Fixed #16905 -- Added extensible checks (nee validation) framework

This is the result of Christopher Medrela's 2013 Summer of Code project.

Thanks also to Preston Holmes, Tim Graham, Anssi Kääriäinen, Florian
Apolloner, and Alex Gaynor for review notes along the way.

Also: Fixes #8579, fixes #3055, fixes #19844.

django/db/models/fields/files.py

@@ -3,6 +3,8 @@ import os
 
 from django import forms
 from django.db.models.fields import Field
+from django.core import checks
+from django.core.exceptions import ImproperlyConfigured
 from django.core.files.base import File
 from django.core.files.storage import default_storage
 from django.core.files.images import ImageFile
@@ -223,9 +225,8 @@ class FileField(Field):
     description = _("File")
 
     def __init__(self, verbose_name=None, name=None, upload_to='', storage=None, **kwargs):
-        for arg in ('primary_key', 'unique'):
-            if arg in kwargs:
-                raise TypeError("'%s' is not a valid argument for %s." % (arg, self.__class__))
+        self._primary_key_set_explicitly = 'primary_key' in kwargs
+        self._unique_set_explicitly = 'unique' in kwargs
 
         self.storage = storage or default_storage
         self.upload_to = upload_to
@@ -235,6 +236,52 @@ class FileField(Field):
         kwargs['max_length'] = kwargs.get('max_length', 100)
         super(FileField, self).__init__(verbose_name, name, **kwargs)
 
+    def check(self, **kwargs):
+        errors = super(FileField, self).check(**kwargs)
+        #errors.extend(self._check_upload_to())
+        errors.extend(self._check_unique())
+        errors.extend(self._check_primary_key())
+        return errors
+
+    def _check_upload_to(self):
+        if not self.upload_to:
+            return [
+                checks.Error(
+                    'The field requires an "upload_to" attribute.',
+                    hint=None,
+                    obj=self,
+                    id='E031',
+                )
+            ]
+        else:
+            return []
+
+    def _check_unique(self):
+        if self._unique_set_explicitly:
+            return [
+                checks.Error(
+                    '"unique" is not a valid argument for %s.' % self.__class__.__name__,
+                    hint=None,
+                    obj=self,
+                    id='E049',
+                )
+            ]
+        else:
+            return []
+
+    def _check_primary_key(self):
+        if self._primary_key_set_explicitly:
+            return [
+                checks.Error(
+                    '"primary_key" is not a valid argument for %s.' % self.__class__.__name__,
+                    hint=None,
+                    obj=self,
+                    id='E050',
+                )
+            ]
+        else:
+            return []
+
     def deconstruct(self):
         name, path, args, kwargs = super(FileField, self).deconstruct()
         if kwargs.get("max_length", None) != 100:
@@ -348,6 +395,27 @@ class ImageField(FileField):
         self.width_field, self.height_field = width_field, height_field
         super(ImageField, self).__init__(verbose_name, name, **kwargs)
 
+    def check(self, **kwargs):
+        errors = super(ImageField, self).check(**kwargs)
+        errors.extend(self._check_image_library_installed())
+        return errors
+
+    def _check_image_library_installed(self):
+        try:
+            from django.utils.image import Image  # NOQA
+        except ImproperlyConfigured:
+            return [
+                checks.Error(
+                    'To use ImageFields, Pillow must be installed.',
+                    hint=('Get Pillow at https://pypi.python.org/pypi/Pillow '
+                          'or run command "pip install pillow".'),
+                    obj=self,
+                    id='E032',
+                )
+            ]
+        else:
+            return []
+
     def deconstruct(self):
         name, path, args, kwargs = super(ImageField, self).deconstruct()
         if self.width_field: