: Kenzo()._is_present()  # optional - kenzo
             FeatureTestResult('kenzo', True)
         """
+        from sage.libs.ecl import ecl_eval
         # Redirection of ECL and Maxima stdout to /dev/null
         # This is also done in the Maxima library, but we
         # also do it here for redundancy.

From eaec4000ec31e884e8e152efbd88e1b15ff3c5a3 Mon Sep 17 00:00:00 2001
From: Matthias Koeppe <mkoeppe@math.ucdavis.edu>
Date: Sat, 13 Nov 2021 11:48:50 -0800
Subject: [PATCH 09/29] src/sage/features/interfaces.py: New

---
 src/sage/features/interfaces.py | 92 +++++++++++++++++++++++++++++++++
 1 file changed, 92 insertions(+)
 create mode 100644 src/sage/features/interfaces.py

diff --git a/src/sage/features/interfaces.py b/src/sage/features/interfaces.py
new file mode 100644
index 00000000000..a3e86c75e8d
--- /dev/null
+++ b/src/sage/features/interfaces.py
@@ -0,0 +1,92 @@
+r"""
+Check for working interpreter interfaces
+"""
+import importlib
+
+from . import Feature, FeatureTestResult, PythonModule
+
+
+class InterfaceFeature(Feature):
+
+    @staticmethod
+    def __classcall__(cls, name, module, description=None):
+        if isinstance(module, str):
+            module = PythonModule(module)
+        return Feature.__classcall__(cls, name, module, description)
+
+    def __init__(self, name, module, description):
+        super().__init__(name, description=description)
+        self.module = module
+
+    def _is_present(self):
+        result = self.module.is_present()
+        if not result:
+            return result
+        m = importlib.import_module(self.module.name)
+        try:
+            interface = getattr(m, self.name)
+            interface('2+3')
+            return FeatureTestResult(self, True)
+        except Exception as exception:
+            return FeatureTestResult(self, False,
+                                     reason=f"Interface {interface} is not functional: {exception}")
+
+# The following are provided by external software only (no SPKG)
+
+class Magma(InterfaceFeature):
+    r"""
+    A :class:`sage.features.Feature` describing whether :class:`sage.interfaces.magma.Magma`
+    is present and functional.
+
+    EXAMPLES::
+
+        sage: from sage.features.interfaces import Magma
+        sage: Magma().is_present()  # random
+        FeatureTestResult('jupymake', False)
+    """
+
+    @staticmethod
+    def __classcall__(cls):
+        return InterfaceFeature(cls, 'magma', 'sage.interfaces.magma')
+
+
+class Matlab(InterfaceFeature):
+
+    @staticmethod
+    def __classcall__(cls):
+        return InterfaceFeature(cls, 'matlab', 'sage.interfaces.matlab')
+
+
+class Mathematica(InterfaceFeature):
+
+    @staticmethod
+    def __classcall__(cls):
+        return InterfaceFeature(cls, 'mathematica', 'sage.interfaces.mathematica')
+
+
+class Maple(InterfaceFeature):
+
+    @staticmethod
+    def __classcall__(cls):
+        return InterfaceFeature(cls, 'maple', 'sage.interfaces.maple')
+
+
+class Macaulay2(InterfaceFeature):
+
+    @staticmethod
+    def __classcall__(cls):
+        return InterfaceFeature(cls, 'macaulay2', 'sage.interfaces.macaulay2')
+
+
+class Octave(InterfaceFeature):
+
+    @staticmethod
+    def __classcall__(cls):
+        return InterfaceFeature(cls, 'octave', 'sage.interfaces.octave')
+
+
+class Scilab(InterfaceFeature):
+
+    @staticmethod
+    def __classcall__(cls):
+        return InterfaceFeature(cls, 'scilab', 'sage.interfaces.scilab')

From 827ca0339457bc5596fce9cb46aefc7fe8d6e002 Mon Sep 17 00:00:00 2001
From: Matthias Koeppe <mkoeppe@math.ucdavis.edu>
Date: Sun, 14 Nov 2021 13:00:22 -0800
Subject: [PATCH 10/29] src/sage/features/interfaces.py: Fix and doctest
 exception handling

---
 src/sage/features/interfaces.py | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/src/sage/features/interfaces.py b/src/sage/features/interfaces.py
index a3e86c75e8d..8df56ab0d3d 100644
--- a/src/sage/features/interfaces.py
+++ b/src/sage/features/interfaces.py
@@ -7,7 +7,26 @@
 
 
 class InterfaceFeature(Feature):
-
+    r"""
+    TESTS::
+
+        sage: from sage.features.interfaces import InterfaceFeature
+        sage: broken = InterfaceFeature("broken_interface", "sage.interfaces.does_not_exist")
+        sage: broken.is_present()
+        FeatureTestResult('sage.interfaces.does_not_exist', False)
+        sage: _.reason
+        "Failed to import `sage.interfaces.does_not_exist`: No module named 'sage.interfaces.does_not_exist'"
+
+        sage: also_broken = InterfaceFeature("also_broken_interface", "sage.interfaces.interface")
+        sage: also_broken.is_present()
+        Traceback (most recent call last):
+        ...
+        FeatureTestResult('also_broken_interface', False)
+        sage: _.reason
+        Traceback (most recent call last):
+        ...
+        "Interface also_broken_interface cannot be imported: module 'sage.interfaces.interface' has no attribute 'also_broken_interface'"
+    """
     @staticmethod
     def __classcall__(cls, name, module, description=None):
         if isinstance(module, str):
@@ -25,6 +44,10 @@ def _is_present(self):
         m = importlib.import_module(self.module.name)
         try:
             interface = getattr(m, self.name)
+        except Exception as exception:
+            return FeatureTestResult(self, False,
+                                     reason=f"Interface {self.name} cannot be imported: {exception}")
+        try:
             interface('2+3')
             return FeatureTestResult(self, True)
         except Exception as exception:

From 52ac51efe425371ab7aab08d3eb964bf4c90cd4c Mon Sep 17 00:00:00 2001
From: Matthias Koeppe <mkoeppe@math.ucdavis.edu>
Date: Sun, 14 Nov 2021 13:00:46 -0800
Subject: [PATCH 11/29] src/sage/features/interfaces.py (all_features): New

---
 src/sage/features/interfaces.py | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/src/sage/features/interfaces.py b/src/sage/features/interfaces.py
index 8df56ab0d3d..2f7a9e7d61c 100644
--- a/src/sage/features/interfaces.py
+++ b/src/sage/features/interfaces.py
@@ -113,3 +113,23 @@ class Scilab(InterfaceFeature):
     @staticmethod
     def __classcall__(cls):
         return InterfaceFeature(cls, 'scilab', 'sage.interfaces.scilab')
+
+
+def all_features():
+    r"""
+    Return features corresponding to interpreter interfaces.
+
+     EXAMPLES::
+
+        sage: from sage.features.interfaces import all_features
+        sage: list(all_features())
+        [Feature(<class 'sage.features.interfaces.Magma'>: sage.interfaces.magma), ...,
+         Feature(<class 'sage.features.interfaces.Scilab'>: sage.interfaces.scilab)]
+    """
+    return [Magma(),
+            Matlab(),
+            Mathematica(),
+            Maple(),
+            Macaulay2(),
+            Octave(),
+            Scilab()]

