[New-bugs-announce] [issue41745] BoundArguments.arguments used in the recommended way to call a callable silently succeeds for nonexistent arguments

Julian Berman report at bugs.python.org
Tue Sep 8 11:40:46 EDT 2020


New submission from Julian Berman <Julian+Python.org at GrayVines.com>:

The following code succeeds "silently", which seems undesirable:

    from inspect import signature
    def two():
        return 2
    bound = signature(two).bind()
    bound.arguments["does_not_exist"] = 12
    two(*bound.args, **bound.kwargs)

where the mechanism for finally calling `two` is the recommended way shown in the docs for `inspect.BoundArguments`: https://docs.python.org/3/library/inspect.html#inspect.BoundArguments.apply_defaults

What's happened there is that:

    print(b.args, b.kwargs)

"silently" ignored the non-existent argument.

Somewhere along the line here it seems like something should complain. I don't see previous discussion of this from quickly searching on the bug tracker, but obviously if I've missed something let me know.

I'm also not really sure what the desirable solution is. To me, it's possibly that BoundArguments should have a fully-managed way to invoke a callable rather than asking a user to unpack *args and *kwargs, and that that mechanism, say arguments.be_passed_to(callable) should do the error checking). Having `.arguments` full-on reject unknown parameters seems like another possibility but there may be reasons that's not a good idea (e.g. if you were to for some reason use a bound arguments object to call some other function that *did* take that additional argument).

----------
components: Library (Lib)
messages: 376578
nosy: Julian
priority: normal
severity: normal
status: open
title: BoundArguments.arguments used in the recommended way to call a callable silently succeeds for nonexistent arguments
type: behavior
versions: Python 3.10, Python 3.8, Python 3.9

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue41745>
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