Give warnings about not being able to parse TOML files if toml isn't …

…installed

coverage/config.py

@@ -260,9 +260,6 @@ def from_file(self, filename, our_file):
         """
         _, ext = os.path.splitext(filename)
         if ext == '.toml':
-            from coverage.optional import toml
-            if toml is None:
-                return False
             cp = TomlConfigParser(our_file)
         else:
             cp = HandyConfigParser(our_file)

coverage/tomlconfig.py

@@ -25,34 +25,43 @@ class TomlConfigParser:
     # pylint: disable=missing-function-docstring
 
     def __init__(self, our_file):
+        self.our_file = our_file
         self.getters = [lambda obj: obj['tool']['coverage']]
-        if our_file:
+        if self.our_file:
             self.getters.append(lambda obj: obj)
 
         self._data = []
 
     def read(self, filenames):
+        # RawConfigParser takes a filename or list of filenames, but we only
+        # ever call this with a single filename.
+        assert isinstance(filenames, path_types)
+        filename = filenames
+        if env.PYVERSION >= (3, 6):
+            filename = os.fspath(filename)
+
         from coverage.optional import toml
         if toml is None:
-            raise RuntimeError('toml module is not installed.')
-
-        if isinstance(filenames, path_types):
-            filenames = [filenames]
-        read_ok = []
-        for filename in filenames:
-            try:
-                with io.open(filename, encoding='utf-8') as fp:
-                    toml_data = fp.read()
-                    toml_data = substitute_variables(toml_data, os.environ)
+            if self.our_file:
+                raise CoverageException("Can't read {!r} without TOML support".format(filename))
+
+        try:
+            with io.open(filename, encoding='utf-8') as fp:
+                toml_data = fp.read()
+            toml_data = substitute_variables(toml_data, os.environ)
+            if toml:
+                try:
                     self._data.append(toml.loads(toml_data))
-            except IOError:
-                continue
-            except toml.TomlDecodeError as err:
-                raise TomlDecodeError(*err.args)
-            if env.PYVERSION >= (3, 6):
-                filename = os.fspath(filename)
-            read_ok.append(filename)
-        return read_ok
+                except toml.TomlDecodeError as err:
+                    raise TomlDecodeError(*err.args)
+            elif re.search(r"^\[tool\.coverage\.", toml_data, flags=re.MULTILINE):
+                # Looks like they meant to read TOML, but we can't.
+                raise CoverageException("Can't read {!r} without TOML support".format(filename))
+            else:
+                return []
+        except IOError:
+            return []
+        return [filename]
 
     def has_option(self, section, option):
         for data in self._data:

tests/test_config.py

@@ -8,6 +8,7 @@
 
 import coverage
 from coverage.misc import CoverageException
+import coverage.optional
 
 from tests.coveragetest import CoverageTest, UsingModulesMixin
 
@@ -651,3 +652,36 @@ def test_nocoveragerc_file_when_specified(self):
         self.assertFalse(cov.config.timid)
         self.assertFalse(cov.config.branch)
         self.assertEqual(cov.config.data_file, ".coverage")
+
+    def test_no_toml_installed_explicit_toml(self):
+        # Can't specify a toml config file if toml isn't installed.
+        with coverage.optional.without('toml'):
+            msg = "Can't read 'cov.toml' without TOML support"
+            with self.assertRaisesRegex(CoverageException, msg):
+                coverage.Coverage(config_file="cov.toml")
+
+    def test_no_toml_installed_pyproject_toml(self):
+        # Can't have coverage config in pyproject.toml without toml installed.
+        self.make_file("pyproject.toml", """\
+            # A toml file!
+            [tool.coverage.run]
+            xyzzy = 17
+            """)
+        with coverage.optional.without('toml'):
+            msg = "Can't read 'pyproject.toml' without TOML support"
+            with self.assertRaisesRegex(CoverageException, msg):
+                coverage.Coverage()
+
+    def test_no_toml_installed_pyproject_no_coverage(self):
+        # It's ok to have non-coverage pyproject.toml without toml installed.
+        self.make_file("pyproject.toml", """\
+            # A toml file!
+            [tool.something]
+            xyzzy = 17
+            """)
+        with coverage.optional.without('toml'):
+            cov = coverage.Coverage()
+            # We get default settings:
+            self.assertFalse(cov.config.timid)
+            self.assertFalse(cov.config.branch)
+            self.assertEqual(cov.config.data_file, ".coverage")