From 7754fae5f46ef3ad9fc2c9c3bfd2a2dac3f76f11 Mon Sep 17 00:00:00 2001
From: Matthias Koeppe <mkoeppe@math.ucdavis.edu>
Date: Sun, 14 Nov 2021 13:25:16 -0800
Subject: [PATCH 12/29] src/sage/features/interfaces.py: Fix and doctest
 exception handling (fixup)

---
 src/sage/features/interfaces.py | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/src/sage/features/interfaces.py b/src/sage/features/interfaces.py
index 2f7a9e7d61c..21b8b01445f 100644
--- a/src/sage/features/interfaces.py
+++ b/src/sage/features/interfaces.py
@@ -19,12 +19,8 @@ class InterfaceFeature(Feature):
 
         sage: also_broken = InterfaceFeature("also_broken_interface", "sage.interfaces.interface")
         sage: also_broken.is_present()
-        Traceback (most recent call last):
-        ...
         FeatureTestResult('also_broken_interface', False)
         sage: _.reason
-        Traceback (most recent call last):
-        ...
         "Interface also_broken_interface cannot be imported: module 'sage.interfaces.interface' has no attribute 'also_broken_interface'"
     """
     @staticmethod

From 8081f29f97a947e39b8ef17e861433847e623dcd Mon Sep 17 00:00:00 2001
From: Matthias Koeppe <mkoeppe@math.ucdavis.edu>
Date: Sun, 14 Nov 2021 13:25:31 -0800
Subject: [PATCH 13/29] src/sage/doctest/external.py: Use
 sage.features.interfaces

---
 src/sage/doctest/external.py | 48 +++++++++---------------------------
 1 file changed, 12 insertions(+), 36 deletions(-)

diff --git a/src/sage/doctest/external.py b/src/sage/doctest/external.py
index e74b3554e50..cdd09eb464b 100644
--- a/src/sage/doctest/external.py
+++ b/src/sage/doctest/external.py
@@ -117,12 +117,8 @@ def has_magma():
         sage: has_magma() # random, optional - magma
         True
     """
-    from sage.interfaces.magma import magma
-    try:
-        magma('2+3')
-        return True
-    except Exception:
-        return False
+    from sage.features.interfaces import Magma
+    return Magma().is_present()
 
 def has_matlab():
     """
@@ -134,12 +130,8 @@ def has_matlab():
         sage: has_matlab() # random, optional - matlab
         True
     """
-    from sage.interfaces.matlab import matlab
-    try:
-        matlab('2+3')
-        return True
-    except Exception:
-        return False
+    from sage.features.interfaces import Matlab
+    return Matlab().is_present()
 
 def has_mathematica():
     """
@@ -151,12 +143,8 @@ def has_mathematica():
         sage: has_mathematica() # random, optional - mathematica
         True
     """
-    from sage.interfaces.mathematica import mathematica
-    try:
-        mathematica('2+3')
-        return True
-    except Exception:
-        return False
+    from sage.features.interfaces import Mathematica
+    return Mathematica().is_present()
 
 def has_maple():
     """
@@ -168,12 +156,8 @@ def has_maple():
         sage: has_maple() # random, optional - maple
         True
     """
-    from sage.interfaces.maple import maple
-    try:
-        maple('2+3')
-        return True
-    except Exception:
-        return False
+    from sage.features.interfaces import Maple
+    return Maple().is_present()
 
 def has_macaulay2():
     """
@@ -185,12 +169,8 @@ def has_macaulay2():
         sage: has_macaulay2() # random, optional - macaulay2
         True
     """
-    from sage.interfaces.macaulay2 import macaulay2
-    try:
-        macaulay2('2+3')
-        return True
-    except Exception:
-        return False
+    from sage.features.interfaces import Macaulay2
+    return Macaulay2().is_present()
 
 def has_octave():
     """
@@ -202,12 +182,8 @@ def has_octave():
         sage: has_octave() # random, optional - octave
         True
     """
-    from sage.interfaces.octave import octave
-    try:
-        octave('2+3')
-        return True
-    except Exception:
-        return False
+    from sage.features.interfaces import Octave
+    return Octave().is_present()
 
 def has_pandoc():
     """

From c41808ba23b81ac2c930fb77ea9ded20a7b7d770 Mon Sep 17 00:00:00 2001
From: Matthias Koeppe <mkoeppe@math.ucdavis.edu>
Date: Sun, 14 Nov 2021 14:03:39 -0800
Subject: [PATCH 14/29] src/sage/features/latex.py (all_features): New

---
 src/sage/features/latex.py | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/src/sage/features/latex.py b/src/sage/features/latex.py
index a0f0d6f4096..0890320b39b 100644
--- a/src/sage/features/latex.py
+++ b/src/sage/features/latex.py
@@ -136,3 +136,10 @@ def __init__(self):
         """
         Executable.__init__(self, "lualatex", executable="lualatex",
                             url="https://www.latex-project.org/")
+
+
+def all_features():
+    return [latex(),
+            pdflatex(),
+            xelatex(),
+            lualatex()]

From 44c19e3145fb47c7d977a326020ecf2be3298776 Mon Sep 17 00:00:00 2001
From: Matthias Koeppe <mkoeppe@math.ucdavis.edu>
Date: Sun, 14 Nov 2021 14:03:53 -0800
Subject: [PATCH 15/29] src/sage/features/mip_backends.py (all_features): New

---
 src/sage/features/mip_backends.py | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/src/sage/features/mip_backends.py b/src/sage/features/mip_backends.py
index bec5d958247..6bf3f936f26 100644
--- a/src/sage/features/mip_backends.py
+++ b/src/sage/features/mip_backends.py
@@ -71,3 +71,9 @@ def __init__(self):
         JoinFeature.__init__(self, 'sage_numerical_backends_coin',
                              [MIPBackend('coin')],
                              spkg='sage_numerical_backends_coin')
+
+
+def all_features():
+    return [CPLEX(),
+            Gurobi(),
+            COIN()]

From fa25155f55cadf4424dd561586b111e4183e2d50 Mon Sep 17 00:00:00 2001
From: Matthias Koeppe <mkoeppe@math.ucdavis.edu>
Date: Sun, 14 Nov 2021 14:04:39 -0800
Subject: [PATCH 16/29] src/sage/doctest/external.py (external_software): Go
 through external_features

