Improve performance of lazy validation message formatting

rest_framework/compat.py

@@ -5,7 +5,6 @@
 import sys
 
 from django.conf import settings
-from django.core import validators
 from django.views.generic import View
 
 try:
@@ -238,34 +237,5 @@ def md_filter_add_syntax_highlight(md):
 INDENT_SEPARATORS = (',', ': ')
 
 
-class CustomValidatorMessage:
-    """
-    We need to avoid evaluation of `lazy` translated `message` in `django.core.validators.BaseValidator.__init__`.
-    https://github.com/django/django/blob/75ed5900321d170debef4ac452b8b3cf8a1c2384/django/core/validators.py#L297
-
-    Ref: https://github.com/encode/django-rest-framework/pull/5452
-    """
-
-    def __init__(self, *args, **kwargs):
-        self.message = kwargs.pop('message', self.message)
-        super().__init__(*args, **kwargs)
-
-
-class MinValueValidator(CustomValidatorMessage, validators.MinValueValidator):
-    pass
-
-
-class MaxValueValidator(CustomValidatorMessage, validators.MaxValueValidator):
-    pass
-
-
-class MinLengthValidator(CustomValidatorMessage, validators.MinLengthValidator):
-    pass
-
-
-class MaxLengthValidator(CustomValidatorMessage, validators.MaxLengthValidator):
-    pass
-
-
 # Version Constants.
 PY36 = sys.version_info >= (3, 6)

rest_framework/fields.py

@@ -12,7 +12,8 @@
 from django.core.exceptions import ObjectDoesNotExist
 from django.core.exceptions import ValidationError as DjangoValidationError
 from django.core.validators import (
-    EmailValidator, RegexValidator, URLValidator, ip_address_validators
+    EmailValidator, MaxLengthValidator, MaxValueValidator, MinLengthValidator,
+    MinValueValidator, RegexValidator, URLValidator, ip_address_validators
 )
 from django.forms import FilePathField as DjangoFilePathField
 from django.forms import ImageField as DjangoImageField
@@ -23,20 +24,17 @@
 from django.utils.duration import duration_string
 from django.utils.encoding import is_protected_type, smart_text
 from django.utils.formats import localize_input, sanitize_separators
-from django.utils.functional import lazy
 from django.utils.ipv6 import clean_ipv6_address
 from django.utils.timezone import utc
 from django.utils.translation import gettext_lazy as _
 from pytz.exceptions import InvalidTimeError
 
 from rest_framework import ISO_8601
-from rest_framework.compat import (
-    MaxLengthValidator, MaxValueValidator, MinLengthValidator,
-    MinValueValidator, ProhibitNullCharactersValidator
-)
+from rest_framework.compat import ProhibitNullCharactersValidator
 from rest_framework.exceptions import ErrorDetail, ValidationError
 from rest_framework.settings import api_settings
 from rest_framework.utils import html, humanize_datetime, json, representation
+from rest_framework.utils.formatting import lazy_format
 
 
 class empty:
@@ -749,12 +747,11 @@ def __init__(self, **kwargs):
         self.min_length = kwargs.pop('min_length', None)
         super().__init__(**kwargs)
         if self.max_length is not None:
-            message = lazy(self.error_messages['max_length'].format, str)(max_length=self.max_length)
+            message = lazy_format(self.error_messages['max_length'], max_length=self.max_length)
             self.validators.append(
                 MaxLengthValidator(self.max_length, message=message))
         if self.min_length is not None:
-            message = lazy(
-                self.error_messages['min_length'].format, str)(min_length=self.min_length)
+            message = lazy_format(self.error_messages['min_length'], min_length=self.min_length)
             self.validators.append(
                 MinLengthValidator(self.min_length, message=message))
 
@@ -915,13 +912,11 @@ def __init__(self, **kwargs):
         self.min_value = kwargs.pop('min_value', None)
         super().__init__(**kwargs)
         if self.max_value is not None:
-            message = lazy(
-                self.error_messages['max_value'].format, str)(max_value=self.max_value)
+            message = lazy_format(self.error_messages['max_value'], max_value=self.max_value)
             self.validators.append(
                 MaxValueValidator(self.max_value, message=message))
         if self.min_value is not None:
-            message = lazy(
-                self.error_messages['min_value'].format, str)(min_value=self.min_value)
+            message = lazy_format(self.error_messages['min_value'], min_value=self.min_value)
             self.validators.append(
                 MinValueValidator(self.min_value, message=message))
 
@@ -953,15 +948,11 @@ def __init__(self, **kwargs):
         self.min_value = kwargs.pop('min_value', None)
         super().__init__(**kwargs)
         if self.max_value is not None:
-            message = lazy(
-                self.error_messages['max_value'].format,
-                str)(max_value=self.max_value)
+            message = lazy_format(self.error_messages['max_value'], max_value=self.max_value)
             self.validators.append(
                 MaxValueValidator(self.max_value, message=message))
         if self.min_value is not None:
-            message = lazy(
-                self.error_messages['min_value'].format,
-                str)(min_value=self.min_value)
+            message = lazy_format(self.error_messages['min_value'], min_value=self.min_value)
             self.validators.append(
                 MinValueValidator(self.min_value, message=message))
 
