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

Floris Bruynooghe floris.bruynooghe at gmail.com
Thu May 20 10:02:07 CEST 2010

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 "", line 1, in > 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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