---
 src/sage/doctest/external.py | 32 +++++++++++++++++++++++++++-----
 1 file changed, 27 insertions(+), 5 deletions(-)

diff --git a/src/sage/doctest/external.py b/src/sage/doctest/external.py
index cdd09eb464b..7e73dc60577 100644
--- a/src/sage/doctest/external.py
+++ b/src/sage/doctest/external.py
@@ -333,6 +333,32 @@ def has_4ti2():
     from sage.features.four_ti_2 import FourTi2
     return FourTi2().is_present()
 
+def external_features():
+    features = []
+    from sage.features.internet import Internet
+    features.append(Internet())
+    import sage.features.latex
+    features.extend(sage.features.latex.all_features())
+    import sage.features.interfaces
+    features.extend(sage.features.interfaces.all_features())
+    from sage.features.pandoc import Pandoc
+    features.append(Pandoc())
+    from sage.features.graphviz import Graphviz
+    features.append(Graphviz())
+    from sage.features.ffmpeg import FFmpeg
+    features.append(FFmpeg())
+    from sage.features.imagemagick import ImageMagick
+    features.append(ImageMagick())
+    from sage.features.dvipng import dvipng
+    features.append(dvipng())
+    from sage.features.pdf2svg import pdf2svg
+    features.append(pdf2svg())
+    from sage.features.rubiks import Rubiks
+    features.append(Rubiks())
+    from sage.features.four_ti_2 import FourTi2
+    features.append(FourTi2())
+    return features
+
 def external_software():
     """
     Return the alphabetical list of external software supported by this module.
@@ -343,11 +369,7 @@ def external_software():
         sage: sorted(external_software) == external_software
         True
     """
-    supported = list()
-    for func in globals():
-        if func.startswith(prefix):
-            supported.append(func[len(prefix):])
-    return sorted(supported)
+    return sorted(f.name for f in external_features())
 
 external_software = external_software()
 

From 945132b9524a6734957e70a00830b6db07f74b65 Mon Sep 17 00:00:00 2001
From: Matthias Koeppe <mkoeppe@math.ucdavis.edu>
Date: Sun, 14 Nov 2021 14:16:05 -0800
Subject: [PATCH 17/29] src/sage/features/interfaces.py: Fix up

---
 src/sage/features/interfaces.py | 23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/src/sage/features/interfaces.py b/src/sage/features/interfaces.py
index 21b8b01445f..231fba136b2 100644
--- a/src/sage/features/interfaces.py
+++ b/src/sage/features/interfaces.py
@@ -66,49 +66,49 @@ class Magma(InterfaceFeature):
 
     @staticmethod
     def __classcall__(cls):
-        return InterfaceFeature(cls, 'magma', 'sage.interfaces.magma')
+        return InterfaceFeature.__classcall__(cls, 'magma', 'sage.interfaces.magma')
 
 
 class Matlab(InterfaceFeature):
 
     @staticmethod
     def __classcall__(cls):
-        return InterfaceFeature(cls, 'matlab', 'sage.interfaces.matlab')
+        return InterfaceFeature.__classcall__(cls, 'matlab', 'sage.interfaces.matlab')
 
 
 class Mathematica(InterfaceFeature):
 
     @staticmethod
     def __classcall__(cls):
-        return InterfaceFeature(cls, 'mathematica', 'sage.interfaces.mathematica')
+        return InterfaceFeature.__classcall__(cls, 'mathematica', 'sage.interfaces.mathematica')
 
 
 class Maple(InterfaceFeature):
 
     @staticmethod
     def __classcall__(cls):
-        return InterfaceFeature(cls, 'maple', 'sage.interfaces.maple')
+        return InterfaceFeature.__classcall__(cls, 'maple', 'sage.interfaces.maple')
 
 
 class Macaulay2(InterfaceFeature):
 
     @staticmethod
     def __classcall__(cls):
-        return InterfaceFeature(cls, 'macaulay2', 'sage.interfaces.macaulay2')
+        return InterfaceFeature.__classcall__(cls, 'macaulay2', 'sage.interfaces.macaulay2')
 
 
 class Octave(InterfaceFeature):
 
     @staticmethod
     def __classcall__(cls):
-        return InterfaceFeature(cls, 'octave', 'sage.interfaces.octave')
+        return InterfaceFeature.__classcall__(cls, 'octave', 'sage.interfaces.octave')
 
 
 class Scilab(InterfaceFeature):
 
     @staticmethod
     def __classcall__(cls):
-        return InterfaceFeature(cls, 'scilab', 'sage.interfaces.scilab')
+        return InterfaceFeature.__classcall__(cls, 'scilab', 'sage.interfaces.scilab')
 
 
 def all_features():
@@ -119,8 +119,13 @@ def all_features():
 
         sage: from sage.features.interfaces import all_features
         sage: list(all_features())
-        [Feature(<class 'sage.features.interfaces.Magma'>: sage.interfaces.magma), ...,
-         Feature(<class 'sage.features.interfaces.Scilab'>: sage.interfaces.scilab)]
+        [Feature('magma'),
+         Feature('matlab'),
+         Feature('mathematica'),
+         Feature('maple'),
+         Feature('macaulay2'),
+         Feature('octave'),
+         Feature('scilab')]
     """
     return [Magma(),
             Matlab(),

From 7fa5b3bf93c320f0d0d388b7c7c240dfe51d8300 Mon Sep 17 00:00:00 2001
From: Matthias Koeppe <mkoeppe@math.ucdavis.edu>
Date: Sun, 14 Nov 2021 14:24:55 -0800
Subject: [PATCH 18/29] src/sage/doctest/external.py (external_features): Add
 CPLEX, Gurobi

---
 src/sage/doctest/external.py | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/sage/doctest/external.py b/src/sage/doctest/external.py
index 7e73dc60577..2736ed09086 100644
--- a/src/sage/doctest/external.py
+++ b/src/sage/doctest/external.py
@@ -341,6 +341,8 @@ def external_features():
     features.extend(sage.features.latex.all_features())
     import sage.features.interfaces
     features.extend(sage.features.interfaces.all_features())
+    from sage.features.mip_backends import CPLEX, Gurobi
+    features.extend([CPLEX(), Gurobi()])
     from sage.features.pandoc import Pandoc
     features.append(Pandoc())
     from sage.features.graphviz import Graphviz

From 132f00dfbe947bb3898debb5ccf659b728624fb9 Mon Sep 17 00:00:00 2001
From: Matthias Koeppe <mkoeppe@math.ucdavis.edu>
Date: Sun, 14 Nov 2021 15:43:31 -0800
Subject: [PATCH 19/29] sage.doctest.{control,external}: Detect optional tags
 via AvailableSoftware

---
 src/sage/doctest/control.py  | 15 ++++-----------
 src/sage/doctest/external.py | 35 ++++++++++++++++-------------------
 src/sage/doctest/parsing.py  |  3 +--
 3 files changed, 21 insertions(+), 32 deletions(-)

diff --git a/src/sage/doctest/control.py b/src/sage/doctest/control.py
index 251e4f34a43..37f7f469d37 100644
--- a/src/sage/doctest/control.py
+++ b/src/sage/doctest/control.py
@@ -409,11 +409,6 @@ def __init__(self, options, args):
                     options.optional.update(system.name
                                             for system in package_systems())
 
