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

[Michael Foord]

On 20/05/2010 10:49, Giampaolo Rodolà wrote:
> 2010/5/20 John Arbash Meinel<john.arbash.meinel at gmail.com>:
>    
>> Giampaolo Rodolà wrote:
>>      
>>>>>> 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?
>>>
>>>        
>> Because you wouldn't want to have
>>
>> A.echo()
>>
>> Say that it takes 1 argument and (-1 given) ?
>>
>> John
>> =:->
>>
>>
>>      
> I see that as a different error type: what you're doing there is
> calling a method of a class which you haven't instantiated in the
> first place.
> Actually the error message returned in this other case is not very
> clear as well:
>
> "unbound method echo() must be called with A instance as first
> argument (got nothing instead)"
>
> It talks about "arguments" while no arguments are actually involved in
> the problem: just a class I forgot to initialize.
>    

Although the pattern of calling an unbound method with an instance as 
the first argument (self) is not uncommon - and if you're doing 
Class.method() that not only looks like what you're doing but is also an 
accurate error message.

I would also expect that accidentally calling unbound methods is a much 
less common error than calling bound methods with the wrong number of 
arguments...

All the best,

Michael Foord

>
> --- Giampaolo
> http://code.google.com/p/pyftpdlib
> http://code.google.com/p/psutil
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