[Python-Dev] TypeError: f() missing 1 required positional argument: 'x' (original) (raw)

Mark Dickinson dickinsm at gmail.com
Thu Sep 20 13:56:38 CEST 2012

I suspect I've missed the boat on this one (certainly for 3.3.0), but here goes. The new TypeError reporting for bad function calls is a huge improvement (thanks Benjamin!), but I have one small nitpick: what is a positional argument? For example:

>>> def f(x): pass
...
>>> f()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: f() missing 1 required positional argument: 'x'

I think it's confusing to describe 'x' as a positional argument. It's a required formal parameter, certainly. But a caller of 'f' could pass 'x' either by position or by 'keyword'.

When running training (generally Python 2.6 or 2.7 based), I frequently have to devote some time to unravelling student confusion between 'arguments passed by keyword' on one hand and 'optional formal parameters' on the other. The outline of the explanation goes something like:

(0) Preamble: be careful to separate out details of function calling from those of function definition; distinguish formal parameters from actual arguments. (1) On the function definition side, formal parameters may be either required or optional. (2) On the function calling side, actual arguments may be passed either positionally or by keyword. (3) The notions in (1) and (2) are entirely orthogonal! (3a) (Although in practice, callers tend to use pass-by-keyword for optional formal parameters.)

That's all for Python 2; Python 3, of course, requires a bit more explanation related to the keyword-only arguments.

There already seems to be a fair amount of confusion in the Python world about point (3); I've seen professional Python training slides that show how to define optional formal parameters under the heading "keyword arguments".

I submit that the word 'positional' in the TypeError message exacerbates this confusion, and that little would be lost by simply dropping it from the exception message.

Thoughts?

Mark

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