Mock call assertions in PyTest

Introduction

Mock call assertions help check if a fake function was called correctly during a test. This makes sure your code talks to other parts as expected.

When you want to test a function that calls another function without running the real one.

When you want to check if a function was called with the right inputs.

When you want to confirm how many times a function was called.

When you want to test error handling by simulating failures in called functions.

Syntax

PyTest

mock_function.assert_called()
mock_function.assert_called_once()
mock_function.assert_called_with(arg1, arg2)
mock_function.assert_called_once_with(arg1, arg2)
mock_function.assert_any_call(arg1)
mock_function.call_count

Use assert_called() to check if the mock was called at least once.

Use assert_called_with() to check the last call's arguments.

Examples

Checks if mock_func was called at least once.

PyTest

mock_func.assert_called()

Checks if mock_func was called exactly once with arguments 5 and 'test'.

PyTest

mock_func.assert_called_once_with(5, 'test')

Checks if mock_func was called at least once with argument 'hello'.

PyTest

mock_func.assert_any_call('hello')

Prints how many times mock_func was called.

PyTest

print(mock_func.call_count)

Sample Program

This test replaces the real greet function with a mock. It calls welcome_user with the mock and checks if the mock was called once with 'Alice'. It also prints how many times the mock was called.

PyTest

from unittest.mock import Mock

def greet(name):
    print(f"Hello, {name}!")

def welcome_user(greet_func, user_name):
    greet_func(user_name)

mock_greet = Mock()
welcome_user(mock_greet, 'Alice')

mock_greet.assert_called_once_with('Alice')
print(f"Call count: {mock_greet.call_count}")

Important Notes

Mock call assertions help test interactions without running real code.

Always check the call arguments to catch mistakes early.

Use call_count to verify how many times a mock was used.

Summary

Mock call assertions check if and how mocks were called.

They help test code interactions safely and clearly.

Common assertions include assert_called_with and assert_called_once_with.