Class fields feature

pyfields/__init__.py

@@ -1,5 +1,5 @@
 from .typing_utils import FieldTypeError
-from .core import field, Field, FieldError, MandatoryFieldInitError, UnsupportedOnNativeFieldError, \
+from .core import field, classfield, Field, FieldError, MandatoryFieldInitError, UnsupportedOnNativeFieldError, \
     ReadOnlyFieldError
 from .validate_n_convert import Converter, ConversionError, DetailedConversionResults, trace_convert
 from .init_makers import inject_fields, make_init, init_fields
@@ -19,7 +19,7 @@
     # submodules
     'core', 'validate_n_convert', 'init_makers', 'helpers',
     # symbols
-    'field', 'Field', 'FieldError', 'MandatoryFieldInitError', 'UnsupportedOnNativeFieldError',
+    'field', 'classfield', 'Field', 'FieldError', 'MandatoryFieldInitError', 'UnsupportedOnNativeFieldError',
     'ReadOnlyFieldError', 'FieldTypeError',
     'Converter', 'ConversionError', 'DetailedConversionResults', 'trace_convert',
     'inject_fields', 'make_init', 'init_fields',

pyfields/core.py

@@ -671,6 +671,45 @@ class (such as `make_init`). If provided, it should be the same name than the on
     :return:
     """
     # Should we create a Native or a Descriptor field ?
+    create_descriptor = _descriptor_needed(check_type, validators, converters, read_only, native)
+
+    # Create the correct type of field
+    if create_descriptor:
+        return DescriptorField(type_hint=type_hint, default=default, default_factory=default_factory,
+                               check_type=check_type, validators=validators, converters=converters,
+                               read_only=read_only, doc=doc, name=name)
+    else:
+        return NativeField(type_hint=type_hint, default=default, default_factory=default_factory,
+                           doc=doc, name=name)
+
+
+def classfield(type_hint=None,        # type: Union[Type[T], Iterable[Type[T]]]
+               check_type=False,      # type: bool
+               default=EMPTY,         # type: T
+               default_factory=None,  # type: Callable[[], T]
+               validators=None,       # type: Validators
+               converters=None,       # type: Converters
+               read_only=False,       # type: bool
+               doc=None,              # type: str
+               name=None,             # type: str
+               native=None            # type: bool
+               ):
+    # type: (...) -> Union[T, Field]
+    # Should we create a Native or a Descriptor field ?
+    create_descriptor = _descriptor_needed(check_type, validators, converters, read_only, native)
+
+    # Create the correct type of field
+    if create_descriptor:
+        return DescriptorClassField(type_hint=type_hint, default=default, default_factory=default_factory,
+                                    check_type=check_type, validators=validators, converters=converters,
+                                    read_only=read_only, doc=doc, name=name)
+    else:
+        return NativeClassField(type_hint=type_hint, default=default, default_factory=default_factory,
+                                doc=doc, name=name)
+
+
+def _descriptor_needed(check_type, validators, converters, read_only, native):
+    """ Should we create a Native or a Descriptor field ? """
     if native is None:
         # default: choose automatically according to user-provided options
         create_descriptor = check_type or (validators is not None) or (converters is not None) or read_only
@@ -687,15 +726,7 @@ class (such as `make_init`). If provided, it should be the same name than the on
         else:
             # explicit `native=False`. Force-use a descriptor
             create_descriptor = True
-
-    # Create the correct type of field
-    if create_descriptor:
-        return DescriptorField(type_hint=type_hint, default=default, default_factory=default_factory,
-                               check_type=check_type, validators=validators, converters=converters,
-                               read_only=read_only, doc=doc, name=name)
-    else:
-        return NativeField(type_hint=type_hint, default=default, default_factory=default_factory,
-                           doc=doc, name=name)
+    return create_descriptor
 
 
 class UnsupportedOnNativeFieldError(FieldError):
@@ -779,6 +810,18 @@ def __get__(self, obj, obj_type):
     #         pass
 
 
+class NativeClassField(NativeField):
+    """
+    A field that is replaced with a native python attribute on first read or write access.
+    Faster but provides not much flexibility (no validator, no type check, no converter)
+    """
+    __slots__ = ()
+
+    def __get__(self, obj, obj_type):
+        # same than super but it acts on the object type
+        return super(NativeClassField, self).__get__(obj_type, obj_type)
+
+
 class DescriptorField(Field):
     """
     General-purpose implementation for fields that require type-checking or validation or converter
@@ -993,6 +1036,25 @@ def __delete__(self, obj):
         delattr(obj, "_" + self.name)
 
 
+class DescriptorClassField(DescriptorField):
+    """
+    A field that is replaced with a native python attribute on first read or write access.
+    Faster but provides not much flexibility (no validator, no type check, no converter)
+    """
+    __slots__ = ()
+
+    def __get__(self, obj, obj_type):
+        # same than super but it acts on the object type
+        return super(DescriptorClassField, self).__get__(obj_type, obj_type)
+
+    def __set__(self,
+                obj,
+                value  # type: T
+                ):
+        # same than super but it acts on the object type
+        return super(DescriptorClassField, self).__set__(obj.__class__, value)
+
+
 def collect_all_fields(cls,
                        include_inherited=True,
                        remove_duplicates=True,

pyfields/tests/test_so.py

@@ -4,7 +4,8 @@
 
 import pytest
 
-from pyfields import ReadOnlyFieldError
+from pyfields import ReadOnlyFieldError, MandatoryFieldInitError, FieldTypeError
+from pyfields.core import DescriptorClassField
 from valid8 import ValidationError
 
 
@@ -206,3 +207,25 @@ class User(object):
     qualname = User.__dict__['username'].qualname
     assert str(exc_info.value) == "Read-only field '%s' has already been initialized on instance %s and cannot be " \
                                   "modified anymore." % (qualname, u)
+
+
+def test_so8_classfields():
+    """ checks answer at xxx (todo: not capable of doing this yet) """
+
+    from pyfields import classfield
+
+    class A(object):
+        s = classfield(type_hint=int, check_type=True)
+
+    class ClassFromA(A):
+        pass
+
+    s_field = A.__dict__['s']
+    assert isinstance(s_field, DescriptorClassField)
+
+    for c in (A, ClassFromA):
+        with pytest.raises(MandatoryFieldInitError):
+            print(c.s)
+
+        with pytest.raises(FieldTypeError):
+            c.s = "hello"