Introduction to py.test

py.test

pytest.org

github.com/pytest-dev/pytest

Introduction

• no-boilerplate test
• discovery
• plain assert
• nice exception formatting
• pdb integration
• output capture
• parametrisation
• markers
• xdist
• plugins
• runs unittest & nose tests too
• …

Simple Tests

def test_int_casting():
    assert int('5') == 5


class TestFloats:

    def test_dot(self):
        assert float('3.3') == 3.3

    def test_e(self):
        assert float('3.1e3') == 3100.0

$ py.test
========================= test session starts =========================
platform linux -- Python 3.4.3, pytest-2.8.0, py-1.4.30, pluggy-0.3.1
rootdir: /tmp/sandbox, inifile:
collected 3 items

test.py ...

====================== 3 passed in 0.01 seconds =======================

Detailed Failure Reports

def test_wrong():
    assert 'Some test about spam' == 'Some blurb about ham'

$ py.test -q
========================= test session starts =========================
platform linux -- Python 3.4.3, pytest-2.8.0, py-1.4.30, pluggy-0.3.1
rootdir: /tmp/sandbox, inifile:
collected 1 items

test_ex.py F

============================== FAILURES ===============================
_____________________________ test_wrong ______________________________

    def test_wrong():
>       assert 'Some text about spam' == 'Some text about ham'
E       assert 'Some text about spam' == 'Some text about ham'
E         - Some text about spam
E         ?                 ^^
E         + Some text about ham
E         ?                 ^

test.py:2: AssertionError
====================== 1 failed in 0.01 seconds =======================

Assertions

def test_assertions():
    x = y = 0
    assert x
    assert x == 1
    assert x != 2
    assert not x
    assert 3 < x < 5 and y < 5
    s = 'the quick fox ...'
    assert 'fox' in s

Expected Exceptions

import pytest

def divide(x, y):
    return x / y

def test_raises():
    with pytest.raises(ZeroDivisionError):
        divide(3, 0)

def test_exc_args():
    with pytest.raises(Exception) as exc:
        raise Exception(42, 'msg')
    assert exc.value.args = (42, 'msg')
    assert exc.type is Exception
    assert exc.tb

Output Capture

def func(a):
    print('input was: {!r}'.format(a))
    return a + 42

def test_func():
    assert func(5) < 5

$ py.test
========================= test session starts =========================
platform linux -- Python 3.4.3, pytest-2.8.0, py-1.4.30, pluggy-0.3.1
rootdir: /tmp/sandbox, inifile:
collected 1 items

test_ex.py F

============================== FAILURES ===============================
______________________________ test_func ______________________________

    def test_func():
>       assert func(5) < 5
E       assert 47 < 5
E        +  where 47 = func(5)

test_ex.py:6: AssertionError
-------------------------- Captured stdout call -----------------------
input was: 5
======================== 1 failed in 0.05 seconds =====================

Common Options

• -s disable output capture
• -x exit on first failure
• -k only run matching tests
• -l show locals
• --pdb enter debugger on errors
• -rsxXw -ra report skipped, xfailed, …
• defaults in pytest.ini

showlocals

$ py.test -ql
F
========================== FAILURES ===========================
__________________________ test_func __________________________

    def test_func():
>       func(3)


test_ex.py:8:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

b = 3

    def func(b):
        a = 1
        a += b
        c = a * b
>       return c / 0
E       ZeroDivisionError: division by zero

a          = 4
b          = 3
c          = 12

test_ex.py:5: ZeroDivisionError

Selecting Tests

def test_foo():
    assert True

def test_bar():
    assert True

$ py.test -v -k foo
===================== test session starts =====================
platform linux -- Python 3.4.3, pytest-2.8.0, py-1.4.30, pluggy
cachedir: .cache
rootdir: /tmp/sandbox, inifile:
collected 2 items

test_ex.py::test_foo PASSED

================ 1 tests deselected by '-kfoo' ================
=========== 1 passed, 1 deselected in 0.12 seconds ============

Skipping, xfail & marks

import os, pytest

@pytest.mark.skipif(os.name != 'posix', reason='Not supported')
def test_pipe():
    r, w = os.pipe()
    assert 1

@pytest.mark.xfail
def test_oops():
    assert 0

@pytest.mark.mymark
def test_foo():
    assert 1

• Select tests by mark: ~-m 'not mymark'~
• Marks also useful in plugins

Output

> py.test -ra
===================== test session starts =====================
platform linux -- Python 3.4.3, pytest-2.8.0, py-1.4.30, pluggy-0.3.1
rootdir: /tmp/sandbox, inifile:
collected 3 items

test_ex.py sx.
=================== short test summary info ===================
SKIP [1] test_ex.py:2: Not supported
XFAIL test_ex.py::test_oops

======= 1 passed, 1 skipped, 1 xfailed in 0.07 seconds ========

Fixtures

• Dependecy injection
• Isolation
• (.setUp() .tearDown())

import pytest

@pytest.fixture
def somevalue():
    return 42

def test_value(somevalue):
    assert somevalue == 42

Fixture finalizer

import pytest

@pytest.fixture
def db(request):
    conn = create_conn()

    def fin():
        destroy_db(conn)

    request.addfinalizer(fin)
    return conn

def test_something_with_db(db):
    assert func(db)

Builtin fixture: tmpdir

def write(fname):
    with open(fname, 'w') as fp:
        fp.write('hello world')

def test_output(tmpdir):
    out_txt = tmpdir.join('out.txt')
    write(str(out_txt))
    assert out_txt.read() == 'hello world'

Builtin fixture: monkeypatch

import sys

def test_platform_win(monkeypatch):
    monkeypatch.setattr(sys, 'platform', 'win32')
    assert sys.platform == 'win32'

def test_platform():
    assert sys.platform == 'posix'

More

• parametrization
• fixtures
  • parametrization
  • scopes
• local plugins: conftest.py
  • command line options
  • extra config setup

Questions?