如何使用 pytest Monkeypatch 来修补类

Question

我想使用 [pytest Monkeypatch][1] 来模拟导入到单独模块中的类。这实际上可能吗？如果可能的话，如何做到这一点？我似乎还没有看到这种具体情况的例子。假设您有一个应用程序并在 Something.py 中导入了 A 类

from something import A #Class is imported

class B :
  def __init__(self) :
   self.instance = A() #class instance is created

  def f(self, value) :
    return self.instance.g(value)

在我的 test.py 中我想在 B 中模拟 A

from something import B

#this is where I would mock A such that
def mock_A :
  def g(self, value) :
    return 2*value

#Then I would call B
c = B()
print(c.g(2)) #would be 4

I see how monkeypatch can be used to patch instances of classes, but how is it done for classes that have not yet been instantiated?  Is it possible?  Thanks!

  [1]: https://docs.pytest.org/en/latest/monkeypatch.html

Answer 1

测试过这个，对我有用：

def test_thing(monkeypatch):
    def patched_g(self, value):
        return value * 2

    monkeypatch.setattr(A, 'g', patched_g)
    b = B()
    assert b.f(2) == 4