pytest_cheatsheet.py

# ========== theory ==========
# 
# Mock and MagicMock are all classes in `unitest.mock` library.
#
# ---------- The MagicMock Class ----------
# A MagicMock instance can:
#   * capture the arguments that the method is called with
#   * count how many times it's called
#   * return values that we specify
#   * return the same or different values each time the mocked method is called
#   * be made to raise errors
#
# ---------- The Mock Class ----------
# `unittest.mock` provides a core Mock class removing the need to create a host of stubs throughout your test suite.
# 
# MagicMock is a subclass of Mock with all the magic methods pre-created and ready to use.
#
# The patch() decorators makes it easy to temporarily replace classes in a particular module with a
# Mock object. By default patch() will create a MagicMock for you.
#
# ---------- pytest-mock ----------
# It provides a `mocker` fixture which is a thin-wrapper around the patching API provided by the mock package. 
# The `mocker` fixture has the same API as `mock.patch`.
# The `mocker` also provides other nice utilities such as `spy` and `stub`.
# Other names like `Mock`, `MagicMock` etc are also accessible from `mocker`.
#
# Consider following examples
#
# In Mock directly
from unittest import mock
m = mock.MagicMock()
assert isinstance(m.foo, mock.MagicMock)
assert isinstance(m.bar, mock.MagicMock)
assert isinstance(m(), mock.MagicMock)
assert m.foo is not m.bar is not m()

# In mocker fixture
from unittest import mock
import pytest

def test_do_something(mocker):
    m = mocker.MagicMock()
    assert isinstance(m.foo, mock.MagicMock)
    assert isinstance(m.bar, mock.MagicMock)
    assert isinstance(m(), mock.MagicMock)
    assert m.foo is not m.bar is not m()

# ---------- Assertion for Mock object ----------
m = MagicMock()
m.assert_called()
m.assert_called_once()
m.assert_called_with()
m.assert_called_once_with()
m.assert_any_call()
m.assert_has_calls()
m.assert_not_called()

# ========== mocker.path() requires a path to the function being patched ==========
# Consider following examples
# Example 1
import pytest

def foo():
    return 'should not be called'

def get_foo():
    return foo()

def test_get_foo(mocker):
    m = mocker.patch(__name__ + '.foo', return_value='bar')     # notice: `__name__`
    assert get_foo() == 'bar'

# Example 2
# foo.py
def foo():
    return 'should not be called'

# bar.py
from foo import foo

def get_foo():
    return foo()

# test_something.py
import pytest
from bar import get_foo

def test_get_foo(mocker):
    m = mocker.patch('bar.foo', return_value='bar')             # notice: `bar.foo`
    assert get_foo() == 'bar'

# ========== assert input for mocked function ==========
import pytest

def do_something(client, name):
    client.greeting(name)

def test_it(mocker):
    obj = mocker.MagicMock()
    do_something(obj, 'Jim')
    obj.greeting.assert_called_with('Jim')
    
# ========== mock standard library ==========
import os

class UnixFS:
    @staticmethod
    def rm(filename):
        os.remove(filename)

def test_unix_fs(mocker):
    m = mocker.patch('os.remove')
    UnixFS.rm('file')
    m.assert_called_once_with('file')

# ========== Pytest assert raise exception ==========
import re
import pytest

def check_email_format(email):
    if not re.match(r"(^[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\.[a-zA-Z0-9-.]+$)", email):
        raise Exception("Invalid email format")
    else:
        return "Email format is ok"

def test_email_exception():
    with pytest.raises(Exception) as e:
        assert check_email_format("bademail.com")
    assert str(e.value) == "Invalid email format"