-                    logger = sys.stderr if options.verbose else None
-                    from sage.features.sagemath import sage_features
-                    options.optional.update(feature.name
-                                            for feature in sage_features(logger=logger))
-
                 # Check that all tags are valid
                 for o in options.optional:
                     if not optionaltag_regex.search(o):
@@ -1247,18 +1242,16 @@ def run(self):
                     pass
 
             self.log("Using --optional=" + self._optional_tags_string())
-            if self.options.optional is True or 'external' in self.options.optional:
-                self.log("External software to be detected: " + ','.join(external_software))
-
+            self.log("External software to be detected: " + ','.join(external_software))
+            available_software._allow_external = self.options.optional is True or 'external' in self.options.optional
             self.add_files()
             self.expand_files_into_sources()
             self.filter_sources()
             self.sort_sources()
             self.run_doctests()
 
-            if self.options.optional is True or 'external' in self.options.optional:
-                self.log("External software detected for doctesting: "
-                         + ','.join(available_software.seen()))
+            self.log("Features detected for doctesting: "
+                     + ','.join(available_software.seen()))
             self.cleanup()
             return self.reporter.error_status
 
diff --git a/src/sage/doctest/external.py b/src/sage/doctest/external.py
index 2736ed09086..882227fdbe1 100644
--- a/src/sage/doctest/external.py
+++ b/src/sage/doctest/external.py
@@ -375,20 +375,6 @@ def external_software():
 
 external_software = external_software()
 
-def _lookup(software):
-    """
-    Test if the software is available on the system.
-
-    EXAMPLES::
-
-        sage: sage.doctest.external._lookup('internet') # random, optional - internet
-        True
-    """
-    if software in external_software:
-        func = globals().get(prefix + software)
-        return func()
-    else:
-        return False
 
 class AvailableSoftware(object):
     """
@@ -437,10 +423,17 @@ def __init__(self):
             sage: S.seen() # random
             []
         """
+        self._allow_external = True
         # For multiprocessing of doctests, the data self._seen should be
         # shared among subprocesses. Thus we use Array class from the
         # multiprocessing module.
-        self._seen = Array('i', len(external_software)) # initialized to zeroes
+        from sage.features.all import all_features
+        self._external_features = set(external_features())
+        features = set(self._external_features)
+        features.update(all_features())
+        self._features = sorted(features, key=lambda feature: feature.name)
+        self._indices = {feature.name: idx for idx, feature in enumerate(self._features)}
+        self._seen = Array('i', len(self._features)) # initialized to zeroes
 
     def __contains__(self, item):
         """
@@ -453,11 +446,13 @@ def __contains__(self, item):
             True
         """
         try:
-            idx = external_software.index(item)
-        except Exception:
+            idx = self._indices[item]
+        except KeyError:
             return False
         if not self._seen[idx]:
-            if _lookup(item):
+            if not self._allow_external and self._features[idx] in self._external_features:
+                self._seen[idx] = -1 # not available
+            elif self._features[idx].is_present():
                 self._seen[idx] = 1 # available
             else:
                 self._seen[idx] = -1 # not available
@@ -493,6 +488,8 @@ def seen(self):
             sage: available_software.seen() # random
             ['internet', 'latex', 'magma']
         """
-        return [external_software[i] for i in range(len(external_software)) if self._seen[i] > 0]
+        return [feature.name
+                for feature, seen in zip(self._features, self._seen)
+                if seen > 0]
 
 available_software = AvailableSoftware()
diff --git a/src/sage/doctest/parsing.py b/src/sage/doctest/parsing.py
index 3346ca14a24..dbb8a8f43a9 100644
--- a/src/sage/doctest/parsing.py
+++ b/src/sage/doctest/parsing.py
@@ -798,8 +798,7 @@ def parse(self, string, *args):
                     if self.optional_tags is not True:
                         extra = optional_tags - self.optional_tags # set difference
                         if extra:
-                            if not('external' in self.optional_tags
-                                   and available_software.issuperset(extra)):
+                            if not available_software.issuperset(extra):
                                 continue
                 elif self.optional_only:
                     self.optionals['sage'] += 1

From 68b3d4186903d2d215b709bb17c55979ac4353b1 Mon Sep 17 00:00:00 2001
From: Matthias Koeppe <mkoeppe@math.ucdavis.edu>
Date: Sun, 14 Nov 2021 16:08:20 -0800
Subject: [PATCH 20/29] src/sage/doctest/external.py (external_features):
 Rewrite as generator, add doctest

---
 src/sage/doctest/external.py | 36 ++++++++++++++++++++++--------------
 1 file changed, 22 insertions(+), 14 deletions(-)

diff --git a/src/sage/doctest/external.py b/src/sage/doctest/external.py
index 882227fdbe1..e95592b86ce 100644
--- a/src/sage/doctest/external.py
+++ b/src/sage/doctest/external.py
@@ -334,32 +334,40 @@ def has_4ti2():
     return FourTi2().is_present()
 
 def external_features():
-    features = []
+    r"""
+    Generate the features that are only to be tested if ``--optional=external`` is used.
+
+    EXAMPLES::
+
+        sage: from sage.doctest.external import external_features
+        sage: next(external_features())
+        Feature('internet')
+    """
     from sage.features.internet import Internet
-    features.append(Internet())
+    yield Internet()
     import sage.features.latex
-    features.extend(sage.features.latex.all_features())
+    yield from sage.features.latex.all_features()
     import sage.features.interfaces
-    features.extend(sage.features.interfaces.all_features())
+    yield from sage.features.interfaces.all_features()
     from sage.features.mip_backends import CPLEX, Gurobi
-    features.extend([CPLEX(), Gurobi()])
+    yield CPLEX()
+    yield Gurobi()
     from sage.features.pandoc import Pandoc
-    features.append(Pandoc())
+    yield Pandoc()
     from sage.features.graphviz import Graphviz
-    features.append(Graphviz())
+    yield Graphviz()
     from sage.features.ffmpeg import FFmpeg
-    features.append(FFmpeg())
+    yield FFmpeg()
     from sage.features.imagemagick import ImageMagick
-    features.append(ImageMagick())
+    yield ImageMagick()
     from sage.features.dvipng import dvipng
-    features.append(dvipng())
+    yield dvipng()
     from sage.features.pdf2svg import pdf2svg
-    features.append(pdf2svg())
+    yield pdf2svg()
     from sage.features.rubiks import Rubiks
-    features.append(Rubiks())
+    yield Rubiks()
     from sage.features.four_ti_2 import FourTi2
-    features.append(FourTi2())
-    return features
+    yield FourTi2()
 
 def external_software():
     """

