"pytest英文文档：快速入门和使用指南"

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The pytest documentation provides comprehensive information on how to use and leverage the features of the pytest framework. It includes instructions on getting started, how to invoke pytest, as well as detailed guides on writing and reporting assertions in tests and using fixtures. In the "Start here" section, readers are introduced to the basics of pytest. The section covers topics such as how to get started with pytest, including installation instructions and an overview of the pytest command line interface. The "How-to guides" section delves into specific topics related to using pytest. One of the topics covered is how to invoke pytest effectively. This guide provides instructions on how to run tests, customize test discovery, and utilize command line options to configure pytest behavior. Another important guide in this section is on writing and reporting assertions in tests. It outlines the syntax and recommended practices for using assertions effectively in pytest tests. It also explains how to customize assertion messages and provides examples for various types of assertions. The third guide in this section focuses on using fixtures. Fixtures are a powerful feature in pytest that allow for the setup and teardown of resources needed for tests. The guide gives a detailed explanation of fixtures, including how to define and use them, and covers advanced topics such as parameterized fixtures and fixture composition. Overall, the pytest documentation serves as a comprehensive resource for users of pytest. It provides clear instructions on getting started, as well as detailed guides on various aspects of using pytest effectively. Whether you are a beginner or an experienced pytest user, the documentation helps you navigate the framework and make the most of its features.

pytest Documentation, Release 0.1

will not re󰝙ect changes to those 󰝘les between the calls. For this reason, making multiple calls to pytest.main() from

the same process (in order to re-run tests, for example) is not recommended.

2.2 How to write and report assertions in tests

2.2.1 Asserting with the assert statement

pytest allows you to use the standard Python assert for verifying expectations and values in Python tests. For

example, you can write the following:

# content of test_assert1.py

def f():

return 3

def test_function():

assert f() == 4

to assert that your function returns a certain value. If this assertion fails you will see the return value of the function call:

$ pytest test_assert1.py

=========================== test session starts ============================

platform linux -- Python 3.x.y, pytest-7.x.y, pluggy-1.x.y

rootdir: /home/sweet/project

collected 1 item

test_assert1.py F [100%]

================================= FAILURES =================================

______________________________ test_function _______________________________

def test_function():

> assert f() == 4

E assert 3 == 4

E + where 3 = f()

test_assert1.py:6: AssertionError

========================= short test summary info ==========================

FAILED test_assert1.py::test_function - assert 3 == 4

============================ 1 failed in 0.12s =============================

pytest has support for showing the values of the most common subexpressions including calls, attributes, comparisons,

and binary and unary operators. (See Demo of Python failure reports with pytest). This allows you to use the idiomatic

python constructs without boilerplate code while not losing introspection information.

However, if you specify a message with the assertion like this:

assert a % 2 == 0, "value was odd, should be even"

then no assertion introspection takes places at all and the message will be simply shown in the traceback.

See Assertion introspection details for more information on assertion introspection.

12 Chapter 2. How-to guides

pytest Documentation, Release 0.1

2.2.2 Assertions about expected exceptions

In order to write assertions about raised exceptions, you can use pytest.raises() as a context manager like this:

import pytest

def test_zero_division():

with pytest.raises(ZeroDivisionError):

1 / 0

and if you need to have access to the actual exception info you may use:

def test_recursion_depth():

with pytest.raises(RuntimeError) as excinfo:

def f():

f()

assert "maximum recursion" in str(excinfo.value)

excinfo is an ExceptionInfo instance, which is a wrapper around the actual exception raised. The main attributes

of interest are .type, .value and .traceback.

You can pass a match keyword parameter to the context-manager to test that a regular expression matches on the string

representation of an exception (similar to the TestCase.assertRaisesRegex method from unittest):

import pytest

def myfunc():

raise ValueError("Exception 123 raised")

def test_match():

with pytest.raises(ValueError, match=r".* 123 .*"):

myfunc()

The regexp parameter of the match method is matched with the re.search function, so in the above example

match='123' would have worked as well.

There’s an alternate form of the pytest.raises() function where you pass a function that will be executed with the

given *args and **kwargs and assert that the given exception is raised:

pytest.raises(ExpectedException, func, *args, **kwargs)

The reporter will provide you with helpful output in case of failures such as no exception or wrong exception.

Note that it is also possible to specify a “raises” argument to pytest.mark.xfail, which checks that the test is

failing in a more speci󰝘c way than just having any exception raised:

@pytest.mark.xfail(raises=IndexError)

def test_f():

f()

Using pytest.raises() is likely to be better for cases where you are testing exceptions your own code is delib-

erately raising, whereas using @pytest.mark.xfail with a check function is probably better for something like

documenting un󰝘xed bugs (where the test describes what “should” happen) or bugs in dependencies.

2.2. How to write and report assertions in tests 13

pytest Documentation, Release 0.1

(continued from previous page)

f2 = Foo(2)

> assert f1 == f2

E assert Comparing Foo instances:

E vals: 1 != 2

test_foocompare.py:12: AssertionError

========================= short test summary info ==========================

FAILED test_foocompare.py::test_compare - assert Comparing Foo instances:

1 failed in 0.12s

2.2.6 Assertion introspection details

Reporting details about a failing assertion is achieved by rewriting assert statements before they are run. Rewritten assert

statements put introspection information into the assertion failure message. pytest only rewrites test modules directly

discovered by its test collection process, so asserts in supporting modules which are not themselves test modules will

not be rewritten.

You can manually enable assertion rewriting for an imported module by calling register_assert_rewrite before you import

it (a good place to do that is in your root conftest.py).

For further information, Benjamin Peterson wrote up Behind the scenes of pytest’s new assertion rewriting.

Assertion rewriting caches les on disk

pytest will write back the rewritten modules to disk for caching. You can disable this behavior (for example to avoid

leaving stale .pyc 󰝘les around in projects that move 󰝘les around a lot) by adding this to the top of your conftest.py

󰝘le:

import sys

sys.dont_write_bytecode = True

Note that you still get the bene󰝘ts of assertion introspection, the only change is that the .pyc 󰝘les won’t be cached on

disk.

Additionally, rewriting will silently skip caching if it cannot write new .pyc 󰝘les, i.e. in a read-only 󰝘lesystem or a

zip󰝘le.

Disabling assert rewriting

pytest rewrites test modules on import by using an import hook to write new pyc 󰝘les. Most of the time this works

transparently. However, if you are working with the import machinery yourself, the import hook may interfere.

If this is the case you have two options:

• Disable rewriting for a speci󰝘c module by adding the string PYTEST_DONT_REWRITE to its docstring.

• Disable rewriting for all modules by using --assert=plain.

16 Chapter 2. How-to guides

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