[Python-Dev] Reasons behind misleading TypeError message when passing the wrong number of arguments to a method

[Floris Bruynooghe]

On Thu, May 20, 2010 at 11:55:02AM +0900, Stephen J. Turnbull wrote:
> Giampaolo Rodolà writes:
>  > >>> class A:
>  > ...     def echo(self, x):
>  > ...             return x
>  > ...
>  > >>> a = A()
>  > >>> a.echo()
>  > Traceback (most recent call last):
>  >   File "<stdin>", line 1, in <module>
>  > TypeError: echo() takes exactly 2 arguments (1 given)
>  > >>>
>  > 
>  > I bet my last 2 cents this has already been raised in past but I want
>  > to give it a try and revamp the subject anyway.
>  > Is there a reason why the error shouldn't be adjusted to state that
>  > *1* argument is actually required instead of 2?
> 
> As a function, it does take two arguments, and can be called
> explicitly that way, no?  Adjustment is not enough, the message needs
> to be substantially rewritten.  Something like
> 
> TypeError: invoked as a method, echo() takes exactly 1 argument (0 given)
> 
> captures the semantics, but is perhaps too verbose.

How about:

TypeError: bound method echo() takes exactly 1 argument (0 given)

That way you can also have: "unbound method echo() ...".  And it's as
semantically correct as the short "echo() takes ..."

Not having looked at the code I don't know how hard it is for the code
that raises this traceback to notice if it's a bound or unbound method
tough.

Regards
Floris

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