Allowed Func subclasses to add kwargs to __repr__().

Thanks Tim Graham for the review.

django/db/models/aggregates.py

@@ -70,12 +70,8 @@ class Count(Aggregate):
             output_field=IntegerField(), **extra
         )
 
-    def __repr__(self):
-        return "{}({}, distinct={})".format(
-            self.__class__.__name__,
-            self.arg_joiner.join(str(arg) for arg in self.source_expressions),
-            'False' if self.extra['distinct'] == '' else 'True',
-        )
+    def _get_repr_options(self):
+        return {'distinct': self.extra['distinct'] != ''}
 
     def convert_value(self, value, expression, connection, context):
         if value is None:
@@ -100,12 +96,8 @@ class StdDev(Aggregate):
         self.function = 'STDDEV_SAMP' if sample else 'STDDEV_POP'
         super().__init__(expression, output_field=FloatField(), **extra)
 
-    def __repr__(self):
-        return "{}({}, sample={})".format(
-            self.__class__.__name__,
-            self.arg_joiner.join(str(arg) for arg in self.source_expressions),
-            'False' if self.function == 'STDDEV_POP' else 'True',
-        )
+    def _get_repr_options(self):
+        return {'sample': self.function == 'STDDEV_SAMP'}
 
     def convert_value(self, value, expression, connection, context):
         if value is None:
@@ -134,12 +126,8 @@ class Variance(Aggregate):
         self.function = 'VAR_SAMP' if sample else 'VAR_POP'
         super().__init__(expression, output_field=FloatField(), **extra)
 
-    def __repr__(self):
-        return "{}({}, sample={})".format(
-            self.__class__.__name__,
-            self.arg_joiner.join(str(arg) for arg in self.source_expressions),
-            'False' if self.function == 'VAR_POP' else 'True',
-        )
+    def _get_repr_options(self):
+        return {'sample': self.function == 'VAR_SAMP'}
 
     def convert_value(self, value, expression, connection, context):
         if value is None:

django/db/models/expressions.py

@@ -531,11 +531,16 @@ class Func(Expression):
 
     def __repr__(self):
         args = self.arg_joiner.join(str(arg) for arg in self.source_expressions)
-        extra = ', '.join(str(key) + '=' + str(val) for key, val in self.extra.items())
+        extra = dict(self.extra, **self._get_repr_options())
         if extra:
+            extra = ', '.join(str(key) + '=' + str(val) for key, val in extra.items())
             return "{}({}, {})".format(self.__class__.__name__, args, extra)
         return "{}({})".format(self.__class__.__name__, args)
 
+    def _get_repr_options(self):
+        """Return a dict of extra __init__() options to include in the repr."""
+        return {}
+
     def get_source_expressions(self):
         return self.source_expressions