Lazy asyncsupport.patch_all()

fixes #765

jinja2/__init__.py

@@ -70,14 +70,3 @@ __all__ = [
     '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 @@ class Environment(object):
 
         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)