Merge pull request #806 from methane/lazy-async-patch

Lazy asyncsupport.patch_all()

CHANGES.rst

@@ -1,3 +1,5 @@
+.. currentmodule:: jinja2
+
 Jinja Changelog
 ===============
 
@@ -7,6 +9,12 @@ Version 2.11
 
 unreleased
 
+- Async support is only loaded the first time an
+  :class:`~environment.Environment` enables it, in order to avoid a
+  slow initial import. (`#765`_)
+
+.. _#765: https://github.com/pallets/jinja/issues/765
+
 
 Version 2.10
 ------------

docs/templates.rst

@@ -1309,11 +1309,9 @@ The general syntax is ``<do something> if <something is true> else <do
 something else>``.
 
 The `else` part is optional.  If not provided, the else block implicitly
-evaluates into an undefined object:
+evaluates into an undefined object::
 
-.. sourcecode:: jinja
-
-    {{ '[%s]' % page.title if page.title }}
+    {{ ('[%s]' % page.title) if page.title }}
 
 
 .. _builtin-filters:

jinja2/__init__.py

@@ -70,14 +70,3 @@
     'evalcontextfilter', 'evalcontextfunction', 'make_logging_undefined',
     'select_autoescape',
 ]
-
-
-def _patch_async():
-    from jinja2.utils import have_async_gen
-    if have_async_gen:
-        from jinja2.asyncsupport import patch_all
-        patch_all()
-
-
-_patch_async()
-del _patch_async

jinja2/asyncsupport.py

@@ -254,3 +254,6 @@ async def make_async_loop_context(iterable, undefined, recurse=None, depth0=0):
         after = _last_iteration
     return AsyncLoopContext(async_iterator, undefined, after, length, recurse,
                             depth0)
+
+
+patch_all()

jinja2/environment.py

@@ -334,6 +334,8 @@ def __init__(self,
 
         self.enable_async = enable_async
         self.is_async = self.enable_async and have_async_gen
+        if self.is_async:
+            import jinja2.asyncsupport  # runs patch_all() once
 
         _environment_sanity_check(self)