Counting the number of times a function is called in a test (pytest)

calls_to_fn_counter.py

import random

class MutableCounter:
    """
    Used by "count_calls".
    """
    n = 0

    def inc(self):
        self.n += 1

    def get(self):
        return self.n


def count_calls(monkeypatch, module, fn) -> MutableCounter:
    """
    Returns a mutable object containing the number of times the given function
    "fn", from the module "module", was called.

    Intended to be used inside pytest functions.
    """
    cnt = MutableCounter()

    def mock_fn(*args, **kwargs):
        nonlocal cnt
        cnt.inc()
        return fn(*args, **kwargs)
    mock_fn.__name__ = fn.__name__
    monkeypatch.setattr(module, fn.__name__, mock_fn)
    return cnt


def test_calls_to_fn(monkeypatch):
    def fn_to_be_tested():
        return random.randint(0, 10)
    calls_cnt = count_calls(monkeypatch, random, random.randint)

    assert calls_cnt.get() == 0

    x = fn_to_be_tested()
    assert 0 <= x <= 10
    assert calls_cnt.get() == 1