@@ -1012,14 +1003,11 @@ def __init__(self, max_digits, decimal_places, coerce_to_string=None, max_value=
         super().__init__(**kwargs)
 
         if self.max_value is not None:
-            message = lazy(
-                self.error_messages['max_value'].format,
-                str)(max_value=self.max_value)
+            message = lazy_format(self.error_messages['max_value'], max_value=self.max_value)
             self.validators.append(
                 MaxValueValidator(self.max_value, message=message))
         if self.min_value is not None:
-            message = lazy(
-                self.error_messages['min_value'].format, str)(min_value=self.min_value)
+            message = lazy_format(self.error_messages['min_value'], min_value=self.min_value)
             self.validators.append(
                 MinValueValidator(self.min_value, message=message))
 
@@ -1357,15 +1345,11 @@ def __init__(self, **kwargs):
         self.min_value = kwargs.pop('min_value', None)
         super().__init__(**kwargs)
         if self.max_value is not None:
-            message = lazy(
-                self.error_messages['max_value'].format,
-                str)(max_value=self.max_value)
+            message = lazy_format(self.error_messages['max_value'], max_value=self.max_value)
             self.validators.append(
                 MaxValueValidator(self.max_value, message=message))
         if self.min_value is not None:
-            message = lazy(
-                self.error_messages['min_value'].format,
-                str)(min_value=self.min_value)
+            message = lazy_format(self.error_messages['min_value'], min_value=self.min_value)
             self.validators.append(
                 MinValueValidator(self.min_value, message=message))
 
@@ -1610,10 +1594,10 @@ def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
         self.child.bind(field_name='', parent=self)
         if self.max_length is not None:
-            message = lazy(self.error_messages['max_length'].format, str)(max_length=self.max_length)
+            message = lazy_format(self.error_messages['max_length'], max_length=self.max_length)
             self.validators.append(MaxLengthValidator(self.max_length, message=message))
         if self.min_length is not None:
-            message = lazy(self.error_messages['min_length'].format, str)(min_length=self.min_length)
+            message = lazy_format(self.error_messages['min_length'], min_length=self.min_length)
             self.validators.append(MinLengthValidator(self.min_length, message=message))
 
     def get_value(self, dictionary):
@@ -1886,8 +1870,7 @@ def __init__(self, model_field, **kwargs):
         max_length = kwargs.pop('max_length', None)
         super().__init__(**kwargs)
         if max_length is not None:
-            message = lazy(
-                self.error_messages['max_length'].format, str)(max_length=self.max_length)
+            message = lazy_format(self.error_messages['max_length'], max_length=self.max_length)
             self.validators.append(
                 MaxLengthValidator(self.max_length, message=message))
 

rest_framework/utils/formatting.py

@@ -65,3 +65,29 @@ def markup_description(description):
         description = escape(description).replace('\n', '<br />')
         description = '<p>' + description + '</p>'
     return mark_safe(description)
+
+
+class lazy_format:
+    """
+    Delay formatting until it's actually needed.
+
+    Useful when the format string or one of the arguments is lazy.
+
+    Not using Django's lazy because it is too slow.
+    """
+    __slots__ = ('format_string', 'args', 'kwargs', 'result')
+
+    def __init__(self, format_string, *args, **kwargs):
+        self.result = None
+        self.format_string = format_string
+        self.args = args
+        self.kwargs = kwargs
+
+    def __str__(self):
+        if self.result is None:
+            self.result = self.format_string.format(*self.args, **self.kwargs)
+            self.format_string, self.args, self.kwargs = None, None, None
+        return self.result
+
+    def __mod__(self, value):
+        return str(self) % value

tests/test_utils.py

@@ -1,3 +1,5 @@
+from unittest import mock
+
 from django.conf.urls import url
 from django.test import TestCase, override_settings
 
@@ -6,6 +8,7 @@
 from rest_framework.serializers import ModelSerializer
 from rest_framework.utils import json
 from rest_framework.utils.breadcrumbs import get_breadcrumbs
+from rest_framework.utils.formatting import lazy_format
 from rest_framework.utils.urls import remove_query_param, replace_query_param
 from rest_framework.views import APIView
 from rest_framework.viewsets import ModelViewSet
@@ -257,3 +260,19 @@ def test_invalid_unicode(self):
         removed_key = 'page'
 
         assert key in remove_query_param(q, removed_key)
+
+
+class LazyFormatTests(TestCase):
+    def test_it_formats_correctly(self):
+        formatted = lazy_format('Does {} work? {answer}: %s', 'it', answer='Yes')
+        assert str(formatted) == 'Does it work? Yes: %s'
+        assert formatted % 'it does' == 'Does it work? Yes: it does'
+
+    def test_it_formats_lazily(self):
+        message = mock.Mock(wraps='message')
+        formatted = lazy_format(message)
+        assert message.format.call_count == 0
+        str(formatted)
+        assert message.format.call_count == 1
+        str(formatted)
+        assert message.format.call_count == 1