Test if python's builtin mul was called using mock

Question

I have a method where I am converting dollars to cents before calling an api.

# money.py
def send_money(dollars):
    cents = dollars * 100
    return send_money_to_api(cents)

* This is not exactly my code. But this is sufficient for demonstration.

dollars's type is decimal.Decimal as it's actually coming from django's DecimalField.

I want to ensure that I am multiplying the amount by 100. I tried testing this using mock

import mock

from money import send_money

@mock.patch('money.operator.__mul__')
@mock.patch('money.send_money_to_api')
def test_money(self, api_mock, mul_mock):
    send_money(20)

    assert api_mock.called
    mul_mock.assert_any_call(20, 100)

It fails saying,

E   ImportError: No module named operator

How do I test this?

What else would it call? Are you worried about someone passing `dollars` as an instance of a class with its own `__mul__`? What is it you're concerned about? — Silvio Mayolo, Jan 10 '18 at 02:27
I am concerned that some developer/I will forget to do the conversion. — Hussain, Jan 10 '18 at 02:29
What is `dollars`'s type? There isn't a builtin `__mul__`, but every objects have their own `__mul__` function. So if you want to mock this function, you need to mock `dollars`'s class. — Sraw, Jan 10 '18 at 02:34
Wait. It's actually `decimal.Decimal` because it comes from `django`'s `DecimalField` — Hussain, Jan 10 '18 at 02:45
Wouldn't it be sufficient to test that `send_money`is called? — wwii, Jan 10 '18 at 02:46

score 0 · Answer 1 · answered Jan 10 '18 at 03:13

I was able to fix this after some great help from comments. Special thanks to @Sraw.

@mock.patch.object(decimal.Decimal, '__mul__')
@mock.patch('money.send_money_to_api')
def test_money(self, api_mock, mul_mock):
    send_money(20)

    assert api_mock.called
    mul_mock.assert_any_call(100)

An advantage of mocking decimal.Decimal instance method __mul__ is I can truly ensure that all the DecimalField values (which are mostly amounts) from database are converted.

Test if python's builtin __mul__ was called using mock

1 Answers1

Test if python's builtin mul was called using mock