From 6f2d22bebad1f9c043499bdc0a5fea6406ac6c7f Mon Sep 17 00:00:00 2001
From: Matthias Koeppe <mkoeppe@math.ucdavis.edu>
Date: Mon, 15 Nov 2021 09:47:40 -0800
Subject: [PATCH 21/29] src/sage/doctest/external.py: Declare pandoc, graphviz,
 ffmpeg, imagemagick, dvipng, pdf2svg, rubiks, 4ti2 feature tests safe (do not
 require 'external' to test them)

---
 src/sage/doctest/external.py | 18 ------------------
 1 file changed, 18 deletions(-)

diff --git a/src/sage/doctest/external.py b/src/sage/doctest/external.py
index e95592b86ce..de69191a5e3 100644
--- a/src/sage/doctest/external.py
+++ b/src/sage/doctest/external.py
@@ -345,29 +345,11 @@ def external_features():
     """
     from sage.features.internet import Internet
     yield Internet()
-    import sage.features.latex
-    yield from sage.features.latex.all_features()
     import sage.features.interfaces
     yield from sage.features.interfaces.all_features()
     from sage.features.mip_backends import CPLEX, Gurobi
     yield CPLEX()
     yield Gurobi()
-    from sage.features.pandoc import Pandoc
-    yield Pandoc()
-    from sage.features.graphviz import Graphviz
-    yield Graphviz()
-    from sage.features.ffmpeg import FFmpeg
-    yield FFmpeg()
-    from sage.features.imagemagick import ImageMagick
-    yield ImageMagick()
-    from sage.features.dvipng import dvipng
-    yield dvipng()
-    from sage.features.pdf2svg import pdf2svg
-    yield pdf2svg()
-    from sage.features.rubiks import Rubiks
-    yield Rubiks()
-    from sage.features.four_ti_2 import FourTi2
-    yield FourTi2()
 
 def external_software():
     """

From 2f50385550e03e6f02d1432864be5dba559b081e Mon Sep 17 00:00:00 2001
From: Matthias Koeppe <mkoeppe@math.ucdavis.edu>
Date: Mon, 15 Nov 2021 10:04:25 -0800
Subject: [PATCH 22/29] src/sage/features/: Add more all_features functions

---
 src/sage/features/csdp.py        | 4 ++++
 src/sage/features/databases.py   | 7 +++++++
 src/sage/features/imagemagick.py | 4 ++++
 3 files changed, 15 insertions(+)

diff --git a/src/sage/features/csdp.py b/src/sage/features/csdp.py
index 1d739f684eb..90fc8ddf4cc 100644
--- a/src/sage/features/csdp.py
+++ b/src/sage/features/csdp.py
@@ -64,3 +64,7 @@ def is_functional(self):
                                          .format(command=" ".join(command)))
 
         return FeatureTestResult(self, True)
+
+
+def all_features():
+    return [CSDP()]
diff --git a/src/sage/features/databases.py b/src/sage/features/databases.py
index 7fff8075914..afb9373cd15 100644
--- a/src/sage/features/databases.py
+++ b/src/sage/features/databases.py
@@ -125,3 +125,10 @@ def __init__(self):
             True
         """
         PythonModule.__init__(self, 'database_knotinfo', spkg='database_knotinfo')
+
+
+def all_features():
+    return [DatabaseConwayPolynomials(),
+            DatabaseCremona(), DatabaseCremona('cremona_mini'),
+            DatabaseJones(),
+            DatabaseKnotInfo()]
diff --git a/src/sage/features/imagemagick.py b/src/sage/features/imagemagick.py
index 7ea27ea40de..8bf53a24f3a 100644
--- a/src/sage/features/imagemagick.py
+++ b/src/sage/features/imagemagick.py
@@ -46,3 +46,7 @@ def __init__(self):
                              [Executable("convert", executable="convert")],
                              spkg="_recommended",
                              url="https://www.imagemagick.org/")
+
+
+def all_features():
+    return [ImageMagick()]

From e2cda415abc4c5af6fea8e4739016d877414091e Mon Sep 17 00:00:00 2001
From: Matthias Koeppe <mkoeppe@math.ucdavis.edu>
Date: Mon, 15 Nov 2021 10:17:54 -0800
Subject: [PATCH 23/29] src/sage/features/: Add remaining missing all_features
 functions

---
 src/sage/features/ffmpeg.py           | 4 ++++
 src/sage/features/four_ti_2.py        | 4 ++++
 src/sage/features/graph_generators.py | 6 ++++++
 src/sage/features/graphviz.py         | 4 ++++
 src/sage/features/internet.py         | 4 ++++
 src/sage/features/kenzo.py            | 3 +++
 src/sage/features/latte.py            | 4 ++++
 src/sage/features/lrs.py              | 4 ++++
 src/sage/features/mcqd.py             | 4 ++++
 src/sage/features/meataxe.py          | 4 ++++
 src/sage/features/pandoc.py           | 4 ++++
 src/sage/features/rubiks.py           | 4 ++++
 src/sage/features/tdlib.py            | 4 ++++
 13 files changed, 53 insertions(+)

diff --git a/src/sage/features/ffmpeg.py b/src/sage/features/ffmpeg.py
index 68245d24c27..ebfda670951 100644
--- a/src/sage/features/ffmpeg.py
+++ b/src/sage/features/ffmpeg.py
@@ -35,3 +35,7 @@ def __init__(self):
         """
         Executable.__init__(self, "ffmpeg", executable="ffmpeg",
                             url="https://www.ffmpeg.org/")
+
+
+def all_packages():
+    return [FFmpeg()]
diff --git a/src/sage/features/four_ti_2.py b/src/sage/features/four_ti_2.py
index 83ac7b6d2ab..1eda0402607 100644
--- a/src/sage/features/four_ti_2.py
+++ b/src/sage/features/four_ti_2.py
@@ -44,3 +44,7 @@ def __init__(self):
                               # same list is tested in build/pkgs/4ti2/spkg-configure.m4
                               for x in ('hilbert', 'markov', 'graver', 'zsolve', 'qsolve',
                                         'rays', 'ppi', 'circuits', 'groebner')])
+
+
+def all_features():
+    return [FourTi2()]
diff --git a/src/sage/features/graph_generators.py b/src/sage/features/graph_generators.py
index a47dca0e5d5..1fcd5764879 100644
--- a/src/sage/features/graph_generators.py
+++ b/src/sage/features/graph_generators.py
@@ -152,3 +152,9 @@ def is_functional(self):
                     reason="Call `{command}` did not produce output that started with `{expected}`.".format(command=" ".join(command), expected=expected))
 
         return FeatureTestResult(self, True)
+
+
+def all_features():
+    return [Plantri(),
+            Buckygen(),
+            Benzene()]
diff --git a/src/sage/features/graphviz.py b/src/sage/features/graphviz.py
index 8df3c8002bc..2eb284da424 100644
--- a/src/sage/features/graphviz.py
+++ b/src/sage/features/graphviz.py
@@ -110,3 +110,7 @@ def __init__(self):
                              [dot(), neato(), twopi()],
                              spkg="graphviz",
                              url="https://www.graphviz.org/")
+
+
+def all_features():
+    return [Graphviz()]
diff --git a/src/sage/features/internet.py b/src/sage/features/internet.py
index 77fa6f0046b..992419e812a 100644
--- a/src/sage/features/internet.py
+++ b/src/sage/features/internet.py
@@ -44,3 +44,7 @@ def _is_present(self):
             return FeatureTestResult(self, True)
         except urllib.error.URLError:
             return FeatureTestResult(self, False)
+
+
+def all_features():
+    return [Internet()]
diff --git a/src/sage/features/kenzo.py b/src/sage/features/kenzo.py
index f1778d9a379..345f1ae388f 100644
--- a/src/sage/features/kenzo.py
+++ b/src/sage/features/kenzo.py
@@ -56,3 +56,6 @@ def _is_present(self):
             return FeatureTestResult(self, False, reason="Unable to make ECL require kenzo")
         return FeatureTestResult(self, True)
 
