[Python-ideas] Re: Improve traceback for common `with` statement mistake

5 Sep 2021

      On Mon, Sep 6, 2021 at 9:37 AM Finn Mason  wrote:
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Hello all,
In Python 3.10 and 3.11, exception tracebacks are being greatly improved. I noticed that there's nothing related to a fairly common (in my personal experience) cryptic traceback relating to the `with` statement:
...
...
...
with ContextManager as ctx:
...     # do something with `ctx`
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: __enter__
I'm seeing a different message, so it looks like something HAS been improved:

Python 3.11.0a0 (heads/main:ed524b4569, Aug 14 2021, 11:29:01) [GCC
8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
...
...
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import contextlib
with contextlib.ExitStack: ...
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'ABCMeta' object does not support the context manager protocol
...
This occurs when one forgets to use a instance of a context manager class and uses the class itself. It's obviously not a very helpful traceback. ("Is it not a context manager?" "Is it the wrong class?") Something like the following would be better.
...
...
...
with ContextManager as ctx:
...     # do something with `ctx`
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: <type Context manager> is not a context manager. Did you mean "with ContextManager()..."?
The actual traceback message should probably be more specific than "<type Context manager> is not a context manager".
Thoughts?
The "did you mean" part depends on or assumes that it can figure out
that calling it would have given a viable context manager. This could
be done by probing whether thing.__enter__ exists, but I'm not sure
that'd be entirely safe. Also, it won't catch those made from
generators:
...
...
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@contextlib.contextmanager
... def foo(): yield
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with foo: ...
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'function' object does not support the context manager protocol
with foo(): ...
...
Ellipsis
foo

In any case, the biggest advantage (IMO) comes from naming the type,
which will make it easy to distinguish the object from its type.

ChrisA

[Python-ideas] Re: Improve traceback for common `with` statement mistake

Chris Angelico