Fixed #28873 -- Used dict.setdefault() to set model and form field defaults.

django/db/models/fields/__init__.py

@@ -1652,7 +1652,7 @@ class EmailField(CharField):
 
     def __init__(self, *args, **kwargs):
         # max_length=254 to be compliant with RFCs 3696 and 5321
-        kwargs['max_length'] = kwargs.get('max_length', 254)
+        kwargs.setdefault('max_length', 254)
         super().__init__(*args, **kwargs)
 
     def deconstruct(self):
@@ -1678,7 +1678,7 @@ class FilePathField(Field):
                  recursive=False, allow_files=True, allow_folders=False, **kwargs):
         self.path, self.match, self.recursive = path, match, recursive
         self.allow_files, self.allow_folders = allow_files, allow_folders
-        kwargs['max_length'] = kwargs.get('max_length', 100)
+        kwargs.setdefault('max_length', 100)
         super().__init__(verbose_name, name, **kwargs)
 
     def check(self, **kwargs):
@@ -2258,7 +2258,7 @@ class URLField(CharField):
     description = _("URL")
 
     def __init__(self, verbose_name=None, name=None, **kwargs):
-        kwargs['max_length'] = kwargs.get('max_length', 200)
+        kwargs.setdefault('max_length', 200)
         super().__init__(verbose_name, name, **kwargs)
 
     def deconstruct(self):

django/db/models/fields/files.py

@@ -226,7 +226,7 @@ class FileField(Field):
         self.storage = storage or default_storage
         self.upload_to = upload_to
 
-        kwargs['max_length'] = kwargs.get('max_length', 100)
+        kwargs.setdefault('max_length', 100)
         super().__init__(verbose_name, name, **kwargs)
 
     def check(self, **kwargs):

django/db/models/fields/related.py

@@ -808,8 +808,7 @@ class ForeignKey(ForeignObject):
             parent_link=parent_link,
             on_delete=on_delete,
         )
-
-        kwargs['db_index'] = kwargs.get('db_index', True)
+        kwargs.setdefault('db_index', True)
 
         super().__init__(to, on_delete, from_fields=['self'], to_fields=[to_field], **kwargs)