+
+def all_features():
+    return [Kenzo()]
diff --git a/src/sage/features/latte.py b/src/sage/features/latte.py
index 7b4d9dda756..496719bd0c8 100644
--- a/src/sage/features/latte.py
+++ b/src/sage/features/latte.py
@@ -65,3 +65,7 @@ def __init__(self):
         JoinFeature.__init__(self, "latte_int",
                              (Latte_count(), Latte_integrate()),
                              description="LattE")
+
+
+def all_features():
+    return [Latte()]
diff --git a/src/sage/features/lrs.py b/src/sage/features/lrs.py
index ab1871a6314..f9deae3caf0 100644
--- a/src/sage/features/lrs.py
+++ b/src/sage/features/lrs.py
@@ -63,3 +63,7 @@ def is_functional(self):
                     expected=" or ".join(expected_list)))
 
         return FeatureTestResult(self, True)
+
+
+def all_features():
+    return [Lrs()]
diff --git a/src/sage/features/mcqd.py b/src/sage/features/mcqd.py
index 5fe2f26e881..ec5809f384b 100644
--- a/src/sage/features/mcqd.py
+++ b/src/sage/features/mcqd.py
@@ -25,3 +25,7 @@ def __init__(self):
         # Will be changed to spkg='sagemath_mcqd' later
         JoinFeature.__init__(self, 'mcqd',
                              [PythonModule('sage.graphs.mcqd', spkg='mcqd')])
+
+
+def all_features():
+    return [Mcqd()]
diff --git a/src/sage/features/meataxe.py b/src/sage/features/meataxe.py
index eb32016ac3e..5bbefe1f0d1 100644
--- a/src/sage/features/meataxe.py
+++ b/src/sage/features/meataxe.py
@@ -24,3 +24,7 @@ def __init__(self):
         # Will be changed to spkg='sagemath_meataxe' later
         JoinFeature.__init__(self, 'meataxe',
                              [PythonModule('sage.matrix.matrix_gfpn_dense', spkg='meataxe')])
+
+
+def all_features():
+    return [Meataxe()]
diff --git a/src/sage/features/pandoc.py b/src/sage/features/pandoc.py
index 566032a643b..4532b49132d 100644
--- a/src/sage/features/pandoc.py
+++ b/src/sage/features/pandoc.py
@@ -35,3 +35,7 @@ def __init__(self):
         """
         Executable.__init__(self, "pandoc", executable="pandoc",
                             url="https://pandoc.org/")
+
+
+def all_features():
+    return [Pandoc()]
diff --git a/src/sage/features/rubiks.py b/src/sage/features/rubiks.py
index 9074fe58c81..6175374ed1b 100644
--- a/src/sage/features/rubiks.py
+++ b/src/sage/features/rubiks.py
@@ -174,3 +174,7 @@ def __init__(self):
         JoinFeature.__init__(self, "rubiks",
                              [cu2(), size222(), optimal(), mcube(), dikcube(), cubex()],
                              spkg="rubiks")
+
+
+def all_features():
+    return [Rubiks()]
diff --git a/src/sage/features/tdlib.py b/src/sage/features/tdlib.py
index cae1f5abc78..ad73807883c 100644
--- a/src/sage/features/tdlib.py
+++ b/src/sage/features/tdlib.py
@@ -18,3 +18,7 @@ def __init__(self):
         # Will be changed to spkg='sagemath_tdlib' later
         JoinFeature.__init__(self, 'tdlib',
                              [PythonModule('sage.graphs.graph_decompositions.tdlib', spkg='tdlib')])
+
+
+def all_features():
+    return [Tdlib()]

From c4deceb2babdadafc3044bb5918c1f8a4467c82e Mon Sep 17 00:00:00 2001
From: Matthias Koeppe <mkoeppe@math.ucdavis.edu>
Date: Mon, 15 Nov 2021 10:43:51 -0800
Subject: [PATCH 24/29] src/sage/doctest/control.py: Replace use of
 'external_software' by new method 'available_software.detectable()'

---
 src/sage/doctest/control.py  | 4 ++--
 src/sage/doctest/external.py | 9 +++++++++
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/src/sage/doctest/control.py b/src/sage/doctest/control.py
index 37f7f469d37..30bc01b143b 100644
--- a/src/sage/doctest/control.py
+++ b/src/sage/doctest/control.py
@@ -38,7 +38,7 @@
 from .forker import DocTestDispatcher
 from .reporting import DocTestReporter
 from .util import Timer, count_noun, dict_difference
-from .external import external_software, available_software
+from .external import available_software
 from .parsing import parse_optional_tags
 
 nodoctest_regex = re.compile(r'\s*(#+|%+|r"+|"+|\.\.)\s*nodoctest')
@@ -1242,8 +1242,8 @@ def run(self):
                     pass
 
             self.log("Using --optional=" + self._optional_tags_string())
-            self.log("External software to be detected: " + ','.join(external_software))
             available_software._allow_external = self.options.optional is True or 'external' in self.options.optional
+            self.log("Features to be detected: " + ','.join(available_software.detectable()))
             self.add_files()
             self.expand_files_into_sources()
             self.filter_sources()
diff --git a/src/sage/doctest/external.py b/src/sage/doctest/external.py
index de69191a5e3..c2ad4ed4949 100644
--- a/src/sage/doctest/external.py
+++ b/src/sage/doctest/external.py
@@ -468,6 +468,14 @@ def issuperset(self, other):
                 return False
         return True
 
+    def detectable(self):
+        """
+        Return the list of names of those features for which testing their presence is allowed.
+        """
+        return [feature.name
+                for feature in self._features
+                if self._allow_external or feature not in self._external_features]
+
     def seen(self):
         """
         Return the list of detected external software.
@@ -482,4 +490,5 @@ def seen(self):
                 for feature, seen in zip(self._features, self._seen)
                 if seen > 0]
 
+
 available_software = AvailableSoftware()

From b8f0a107bd1bc621ecf5d7cacddae0e2110b8427 Mon Sep 17 00:00:00 2001
From: Matthias Koeppe <mkoeppe@math.ucdavis.edu>
Date: Mon, 15 Nov 2021 14:26:57 -0800
Subject: [PATCH 25/29] src/sage/features/normaliz.py: Add all_features

---
 src/sage/features/normaliz.py | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/sage/features/normaliz.py b/src/sage/features/normaliz.py
index ceb67875b01..ae3879c1409 100644
--- a/src/sage/features/normaliz.py
+++ b/src/sage/features/normaliz.py
@@ -26,3 +26,7 @@ def __init__(self):
         """
         JoinFeature.__init__(self, 'pynormaliz',
                              [PythonModule('PyNormaliz', spkg="pynormaliz")])
+
+
+def all_features():
+    return [PyNormaliz()]

From 944bb74f0bf0260fbc262152005745edfcdd3ad9 Mon Sep 17 00:00:00 2001
From: Matthias Koeppe <mkoeppe@math.ucdavis.edu>
Date: Mon, 15 Nov 2021 16:13:54 -0800
Subject: [PATCH 26/29] src/sage/features/interfaces.py: Add doctests

---
 src/sage/features/interfaces.py | 91 ++++++++++++++++++++++++++++++++-
 1 file changed, 90 insertions(+), 1 deletion(-)

diff --git a/src/sage/features/interfaces.py b/src/sage/features/interfaces.py
index 231fba136b2..9991cfc690c 100644
--- a/src/sage/features/interfaces.py
+++ b/src/sage/features/interfaces.py
@@ -8,6 +8,8 @@
 
 class InterfaceFeature(Feature):
     r"""
+    A :class:`Feature` describing whether an :class:`~sage.interfaces.interface.Interface` is present and functional.
+
     TESTS::
 
         sage: from sage.features.interfaces import InterfaceFeature
@@ -25,15 +27,41 @@ class InterfaceFeature(Feature):
     """
     @staticmethod
     def __classcall__(cls, name, module, description=None):
+        """
+        TESTS::
+
+            sage: from sage.features import PythonModule
+            sage: from sage.features.interfaces import InterfaceFeature
+            sage: f = InterfaceFeature("test_interface", "sage.interfaces.interface")
+            sage: f is InterfaceFeature("test_interface", PythonModule("sage.interfaces.interface"))
+            True
+        """
         if isinstance(module, str):
             module = PythonModule(module)
         return Feature.__classcall__(cls, name, module, description)
 
     def __init__(self, name, module, description):
+        """
+        TESTS::
+
+            sage: from sage.features.interfaces import InterfaceFeature
+            sage: f = InterfaceFeature("test_interface", "sage.interfaces.interface")
+            sage: isinstance(f, InterfaceFeature)
+            True
+        """
         super().__init__(name, description=description)
         self.module = module
 
     def _is_present(self):
+        """
+        TESTS::
+
+            sage: from sage.features.interfaces import InterfaceFeature
+            sage: from sage.interfaces.sage0 import Sage
+            sage: f = InterfaceFeature("sage0", "sage.interfaces.sage0")
+            sage: f.is_present()
+            FeatureTestResult('sage0', True)
+        """
         result = self.module.is_present()
         if not result:
             return result
@@ -50,6 +78,7 @@ def _is_present(self):
             return FeatureTestResult(self, False,
                                      reason=f"Interface {interface} is not functional: {exception}")
 
+
 # The following are provided by external software only (no SPKG)
 
 class Magma(InterfaceFeature):
@@ -61,7 +90,7 @@ class Magma(InterfaceFeature):
 
         sage: from sage.features.interfaces import Magma
         sage: Magma().is_present()  # random
-        FeatureTestResult('jupymake', False)
+        FeatureTestResult('magma', False)
     """
 
     @staticmethod
@@ -70,6 +99,16 @@ def __classcall__(cls):
 
 
 class Matlab(InterfaceFeature):
+    r"""
+    A :class:`sage.features.Feature` describing whether :class:`sage.interfaces.matlab.Matlab`
+    is present and functional.
+
+    EXAMPLES::
+
+        sage: from sage.features.interfaces import Matlab
+        sage: Matlab().is_present()  # random
+        FeatureTestResult('matlab', False)
+    """
 
     @staticmethod
     def __classcall__(cls):
@@ -77,6 +116,16 @@ def __classcall__(cls):
 
 
 class Mathematica(InterfaceFeature):
+    r"""
+    A :class:`sage.features.Feature` describing whether :class:`sage.interfaces.mathematica.Mathematica`
+    is present and functional.
+
+    EXAMPLES::
+
+        sage: from sage.features.interfaces import Mathematica
+        sage: Mathematica().is_present()  # random
+        FeatureTestResult('mathematica', False)
+    """
 
     @staticmethod
     def __classcall__(cls):
@@ -84,6 +133,16 @@ def __classcall__(cls):
 
 
 class Maple(InterfaceFeature):
+    r"""
+    A :class:`sage.features.Feature` describing whether :class:`sage.interfaces.maple.Maple`
+    is present and functional.
+
+    EXAMPLES::
+
+        sage: from sage.features.interfaces import Maple
+        sage: Maple().is_present()  # random
+        FeatureTestResult('maple', False)
+    """
 
     @staticmethod
     def __classcall__(cls):
@@ -91,6 +150,16 @@ def __classcall__(cls):
 
 
 class Macaulay2(InterfaceFeature):
+    r"""
+    A :class:`sage.features.Feature` describing whether :class:`sage.interfaces.macaulay2.Macaulay2`
+    is present and functional.
+
+    EXAMPLES::
+
+        sage: from sage.features.interfaces import Macaulay2
+        sage: Macaulay2().is_present()  # random
+        FeatureTestResult('macaulay2', False)
+    """
 
     @staticmethod
     def __classcall__(cls):
@@ -98,6 +167,16 @@ def __classcall__(cls):
 
 
 class Octave(InterfaceFeature):
+    r"""
+    A :class:`sage.features.Feature` describing whether :class:`sage.interfaces.octave.Octave`
+    is present and functional.
+
+    EXAMPLES::
+
+        sage: from sage.features.interfaces import Octave
+        sage: Octave().is_present()  # random
+        FeatureTestResult('octave', False)
+    """
 
     @staticmethod
     def __classcall__(cls):
@@ -105,6 +184,16 @@ def __classcall__(cls):
 
 
 class Scilab(InterfaceFeature):
+    r"""
+    A :class:`sage.features.Feature` describing whether :class:`sage.interfaces.scilab.Scilab`
+    is present and functional.
+
+    EXAMPLES::
+
+        sage: from sage.features.interfaces import Scilab
+        sage: Scilab().is_present()  # random
+        FeatureTestResult('scilab', False)
+    """
 
     @staticmethod
     def __classcall__(cls):

