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Async support does not work with Native environment #1362

remidebette opened this issue Mar 2, 2021 · 4 comments · Fixed by #1363

Async support does not work with Native environment #1362

remidebette opened this issue Mar 2, 2021 · 4 comments · Fixed by #1363


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We have been starting to porting existing Flask code to an asyncio based server (Quart).
Part of it makes use of the Jinja Native environment.

The render_async works very well, but not in conjunction with Native.

Here is replicable code in a python -m asyncio console:

from jinja2.nativetypes import NativeEnvironment

def render(s, my_vars):
    native_environment = NativeEnvironment()
    template = native_environment.from_string(s)
    return template.render(**my_vars)

async def render_async(s, my_vars):
    native_environment = NativeEnvironment(enable_async=True)
    template = native_environment.from_string(s)
    return await template.render_async(**my_vars)

global_vars = {}

number_list = [1, 2, 3, 4, 5, 6, 7, 8, 9]
global_vars["number_list"] = number_list

out = render("{{35}}", global_vars)
# int: 35

out = await render_async("{{35}}", global_vars)
# str: '35'

out = render("{{number_list}}", global_vars)
# list: [1, 2, 3, 4, 5, 6, 7, 8, 9]

out = await render_async("{{number_list}}", global_vars)
# Traceback (most recent call last):
#   File "/Users/A492YC/.pyenv/versions/3.8.1/lib/python3.8/concurrent/futures/", line 439, in result
#     return self.__get_result()
#   File "/Users/A492YC/.pyenv/versions/3.8.1/lib/python3.8/concurrent/futures/", line 388, in __get_result
#     raise self._exception
#   File "<console>", line 1, in <module>
#   File "<console>", line 4, in render_async
#   File "/Users/A492YC/.virtualenvs/jinja_test-SmX6WV4i/lib/python3.8/site-packages/jinja2/", line 65, in render_async
#     return self.environment.handle_exception()
#   File "/Users/A492YC/.virtualenvs/jinja_test-SmX6WV4i/lib/python3.8/site-packages/jinja2/", line 832, in handle_exception
#     reraise(*rewrite_traceback_stack(source=source))
#   File "/Users/A492YC/.virtualenvs/jinja_test-SmX6WV4i/lib/python3.8/site-packages/jinja2/", line 28, in reraise
#     raise value.with_traceback(tb)
#   File "/Users/A492YC/.virtualenvs/jinja_test-SmX6WV4i/lib/python3.8/site-packages/jinja2/", line 26, in concat_async
#     return concat(rv)
# TypeError: sequence item 0: expected str instance, list found


In Native mode, instead of forcefully concatenating outputs as strings, outputs of the render_async() should be on par with the render()


  • Python version: 3.8.1
  • Jinja version: 2.11.3
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davidism commented Mar 2, 2021

If you have time, I'd appreciate if you could look into fixing this. I'm unlikely to address this any time remotely soon.

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Contributor Author


It is my first time contributing so I am not sure I did it properly.
If the code change is acceptable, would you consider having an intermediate version for it, since this is a bug fix?

Thank you

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davidism commented Mar 2, 2021

Looks good overall. It will be in 3.0, I'm only doing security fixes for 2.x.

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Contributor Author

Ok, would you happen to know the timeline for the 3.0?

@davidism davidism added this to the 3.0.0 milestone Mar 8, 2021
@davidism davidism added the async label Mar 8, 2021
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2 participants