python - 修补时模块级代码运行两次

Question

考虑以下简单代码：

测试代码.py

def f():
    return 'unpatched'

import patch_stuff

patch_stuff.patch_it_up()
print f()

patch_stuff.py

from mock import patch

def patch_it_up():
    p = patch('test_code.f')
    m = p.start()
    m.return_value = 'patched'

我希望运行的输出python test_code.py是

patched

但是输出是：

unpatched
patched

怎么来的？

Answer 1

There are two problems in your code. First, the test_code.py file is called twice - first when you call it, and then again when mock imports it for the purpose of creating a patch. So you should change it as follows:

def f():
    return 'unpatched'

import patch_stuff

if __name__ == "__main__":
    patch_stuff.patch_it_up()
    print f()

This will print only the 'unpatched' string. Which leads to the second thing: the documentation suggests that this not how the patchers work. When you call the start method it returns the patched object to be used. So the expected output can be achieved with the following modifications:

patch_stuff.py

from mock import patch

def patch_it_up():
    p = patch('test_code.f')
    m = p.start()
    m.return_value = 'patched'
    return m

test_code.py

def f():
    return 'unpatched'

import patch_stuff

if __name__ == "__main__":
    m = patch_stuff.patch_it_up()
    print m()

This will print the expected 'patched' string.

---

This makes this scenario not very practical, but all the examples in the documentation show only the possibility of modifying modules imported in the current context. E.g., this will also work:

from mock import patch

def patch_it_up():
    import test_code
    p = patch('test_code.f')
    m = p.start()
    m.return_value = 'patched'
    print "inside patch_it_up:", test_code.f()