From 91f0bcc173d40c15abfba67d7c01a453726a1a02 Mon Sep 17 00:00:00 2001
From: Matthias Koeppe <mkoeppe@math.ucdavis.edu>
Date: Mon, 15 Nov 2021 16:46:49 -0800
Subject: [PATCH 27/29] src/sage/doctest: Update doctest outputs

---
 src/sage/doctest/control.py  |  9 ++++++---
 src/sage/doctest/external.py | 16 ++--------------
 src/sage/doctest/forker.py   |  2 ++
 3 files changed, 10 insertions(+), 17 deletions(-)

diff --git a/src/sage/doctest/control.py b/src/sage/doctest/control.py
index 30bc01b143b..88bdb6e49b8 100644
--- a/src/sage/doctest/control.py
+++ b/src/sage/doctest/control.py
@@ -923,7 +923,7 @@ def run_doctests(self):
             ----------------------------------------------------------------------
             Total time for all tests: ... seconds
                 cpu time: ... seconds
-                cumulative wall time: ... seconds
+                cumulative wall time: ... seconds...
         """
         nfiles = 0
         nother = 0
@@ -999,6 +999,7 @@ def cleanup(self, final=True):
             Total time for all tests: ... seconds
                 cpu time: ... seconds
                 cumulative wall time: ... seconds
+            Features detected...
             0
             sage: DC.cleanup()
         """
@@ -1189,6 +1190,7 @@ def run(self):
             Total time for all tests: ... seconds
                 cpu time: ... seconds
                 cumulative wall time: ... seconds
+            Features detected...
             0
 
         We check that :trac:`25378` is fixed (testing external packages
@@ -1202,7 +1204,7 @@ def run(self):
             sage: DC.run()
             Running doctests with ID ...
             Using --optional=external,sage
-            External software to be detected: ...
+            Features to be detected: ...
             Doctesting 1 file.
             sage -t ....py
                 [0 tests, ... s]
@@ -1212,7 +1214,7 @@ def run(self):
             Total time for all tests: ... seconds
                 cpu time: ... seconds
                 cumulative wall time: ... seconds
-            External software detected for doctesting:...
+            Features detected...
             0
 
         """
@@ -1279,6 +1281,7 @@ def run_doctests(module, options=None):
         Total time for all tests: ... seconds
             cpu time: ... seconds
             cumulative wall time: ... seconds
+        Features detected...
     """
     import sys
     sys.stdout.flush()
diff --git a/src/sage/doctest/external.py b/src/sage/doctest/external.py
index c2ad4ed4949..ad78384c67a 100644
--- a/src/sage/doctest/external.py
+++ b/src/sage/doctest/external.py
@@ -375,28 +375,16 @@ class AvailableSoftware(object):
 
         sage: from sage.doctest.external import external_software, available_software
         sage: external_software
-        ['4ti2',
-         'cplex',
-         'dvipng',
-         'ffmpeg',
-         'graphviz',
+        ['cplex',
          'gurobi',
-         'imagemagick',
          'internet',
-         'latex',
-         'lualatex',
          'macaulay2',
          'magma',
          'maple',
          'mathematica',
          'matlab',
          'octave',
-         'pandoc',
-         'pdf2svg',
-         'pdflatex',
-         'rubiks',
-         'scilab',
-         'xelatex']
+         'scilab']
         sage: 'internet' in available_software # random, optional - internet
         True
         sage: available_software.issuperset(set(['internet','latex'])) # random, optional - internet latex
diff --git a/src/sage/doctest/forker.py b/src/sage/doctest/forker.py
index 3307d50939e..e36c7bdbafb 100644
--- a/src/sage/doctest/forker.py
+++ b/src/sage/doctest/forker.py
@@ -2056,6 +2056,7 @@ def __init__(self, source, options, funclist=[]):
             Total time for all tests: ... seconds
                 cpu time: ... seconds
                 cumulative wall time: ... seconds
+            Features detected...
         """
         multiprocessing.Process.__init__(self)
 
@@ -2101,6 +2102,7 @@ def run(self):
             Total time for all tests: ... seconds
                 cpu time: ... seconds
                 cumulative wall time: ... seconds
+            Features detected...
         """
         os.setpgid(os.getpid(), os.getpid())
 

From 12f7c98f790982e4ed6ab4126d2b0c80efef8bf7 Mon Sep 17 00:00:00 2001
From: Matthias Koeppe <mkoeppe@math.ucdavis.edu>
Date: Mon, 15 Nov 2021 17:00:38 -0800
Subject: [PATCH 28/29] src/sage/features/polymake.py: Add all_features

---
 src/sage/features/polymake.py | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/sage/features/polymake.py b/src/sage/features/polymake.py
index 1e6d7b81b99..cbcd8badc0e 100644
--- a/src/sage/features/polymake.py
+++ b/src/sage/features/polymake.py
@@ -23,3 +23,7 @@ def __init__(self):
         """
         JoinFeature.__init__(self, "jupymake",
                              [PythonModule("JuPyMake", spkg="jupymake")])
+
+
+def all_features():
+    return [JuPyMake()]

From c9312dbe74009795f2ea1a20cb5f4c2b087415c7 Mon Sep 17 00:00:00 2001
From: Matthias Koeppe <mkoeppe@math.ucdavis.edu>
Date: Mon, 15 Nov 2021 17:06:02 -0800
Subject: [PATCH 29/29] src/sage/features: Add more all_features functions

---
 src/sage/features/dvipng.py  | 4 ++++
 src/sage/features/ffmpeg.py  | 2 +-
 src/sage/features/pdf2svg.py | 4 ++++
 3 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/src/sage/features/dvipng.py b/src/sage/features/dvipng.py
index 27c5bddc059..a22bdbf83b0 100644
--- a/src/sage/features/dvipng.py
+++ b/src/sage/features/dvipng.py
@@ -34,3 +34,7 @@ def __init__(self):
         """
         Executable.__init__(self, "dvipng", executable="dvipng",
                             url="https://savannah.nongnu.org/projects/dvipng/")
+
+
+def all_features():
+    return [dvipng()]
diff --git a/src/sage/features/ffmpeg.py b/src/sage/features/ffmpeg.py
index ebfda670951..ea26e38145d 100644
--- a/src/sage/features/ffmpeg.py
+++ b/src/sage/features/ffmpeg.py
@@ -37,5 +37,5 @@ def __init__(self):
                             url="https://www.ffmpeg.org/")
 
 
-def all_packages():
+def all_features():
     return [FFmpeg()]
diff --git a/src/sage/features/pdf2svg.py b/src/sage/features/pdf2svg.py
index f0b634f5ff4..fc964611901 100644
--- a/src/sage/features/pdf2svg.py
+++ b/src/sage/features/pdf2svg.py
@@ -35,3 +35,7 @@ def __init__(self):
         Executable.__init__(self, "pdf2svg", executable="pdf2svg",
                             spkg='pdf2svg',
                             url="http://www.cityinthesky.co.uk/opensource/pdf2svg/")
+
+
+def all_features():
+    return [pdf2svg()]