[New-bugs-announce] [issue24205] signature.bind error messages are sub-optimal

[R. David Murray]

New submission from R. David Murray:

I have an application where I'm calling a handler function with passed in arguments.  I want to generate an error if the handler is called with the wrong arguments.  I can't just catch TypeError since a TypeError could easily result from some programming error in the handler, rather than an error in the calling args.  So, doing this seems obvious:

    sig = signature(handler)
    try:
        bound = sig.bind(message, payload, *args, **kw)
    except TypeError as exc:
        print("Invalid handler call: {}".format(str(exc))
    handler(*bound.args, **bound.kwargs)

Now, suppose I have a function like:

    def foo(message, payload, anarg, akeyword='bar'):
        pass

If I call it directly with an invalid keyword argument I get:

    >>> foo(1, 2, 3, badword=7)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: foo() got an unexpected keyword argument 'badword'

However, bind gives me:

    >>> sig.bind(1, 2, 3, badword=7)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "/home/rdmurray/python/p35/Lib/inspect.py", line 2818, in bind
        return args[0]._bind(args[1:], kwargs)
      File "/home/rdmurray/python/p35/Lib/inspect.py", line 2809, in _bind
        raise TypeError('too many keyword arguments')
    TypeError: too many keyword arguments

Similarly, for a missing argument I get:

    >>> foo(1, 2)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: foo() missing 1 required positional argument: 'anarg'

While bind gives:

    >>> sig.bind(1, 2)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "/home/rdmurray/python/p35/Lib/inspect.py", line 2818, in bind
        return args[0]._bind(args[1:], kwargs)
      File "/home/rdmurray/python/p35/Lib/inspect.py", line 2737, in _bind
        raise TypeError(msg) from None
    TypeError: 'anarg' parameter lacking default value

So, using this to replace catching the TypeError from incorrectly calling a function does not work.  Nor are the messages in fact accurate.  Is there any chance we could make bind's error handling work like regular function binding?  That's certainly what I expected would happen!

----------
components: Library (Lib)
messages: 243287
nosy: r.david.murray, yselivanov
priority: normal
severity: normal
status: open
title: signature.bind error messages are sub-optimal
type: behavior
versions: Python 3.5

_______________________________________
Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue24205>
_______________________________________