Is the Python Module Counter() Using C?

Question

When I use the function:

from collections import Counter

Is Counter() sourced using C structures? Is there any convenient way to determine that generally for this and other function?

Give that Python is open sourced generally what should I be looking for in the source code if it is the best way to determine if a given function uses C structures?

Answer 1

There's no direct way to assert this for classes, but there's a way to determine if a function or method was written in C. So any class written in C probably defines at least one explicit or implicit function in C (if not it will be difficult) and you can check with inspect.isbuiltin:

>>> import collections
>>> import inspect
>>> def is_c_class(cls):
...     return any(inspect.isbuiltin(getattr(cls, methname, None)) 
...                for methname in cls.__dict__)

>>> is_c_class(collections.Counter)
False

>>> is_c_class(dict)
True

However that's not all there is to it because collections.Counter calls collections._count_elements which is a C function:

>>> inspect.isbuiltin(collections._count_elements)
True

So you should always check the source code (Pythons repository is on github and also the implementation for Counter) to be sure.

---

Note that the above mentioned check using isbuiltin has some flaws. For example you could have a class attribute that is a C function:

>>> class Test(object):
...     c_function = all  # builtin all function is a C function

>>> is_c_class(Test)
True

So don't rely on it always giving correct results, treat it as approximation.