Simplified some imports for database functions.

Used more specific modules to reduce the risk of circular imports.

django/db/models/functions/comparison.py

@@ -1,5 +1,5 @@
 """Database functions that do comparisons or type conversions."""
-from django.db.models import Func
+from django.db.models.expressions import Func
 
 
 class Cast(Func):

django/db/models/functions/datetime.py

@@ -1,12 +1,12 @@
 from datetime import datetime
 
 from django.conf import settings
-from django.db.models import (
-    DateField, DateTimeField, DurationField, Field, Func, IntegerField,
-    TimeField, Transform, fields,
+from django.db.models.expressions import Func
+from django.db.models.fields import (
+    DateField, DateTimeField, DurationField, Field, IntegerField, TimeField,
 )
 from django.db.models.lookups import (
-    YearExact, YearGt, YearGte, YearLt, YearLte,
+    Transform, YearExact, YearGt, YearGte, YearLt, YearLte,
 )
 from django.utils import timezone
 
@@ -157,7 +157,7 @@ ExtractIsoYear.register_lookup(YearLte)
 
 class Now(Func):
     template = 'CURRENT_TIMESTAMP'
-    output_field = fields.DateTimeField()
+    output_field = DateTimeField()
 
     def as_postgresql(self, compiler, connection, **extra_context):
         # PostgreSQL's CURRENT_TIMESTAMP means "the time at the start of the

django/db/models/functions/math.py

@@ -1,10 +1,10 @@
 import math
 import sys
 
-from django.db.models import (
-    DecimalField, FloatField, Func, IntegerField, Transform,
-)
+from django.db.models.expressions import Func
+from django.db.models.fields import DecimalField, FloatField, IntegerField
 from django.db.models.functions import Cast
+from django.db.models.lookups import Transform
 
 
 class DecimalInputMixin:

django/db/models/functions/text.py

@@ -1,5 +1,7 @@
-from django.db.models import Func, IntegerField, Transform, Value, fields
+from django.db.models.expressions import Func, Value
+from django.db.models.fields import IntegerField
 from django.db.models.functions import Coalesce
+from django.db.models.lookups import Transform
 
 
 class BytesToCharFieldConversionMixin:
@@ -124,7 +126,7 @@ class Length(Transform):
     """Return the number of characters in the expression."""
     function = 'LENGTH'
     lookup_name = 'length'
-    output_field = fields.IntegerField()
+    output_field = IntegerField()
 
     def as_mysql(self, compiler, connection, **extra_context):
         return super().as_sql(compiler, connection, function='CHAR_LENGTH', **extra_context)
@@ -207,7 +209,7 @@ class StrIndex(Func):
     """
     function = 'INSTR'
     arity = 2
-    output_field = fields.IntegerField()
+    output_field = IntegerField()
 
     def as_postgresql(self, compiler, connection, **extra_context):
         return super().as_sql(compiler, connection, function='STRPOS', **extra_context)

django/db/models/functions/window.py

@@ -1,4 +1,5 @@
-from django.db.models import FloatField, Func, IntegerField
+from django.db.models.expressions import Func
+from django.db.models.fields import FloatField, IntegerField
 
 __all__ = [
     'CumeDist', 'DenseRank', 'FirstValue', 'Lag', 'LastValue', 'Lead',