python - 我可以修补 Python 的断言以获取 py.test 提供的输出吗?
问题描述
Pytest 的失败断言输出比 Python 中的默认输出更具信息性和实用性。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 来打印堆栈跟踪,同时仍将我的程序运行为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 =====================
期望的结果
$ 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this,但它仅适用于该函数中的断言并发送垃圾邮件跟踪:
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
解决方案
免责声明
虽然肯定有一种方法可以重用pytest
代码以所需格式打印回溯,但您需要使用的东西不是公共 API 的一部分,因此生成的解决方案将太脆弱,需要调用不相关的pytest
代码(用于初始化目的) 并且可能会中断包更新。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:
用自定义替换默认值
sys.excepthook
: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
而不是
hello world
,将打印格式化的异常信息。我们ExceptionInfo.getrepr()
为此使用。要访问断言中的附加信息,请
pytest
重写语句(您可以在这篇旧文章assert
中获得一些关于重写后它们的外观的粗略信息)。为此,请注册PEP 302中指定的自定义导入挂钩。钩子是最有问题的部分,因为它与对象紧密耦合,我还注意到一些模块导入会导致问题(我猜它不会失败只是因为在注册钩子时模块已经导入;将尝试编写在运行中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pytest
Config
pytest
pytest
AssertionRewriter
. 这个 AST tree walker 类是断言重写中必不可少的部分,但AssertionRewritingHook
并不那么重要。
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
将有修改的回溯打印。调用后导入的每个模块install_hooks()
都会在导入时重写断言。
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
我会写一个自己的版本AssertionRewritingHook
,没有整个不相关的pytest
东西。然而,AssertionRewriter
看起来非常可重用;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。
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