在 python 单元测试中模拟类属性的更好方法

2022-01-08 00:00:00 python mocking unit-testing python-mock

问题描述

我有一个定义类属性的基类和一些依赖它的子类，例如

I have a base class that defines a class attribute and some child classes that depend on it, e.g.

class Base(object):
    assignment = dict(a=1, b=2, c=3)

我想用不同的作业对这个类进行单元测试，例如空字典，单项等.当然这非常简化，不是重构我的类或测试的问题

I want to unittest this class with different assignments, e.g. empty dictionary, single item, etc. This is extremely simplified of course, it's not a matter of refactoring my classes or tests

我想出的(pytest)测试最终是有效的

The (pytest) tests I have come up with, eventually, that work are

from .base import Base

def test_empty(self):
    with mock.patch("base.Base.assignment") as a:
        a.__get__ = mock.Mock(return_value={})
        assert len(Base().assignment.values()) == 0

def test_single(self):
    with mock.patch("base.Base.assignment") as a:
        a.__get__ = mock.Mock(return_value={'a':1})
        assert len(Base().assignment.values()) == 1

这感觉相当复杂和 hacky - 我什至不完全理解它为什么起作用(虽然我熟悉描述符).mock 是否会自动将类属性转换为描述符?

This feels rather complicated and hacky - I don't even fully understand why it works (I am familiar with descriptors though). Does mock automagically transform class attributes into descriptors?

感觉更合乎逻辑的解决方案不起作用:

A solution that would feel more logical does not work:

def test_single(self):
    with mock.patch("base.Base") as a:
        a.assignment = mock.PropertyMock(return_value={'a':1})
        assert len(Base().assignment.values()) == 1

或者只是

def test_single(self):
    with mock.patch("base.Base") as a:
        a.assignment = {'a':1}
        assert len(Base().assignment.values()) == 1

我尝试过的其他变体也不起作用(作业在测试中保持不变).

Other variants that I've tried don't work either (assignments remains unchanged in the test).

模拟类属性的正确方法是什么?有没有比上述更好/更容易理解的方法?

What's the proper way to mock a class attribute? Is there a better / more understandable way than the one above?

解决方案

base.Base.assignment 被简单地替换为 Mock 对象.您通过添加 __get__ 方法使其成为描述符.

base.Base.assignment is simply replaced with a Mock object. You made it a descriptor by adding a __get__ method.

这有点冗长，有点不必要；您可以直接设置 base.Base.assignment:

It's a little verbose and a little unnecessary; you could simply set base.Base.assignment directly:

def test_empty(self):
    Base.assignment = {}
    assert len(Base().assignment.values()) == 0

当然，这在使用测试并发时不太安全.

This isn't too safe when using test concurrency, of course.

要使用 PropertyMock，我会使用:

with patch('base.Base.assignment', new_callable=PropertyMock) as a:
    a.return_value = {'a': 1}

甚至:

with patch('base.Base.assignment', new_callable=PropertyMock, 
           return_value={'a': 1}):

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