使用 pytest 测试 __init__.py 中可选依赖项的导入：Python 3.5 /3.6 的行为不同

Question

我有一个用于 python 3.5 和 3.6 的包，它具有可选的依赖项，我想要在任一版本上运行的测试 (pytest)。

我在下面做了一个由两个文件组成的简化示例，一个简单__init__.py的示例，其中导入了可选包“请求”（只是一个示例），并设置了一个标志来指示请求的可用性。

mypackage/
??? mypackage
?   ??? __init__.py
??? test_init.py

该__init__.py文件的内容：

#!/usr/bin/env python
# -*- coding: utf-8 -*-

requests_available = True

try:
    import requests
except ImportError:
    requests_available = False

该test_init.py文件的内容：

#!/usr/bin/env python
# -*- coding: utf-8 -*-
import pytest, sys

def test_requests_missing(monkeypatch):
    import mypackage
    import copy
    fakesysmodules = copy.copy(sys.modules)
    fakesysmodules["requests"] = None
    monkeypatch.delitem(sys.modules,"requests")
    monkeypatch.setattr("sys.modules", fakesysmodules)
    from importlib import reload
    reload(mypackage)
    assert mypackage.requests_available == False


if __name__ == '__main__':
    pytest.main([__file__, "-vv", "-s"])

该test_requests_missing测试适用于 Python 3.6.5：

runfile('/home/bjorn/python_packages/mypackage/test_init.py', wdir='/home/bjorn/python_packages/mypackage')
============================= test session starts ==============================
platform linux -- Python 3.6.5, pytest-3.6.1, py-1.5.2, pluggy-0.6.0 -- /home/bjorn/anaconda3/envs/bjorn36/bin/python
cachedir: .pytest_cache
rootdir: /home/bjorn/python_packages/mypackage, inifile:
plugins: requests-mock-1.5.0, mock-1.10.0, cov-2.5.1, nbval-0.9.0, hypothesis-3.38.5
collecting ... collected 1 item

test_init.py::test_requests_missing PASSED

=========================== 1 passed in 0.02 seconds ===========================

但不是在 Python 3.5.4 上：

runfile('/home/bjorn/python_packages/mypackage/test_init.py', wdir='/home/bjorn/python_packages/mypackage')
========================================================= test session starts ==========================================================
platform linux -- Python 3.5.4, pytest-3.6.1, py-1.5.2, pluggy-0.6.0 -- /home/bjorn/anaconda3/envs/bjorn35/bin/python
cachedir: .pytest_cache
rootdir: /home/bjorn/python_packages/mypackage, inifile:
plugins: requests-mock-1.5.0, mock-1.10.0, cov-2.5.1, nbval-0.9.1, hypothesis-3.38.5
collecting ... collected 1 item

test_init.py::test_requests_missing FAILED

=============================================================== FAILURES ===============================================================
________________________________________________________ test_requests_missing _________________________________________________________

monkeypatch = <_pytest.monkeypatch.MonkeyPatch object at 0x7f9a2953acc0>

    def test_requests_missing(monkeypatch):
        import mypackage
        import copy
        fakesysmodules = copy.copy(sys.modules)
        fakesysmodules["requests"] = None
        monkeypatch.delitem(sys.modules,"requests")
        monkeypatch.setattr("sys.modules", fakesysmodules)
        from importlib import reload
>       reload(mypackage)

test_init.py:13: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
../../anaconda3/envs/bjorn35/lib/python3.5/importlib/__init__.py:166: in reload
    _bootstrap._exec(spec, module)
<frozen importlib._bootstrap>:626: in _exec
    ???
<frozen importlib._bootstrap_external>:697: in exec_module
    ???
<frozen importlib._bootstrap>:222: in _call_with_frames_removed
    ???
mypackage/__init__.py:8: in <module>
    import requests
../../anaconda3/envs/bjorn35/lib/python3.5/site-packages/requests/__init__.py:97: in <module>
    from . import utils

.... VERY LONG OUTPUT ....

    from . import utils
../../anaconda3/envs/bjorn35/lib/python3.5/site-packages/requests/__init__.py:97: in <module>
    from . import utils
<frozen importlib._bootstrap>:968: in _find_and_load
    ???
<frozen importlib._bootstrap>:953: in _find_and_load_unlocked
    ???
<frozen importlib._bootstrap>:896: in _find_spec
    ???
<frozen importlib._bootstrap_external>:1171: in find_spec
    ???
<frozen importlib._bootstrap_external>:1145: in _get_spec
    ???
<frozen importlib._bootstrap_external>:1273: in find_spec
    ???
<frozen importlib._bootstrap_external>:1245: in _get_spec
    ???
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

name = 'requests', location = '/home/bjorn/anaconda3/envs/bjorn35/lib/python3.5/site-packages/requests/__init__.py'

>   ???
E   RecursionError: maximum recursion depth exceeded

<frozen importlib._bootstrap_external>:575: RecursionError
======================================================= 1 failed in 2.01 seconds =======================================================

我有两个问题：

1. 为什么我会看到这种差异？相关软件包在 3.5 和 3.6 上的版本似乎相同。

2. 有没有更好的方法来做我想做的事？我现在的代码是从网上找到的例子拼接在一起的。我试图修补导入机制以避免“重新加载”，但我没有成功。

Answer 1

我要么模拟该__import__函数（在语句后面调用的函数import modname），要么通过添加自定义元路径查找器来自定义导入机制。例子：

改变sys.meta_path

添加一个自定义MetaPathFinder实现，该实现ImportError在尝试导入任何包时引发pkgnames：

class PackageDiscarder:
    def __init__(self):
        self.pkgnames = []
    def find_spec(self, fullname, path, target=None):
        if fullname in self.pkgnames:
            raise ImportError()


@pytest.fixture
def no_requests():
    sys.modules.pop('requests', None)
    d = PackageDiscarder()
    d.pkgnames.append('requests')
    sys.meta_path.insert(0, d)
    yield
    sys.meta_path.remove(d)


@pytest.fixture(autouse=True)
def cleanup_imports():
    yield
    sys.modules.pop('mypackage', None)


def test_requests_available():
    import mypackage
    assert mypackage.requests_available


@pytest.mark.usefixtures('no_requests2')
def test_requests_missing():
    import mypackage
    assert not mypackage.requests_available

夹具在调用时no_requests会发生变化sys.meta_path，因此自定义元路径查找器会从可以导入的包名称中过滤requests掉包名称（我们不能在任何导入上引发，否则pytest它本身会中断）。cleanup_imports只是为了确保mypackage在每次测试中都会重新导入。

嘲笑__import__

import builtins
import sys
import pytest


@pytest.fixture
def no_requests(monkeypatch):
    import_orig = builtins.__import__
    def mocked_import(name, globals, locals, fromlist, level):
        if name == 'requests':
            raise ImportError()
        return import_orig(name, locals, fromlist, level)
    monkeypatch.setattr(builtins, '__import__', mocked_import)


@pytest.fixture(autouse=True)
def cleanup_imports():
    yield
    sys.modules.pop('mypackage', None)


def test_requests_available():
    import mypackage
    assert mypackage.requests_available


@pytest.mark.usefixtures('no_requests')
def test_requests_missing():
    import mypackage
    assert not mypackage.requests_available

在这里，fixtureno_requests负责用尝试__import__时引发的函数替换该函数import requests，在其余导入中表现良好。