Clean up Django 2 path backslashes

In Django 2, routes defines via urls.path are aggresively escaped when converted into regex.

This is a naive fix which unescapes all characters outside capture groups, but in the context of OpenAPI is okay because regular expressions inside paths are not supported anyway.

This issue affects django-rest-framework as well, as outlined in encode/django-rest-framework#5672, encode/django-rest-framework#5675.

docs/changelog.rst

@@ -8,6 +8,8 @@ Changelog
 *********
 
 - **FIX:** fixed a crash caused by having read-only Serializers nested by reference
+- **FIX:** removed erroneous backslashes in paths when routes are generated using Django 2
+  `path() <https://docs.djangoproject.com/en/2.0/ref/urls/#django.urls.path>`_
 - **IMPROVEMENT:** updated ``swagger-ui`` to version 3.7.0
 
 *********

docs/conf.py

@@ -168,6 +168,7 @@ nitpick_ignore = [
 
     ('py:class', 'ruamel.yaml.dumper.SafeDumper'),
     ('py:class', 'rest_framework.renderers.BaseRenderer'),
+    ('py:class', 'rest_framework.schemas.generators.EndpointEnumerator'),
     ('py:class', 'rest_framework.views.APIView'),
 
     ('py:class', 'OpenAPICodecYaml'),

src/drf_yasg/generators.py

@@ -1,21 +1,63 @@
+import re
 from collections import defaultdict, OrderedDict
 
 import django.db.models
 import uritemplate
 from coreapi.compat import force_text
-from rest_framework.schemas.generators import SchemaGenerator
+from rest_framework.schemas.generators import SchemaGenerator, EndpointEnumerator as _EndpointEnumerator
 from rest_framework.schemas.inspectors import get_pk_description
 
 from . import openapi
 from .inspectors import SwaggerAutoSchema
 from .openapi import ReferenceResolver
 
+PATH_PARAMETER_RE = re.compile(r'{(?P<parameter>\w+)}')
+
+
+class EndpointEnumerator(_EndpointEnumerator):
+    def get_path_from_regex(self, path_regex):
+        return self.unescape_path(super(EndpointEnumerator, self).get_path_from_regex(path_regex))
+
+    def unescape(self, s):
+        """Unescape all backslash escapes from `s`.
+
+        :param str s: string with backslash escapes
+        :rtype: str
+        """
+        # unlike .replace('\\', ''), this corectly transforms a double backslash into a single backslash
+        return re.sub(r'\\(.)', r'\1', s)
+
+    def unescape_path(self, path):
+        """Remove backslashes from all path components outside {parameters}. This is needed because
+        Django>=2.0 ``path()``/``RoutePattern`` aggresively escapes all non-parameter path components.
+
+        **NOTE:** this might destructively affect some url regex patterns that contain metacharacters (e.g. \w, \d)
+        outside path parameter groups; if you are in this category, God help you
+
+        :param str path: path possibly containing
+        :return: the unescaped path
+        :rtype: str
+        """
+        original_path = path
+        clean_path = ''
+        while path:
+            match = PATH_PARAMETER_RE.search(path)
+            if not match:
+                clean_path += self.unescape(path)
+                break
+            clean_path += self.unescape(path[:match.start()])
+            clean_path += match.group()
+            path = path[match.end():]
+
+        return clean_path
+
 
 class OpenAPISchemaGenerator(object):
     """
     This class iterates over all registered API endpoints and returns an appropriate OpenAPI 2.0 compliant schema.
     Method implementations shamelessly stolen and adapted from rest_framework SchemaGenerator.
     """
+    endpoint_enumerator_class = EndpointEnumerator
 
     def __init__(self, info, version, url=None, patterns=None, urlconf=None):
         """
@@ -79,8 +121,8 @@ class OpenAPISchemaGenerator(object):
         :return: {path: (view_class, list[(http_method, view_instance)])
         :rtype: dict
         """
-        inspector = self._gen.endpoint_inspector_cls(self._gen.patterns, self._gen.urlconf)
-        endpoints = inspector.get_api_endpoints()
+        enumerator = self.endpoint_enumerator_class(self._gen.patterns, self._gen.urlconf)
+        endpoints = enumerator.get_api_endpoints()
 
         view_paths = defaultdict(list)
         view_cls = {}

testproj/snippets/urls.py

@@ -1,8 +1,17 @@
-from django.conf.urls import url
+import django
 
 from . import views
 
+if django.VERSION[:2] >= (2, 0):
+    from django.urls import path
+
     urlpatterns = [
-    url(r'$', views.SnippetList.as_view()),
-    url(r'^(?P<pk>[0-9]+)/$', views.SnippetDetail.as_view()),
+        path('', views.SnippetList.as_view()),
+        path('<int:pk>/', views.SnippetDetail.as_view()),
+    ]
+else:
+    from django.conf.urls import url
+    urlpatterns = [
+        url('^$', views.SnippetList.as_view()),
+        url(r'^(?P<pk>\d+)/$', views.SnippetDetail.as_view()),
     ]

testproj/users/urls.py

@@ -4,5 +4,5 @@ from users import views
 
 urlpatterns = [
     url(r'^$', views.UserList.as_view()),
-    url(r'^(?P<pk>[0-9]+)/$', views.user_detail),
+    url(r'^(?P<pk>\d+)/$', views.user_detail),
 ]