Pytest Notes

pytest.py

# filter tests by name (works for standalone test functions or method names on a test class)
#
# pytest <test_file.py> -k test_foo
# pytest <test_file.py> -k 'test_foo or test_bar'

# run the same test multiple times with different parameters

@pytest.mark.parametrize("p1, p2", [(1, 2), (3, 4), (5,6)])
def test_foo(p1, p2):
    assert bar(p1) == p2
  
# capture the exception
def test_foo():
    with pytest.raises(SomeError) as exc_info:
        do_it()
    
    assert exc_info.value.attr == "expected"
  
# mocking (from mock import Mock)

# create mock that responds to everything
mock = Mock()

# create mock that only responds to the Clazz API
mock = Mock(spec=Clazz)

# stub an attribute
mock.attr_name = 'foo'

# stub a return value
mock.method_name.return_value = 'bar'

# stub an exception
mock.do_something.side_effect = RuntimeError("Problem")

# stub out a context manager
mgr.return_value.__enter__.return_value = 'foo'
mgr.return_value.__exit__.side_effect = RuntimeError("Problem")

# @patch a function that was imported into a module
from foo.bar import func

class ClassUnderTest:
    def foo(self):
        return func(2) - 12

@patch("my.module.file.func")
def test_foo(self, mock):
    mock.return_value = 12
    assert ClassUnderTest().foo() == 0

# validate some (but not all) of the parameters to mocked function
mock.assert_called_once()
assert mock.call_args_list[0].args[1] == "foo" # validates the second argument from the first invocation 
assert mock.call_args_list[0].args[2] == "bar" # validates the second argument from the first invocation 
assert mock.call_args_list[0].kwargs["foo"] == "bar" # validates the foo kwarg from the first invocation 

# if you don't have any kwargs (or don't care about kwargs), you can do this instead
assert mock.call_args[0][2] == "bar" # validates the second argument from the first invocation 


# match any parameter
mock.assert_called_once_with(mock.ANY)

# custom matcher
class CustomMatcher:
    def __init__(self, expected):
        self.expected = expected
    def __eq__(self, actual):
        return self.expected == actual.values[0].prop_name

mock.assert_called_once_with(CustomMatcher(expected_value))

# using a custom matcher in a test spy verification
mock.assert_has_calls(call('one', CustomerMatcher('foo')), call('two', CustomMatcher('bar')))

# order independent verification of a mock with multiple calls
mock.some_async_method.assert_has_calls([call(arg1), call(arg2), call(arg3)], any_order=True)

# mock a method to return different values across multiple invocations
def fn(arg):
    if arg == 1:
        return value1
    else:
        return value2
mock.some_method.side_effect = fn

# mock a generator method
mock.some_method.return_value = iter([1, 2, 3])

# mocking out an instance method on a class
class Foo:
    def __init__(self):
        self.value = 12
    def do_it(self, x):
        return self.value + x

from mock import patch

with patch.object(Foo, "do_it", return_value=123) as mock_do_it:  # can also be used as a decorator
    f = Foo()
    
    r = f.do_it(12)
    
    assert r == 123
    mock_do_it.assert_called_once_with(12)


# mocking out onject initialization (not working - python3 only)
from unittest.mock import Mock, patch
import decimal

with patch("decimal.Decimal.__new__") as mock_new:
    mock_decimal = Mock(spec=decimal.Decimal)
    mock_decimal.sqrt = -3
    mock_new.return_value = mock_decimal    
    
    assert decimal.Decimal("123").sqrt == -3