我可以修补 Python 的断言以获得 py.test 提供的输出吗？

Question

Pytest 对失败断言的输出比 Python 中的默认值提供更多信息和有用。我想在正常运行我的 Python 程序时利用它，而不仅仅是在执行测试时。有没有办法从我的脚本中覆盖 Python 的assert行为，以使用 pytest 打印堆栈跟踪，同时仍将我的程序作为python script/pytest_assert.py?

示例程序

def test_foo():
  foo = 12
  bar = 42
  assert foo == bar

if __name__ == '__main__':
  test_foo()

$ python script/pytest_assert.py

Traceback (most recent call last):
  File "script/pytest_assert.py", line 8, in <module>
    test_foo()
  File "script/pytest_assert.py", line 4, in test_foo
    assert foo == bar
AssertionError

$ pytest script/pytest_assert.py

======================== test session starts ========================
platform linux -- Python 3.5.3, pytest-3.3.2, py-1.5.2, pluggy-0.6.0
rootdir: /usr/local/google/home/danijar, inifile:
collected 1 item                                                    

script/pytest_assert.py F                                     [100%]

============================= FAILURES ==============================
_____________________________ test_foo ______________________________

    def test_foo():
      foo = 12
      bar = 42
>     assert foo == bar
E     assert 12 == 42

script/pytest_assert.py:4: AssertionError
===================== 1 failed in 0.02 seconds =====================

想要的结果

$ python script/pytest_assert.py

Traceback (most recent call last):
  File "script/pytest_assert.py", line 8, in <module>
    test_foo()

    def test_foo():
      foo = 12
      bar = 42
>     assert foo == bar
E     assert 12 == 42

script/pytest_assert.py:4: AssertionError

进度更新

我得到的最接近的是这个，但它仅适用于该函数中的断言并垃圾邮件跟踪：

import ast
import inspect

from _pytest import assertion


def test_foo():
  foo = []
  foo.append(13)
  foo = foo[-1]
  bar = 42
  assert foo == bar, 'message'


if __name__ == '__main__':
  tree = ast.parse(inspect.getsource(test_foo))
  assertion.rewrite.rewrite_asserts(tree)
  code = compile(tree, '<name>', 'exec')
  ns = {}
  exec(code, ns)
  ns[test_foo.__name__]()

$ python script/pytest_assert.py
Traceback (most recent call last):
  File "script/pytest_assert.py", line 21, in <module>
    ns[test_foo.__name__]()
  File "<name>", line 6, in test_foo
AssertionError: message
assert 13 == 42

Answer 1

免责声明

虽然肯定有一种方法可以重用pytest代码以所需的格式打印回溯，但您需要使用的东西不是公共 API 的一部分，因此最终的解决方案将过于脆弱，需要调用非相关pytest代码（用于初始化目的） ) 并且可能会中断软件包更新。最好的办法是重写关键部分，以pytest代码为例。

笔记

基本上，下面的概念验证代码做了三件事：

1. 用sys.excepthook自定义替换默认值：这是更改默认回溯格式所必需的。例子：

   import sys
   
   orig_hook = sys.excepthook
   
   def myhook(*args):
       orig_hook(*args)
       print('hello world')
   
   if __name__ == '__main__':
       sys.excepthook = myhook
       raise ValueError()
   

   将输出：

   Traceback (most recent call last):
     File "example.py", line 11, in <module>
       raise ValueError()
   ValueError
   hello world
   

2. hello world将打印格式化的异常信息而不是。我们用ExceptionInfo.getrepr()它。

3. 要访问断言中的附加信息，请pytest重写assert语句（您可以在这篇旧文章中获得有关重写后的外观的粗略信息）。为此，请pytest注册PEP 302 中指定的自定义导入挂钩。钩子是最有问题的部分，因为它与Config对象紧密耦合，我也注意到一些模块导入会导致问题（我猜它不会失败，pytest只是因为在钩子注册时模块已经导入；将尝试编写在pytest运行时重现问题并创建新问题的测试）。因此，我建议编写一个自定义导入钩子来调用AssertionRewriter. 这个 AST tree walker 类是断言重写中必不可少的部分，而AssertionRewritingHook不是那么重要。

代码

so-51839452
??? hooks.py
??? main.py
??? pytest_assert.py

hooks.py

import sys

from pluggy import PluginManager
import _pytest.assertion.rewrite
from _pytest._code.code import ExceptionInfo
from _pytest.config import Config, PytestPluginManager


orig_excepthook = sys.excepthook

def _custom_excepthook(type, value, tb):
    orig_excepthook(type, value, tb)  # this is the original traceback printed
    # preparations for creation of pytest's exception info
    tb = tb.tb_next  # Skip *this* frame
    sys.last_type = type
    sys.last_value = value
    sys.last_traceback = tb

    info = ExceptionInfo(tup=(type, value, tb, ))

    # some of these params are configurable via pytest.ini
    # different params combination generates different output
    # e.g. style can be one of long|short|no|native
    params = {'funcargs': True, 'abspath': False, 'showlocals': False,
              'style': 'long', 'tbfilter': False, 'truncate_locals': True}
    print('------------------------------------')
    print(info.getrepr(**params))  # this is the exception info formatted
    del type, value, tb  # get rid of these in this frame


def _install_excepthook():
    sys.excepthook = _custom_excepthook


def _install_pytest_assertion_rewrite():
    # create minimal config stub so AssertionRewritingHook is happy
    pluginmanager = PytestPluginManager()
    config = Config(pluginmanager)
    config._parser._inidict['python_files'] = ('', '', [''])
    config._inicache = {'python_files': None, 'python_functions': None}
    config.inicfg = {}

    # these modules _have_ to be imported, or AssertionRewritingHook will complain
    import py._builtin
    import py._path.local
    import py._io.saferepr

    # call hook registration
    _pytest.assertion.install_importhook(config)

# convenience function
def install_hooks():
    _install_excepthook()
    _install_pytest_assertion_rewrite()

main.py

调用后hooks.install_hooks()，main.py将修改回溯打印。install_hooks()调用后导入的每个模块都将在导入时重写断言。

from hooks import install_hooks

install_hooks()

import pytest_assert


if __name__ == '__main__':
    pytest_assert.test_foo()

pytest_assert.py

def test_foo():
    foo = 12
    bar = 42
    assert foo == bar

示例输出

$ python main.py
Traceback (most recent call last):
  File "main.py", line 9, in <module>
    pytest_assert.test_foo()
  File "/Users/hoefling/projects/private/stackoverflow/so-51839452/pytest_assert.py", line 4, in test_foo
    assert foo == bar
AssertionError
------------------------------------
def test_foo():
        foo = 12
        bar = 42
>       assert foo == bar
E       AssertionError

pytest_assert.py:4: AssertionError

总结

我会去写一个自己的版本AssertionRewritingHook，没有整个不相关的pytest东西。该AssertionRewriter然而，看起来非常可重复使用; 虽然它需要一个Config实例，但它只用于警告打印，可以留给None.

一旦你有了它，编写你自己的函数来正确格式化异常，替换sys.excepthook你就完成了。