Inherit from collections.Counter: 'fromkeys' is abstract

Question

I have a python class that inherit from collections.Counter:

class Analyzer(collections.Counter):
   pass

When I use pylint on this code, its answer is:

W: Method 'fromkeys' is abstract in class 'Counter' but is not overridden (abstract-method)

I checked the implementation of collections.Counter on my machine, and effectively, this method is not implemented (and a comment helps to understand why):

class Counter(dict):
    ...
    @classmethod
    def fromkeys(cls, iterable, v=None):
        # There is no equivalent method for counters because setting v=1
        # means that no element can have a count greater than one.
        raise NotImplementedError(
            'Counter.fromkeys() is undefined.  Use Counter(iterable) instead.')

However, I don't really know how to implement this method, if Counter itself does not…

What is the way to solve this warning in this situation?

Just ignore the warning? Pylint is being wrong, and misinterpreting things. — Martijn Pieters, Jun 14 '17 at 12:28
I mean, a naive solution might be to implement it in the say way as in `collections.Counter`. But this shouldn't even be necessary... — Jared Goguen, Jun 14 '17 at 12:32
@JaredGoguen Or, instead of re-implement it, just reuse the leaking implementation: `fromkeys = collections.Counter.fromkeys`. But as you said, this shouldn't even be necessary. — audeoudh, Jun 14 '17 at 13:21

score 1 · Answer 1 · answered Jun 14 '17 at 14:38

1

This question should answer some of the questions here. Basically, pylint checks for NotImplementedError exceptions being raised to determine whether a method is abstract (a false positive in this case). Adding the comment #pylint: disable=W0223 will disable this check.

A similar issue is also raised in this question.

answered Jun 14 '17 at 14:38

Jared Goguen

8,772
2
18
36

However, if pylint consider `Counter` as an abstract class because `fromkeys` raises `NotImplementedError`, it must consider incorrect to instanciate it. Isn't it? – audeoudh Jun 15 '17 at 11:53

score 0 · Accepted Answer · answered Jun 15 '17 at 11:53

There is two separate way of thinking.

Consider Counter as abstract (as pylint does, as Jared explained). Then, the class Analyzer must implement fromkeys or also be abstract. But then, one should not be able to instanciate Counter.
Consider Counter as concrete, even if you cannot use its fromkeys method. Then, pylint's warning must be disabled (as it is wrong in this case, see Jared's answer to kown how), and the class Analyzer is also concrete and does not need to implement this method.

Inherit from collections.Counter: 'fromkeys' is abstract

2 Answers2