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[Python-Dev] Error message for wrong number of arguments

Jeroen Demeyer J.Demeyer at UGent.be
Mon Jul 30 08:12:22 EDT 2018 
Hello,

I noticed an inconsistency in the error messages for the number of 
arguments to a method call. For Python methods, the "self" argument is 
counted. For built-in methods, the "self" argument is *not* counted:

 >>> class mylist(list):
...     def append(self, val): super().append(val)

 >>> f = list().append
 >>> f(1,2)
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
TypeError: append() takes exactly one argument (2 given)

 >>> g = mylist().append
 >>> g(1,2)
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
TypeError: append() takes 2 positional arguments but 3 were given

I think it has been argued before that it's a feature that self is 
counted. So I consider the error message for list().append a bug. This 
is one of the many oddities I noticed while working on improving 
built-in functions.

Would you agree to change the error message for built-in methods to be 
closer to Python methods?


Jeroen.
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