如何模拟 Python 静态方法和类方法

Question

如何模拟具有未绑定方法的类？比如这个类有一个@classmethod和一个@staticmethod：

class Calculator(object):
    def __init__(self, multiplier):
        self._multiplier = multiplier
    def multiply(self, n):
        return self._multiplier * n
    @classmethod
    def increment(cls, n):
        return n + 1
    @staticmethod
    def decrement(n):
        return n - 1

calculator = Calculator(2)
assert calculator.multiply(3) == 6    
assert calculator.increment(3) == 4
assert calculator.decrement(3) == 2
assert Calculator.increment(3) == 4
assert Calculator.decrement(3) == 2

上面几乎描述了我的问题。以下是一个工作示例，演示了我尝试过的事情。

类Machine 包含Calculator 的一个实例。我将使用Calculator 的模拟来测试Machine。为了演示我的问题，Machine 通过Calculator 的实例和Calculator 类调用未绑定的方法：

class Machine(object):
    def __init__(self, calculator):
        self._calculator = calculator
    def mult(self, n):
        return self._calculator.multiply(n)
    def incr_bound(self, n):
        return self._calculator.increment(n)
    def decr_bound(self, n):
        return self._calculator.decrement(n)
    def incr_unbound(self, n):
        return Calculator.increment(n)
    def decr_unbound(self, n):
        return Calculator.decrement(n)

machine = Machine(Calculator(3))
assert machine.mult(3) == 9

assert machine.incr_bound(3) == 4
assert machine.incr_unbound(3) == 4

assert machine.decr_bound(3) == 2
assert machine.decr_unbound(3) == 2

上面的所有功能代码都可以正常工作。接下来是不起作用的部分。

我创建了一个模拟 Calculator 用于测试 Machine：

from mock import Mock

def MockCalculator(multiplier):
    mock = Mock(spec=Calculator, name='MockCalculator')

    def multiply_proxy(n):
        '''Multiply by 2*multiplier instead so we can see the difference'''
        return 2 * multiplier * n
    mock.multiply = multiply_proxy

    def increment_proxy(n):
        '''Increment by 2 instead of 1 so we can see the difference'''
        return n + 2
    mock.increment = increment_proxy

    def decrement_proxy(n):
        '''Decrement by 2 instead of 1 so we can see the difference'''
        return n - 2
    mock.decrement = decrement_proxy

    return mock

在下面的单元测试中，绑定方法使用MockCalculator，正如我所希望的那样。但是，对Calculator.increment() 和Calculator.decrement() 的调用仍然使用Calculator：

import unittest

class TestMachine(unittest.TestCase):
    def test_bound(self):
        '''The bound methods of Calculator are replaced with MockCalculator'''
        machine = Machine(MockCalculator(3))
        self.assertEqual(machine.mult(3), 18)
        self.assertEqual(machine.incr_bound(3), 5)
        self.assertEqual(machine.decr_bound(3), 1)

    def test_unbound(self):
        '''Machine.incr_unbound() and Machine.decr_unbound() are still using
        Calculator.increment() and Calculator.decrement(n), which is wrong.
        '''
        machine = Machine(MockCalculator(3))
        self.assertEqual(machine.incr_unbound(3), 4)    # I wish this was 5
        self.assertEqual(machine.decr_unbound(3), 2)    # I wish this was 1

所以我尝试修补Calculator.increment() 和Calculator.decrement()：

def MockCalculatorImproved(multiplier):
    mock = Mock(spec=Calculator, name='MockCalculatorImproved')

    def multiply_proxy(n):
        '''Multiply by 2*multiplier instead of multiplier so we can see the difference'''
        return 2 * multiplier * n
    mock.multiply = multiply_proxy
    return mock

def increment_proxy(n):
    '''Increment by 2 instead of 1 so we can see the difference'''
    return n + 2

def decrement_proxy(n):
    '''Decrement by 2 instead of 1 so we can see the difference'''
    return n - 2


from mock import patch

@patch.object(Calculator, 'increment', increment_proxy)
@patch.object(Calculator, 'decrement', decrement_proxy)
class TestMachineImproved(unittest.TestCase):
    def test_bound(self):
        '''The bound methods of Calculator are replaced with MockCalculator'''
        machine = Machine(MockCalculatorImproved(3))
        self.assertEqual(machine.mult(3), 18)
        self.assertEqual(machine.incr_bound(3), 5)
        self.assertEqual(machine.decr_bound(3), 1)

    def test_unbound(self):
        '''machine.incr_unbound() and Machine.decr_unbound() should use
        increment_proxy() and decrement_proxy(n).
        '''
        machine = Machine(MockCalculatorImproved(3))
        self.assertEqual(machine.incr_unbound(3), 5)
        self.assertEqual(machine.decr_unbound(3), 1)

即使在修补之后，未绑定的方法仍需要 Calculator 的实例作为参数：

TypeError: unbound method increment_proxy() 必须以 Calculator 实例作为第一个参数调用（取而代之的是 int 实例）

如何模拟类方法Calculator.increment() 和静态方法Calculator.decrement()？

Answer 1

你修补了错误的对象。您必须修补来自 Machine 类的 Calculator，而不是一般的 Calculator 类。阅读它here。

from mock import patch
import unittest

from calculator import Calculator
from machine import Machine


class TestMachine(unittest.TestCase):
    def my_mocked_mult(self, multiplier):
        return 2 * multiplier * 3
    def test_bound(self):
        '''The bound methods of Calculator are replaced with MockCalculator'''
        machine = Machine(Calculator(3))
        with patch.object(machine, "mult") as mocked_mult:
            mocked_mult.side_effect = self.my_mocked_mult
            self.assertEqual(machine.mult(3), 18)
            self.assertEqual(machine.incr_bound(3), 5)
            self.assertEqual(machine.decr_bound(3), 1)

    def test_unbound(self):
        '''Machine.incr_unbound() and Machine.decr_unbound() are still using
        Calculator.increment() and Calculator.decrement(n), which is wrong.
        '''
        machine = Machine(Calculator(3))
        self.assertEqual(machine.incr_unbound(3), 4)    # I wish this was 5
        self.assertEqual(machine.decr_unbound(3), 2)    # I wish this was 1

Answer 2

一种方法是

def test_increment(mocker):
    mocker.patch.object(Calculator, attribute='increment', return_value=10)
    ...actual test code...

Answer 3

我刚刚做了一些可以转化为您的案例的事情，如下所示：

class Calculator_Mock(object):
    def __init__(self, multiplier):
        ... # add whatever you need here

    def multiply(self, n):
        ... # add whatever you need here

    @classmethod
    def increment(self, n):
        ... # add whatever you need here

然后，在您的测试中，像这样简单：

class TestCalculator(TestCase):

    def test_increment_or_whatever(self):
        with patch.object(Calculator,
                          "increment",
                          return_value=Calculator_Mock.increment()) as increment_mock:
        ... # call whatever your calls Calculator.increment, the mock should run instead the Calculator.increment

Answer 4

C#、Java 和 C++ 程序员倾向于在 Python 中过度使用类和静态方法。 Pythonic 方法是使用模块函数。

首先，这里是重构的待测软件，方法increment() 和decrement() 作为模块函数。界面确实发生了变化，但功能是一样的：

# Module machines

class Calculator(object):
    def __init__(self, multiplier):
        self._multiplier = multiplier
    def multiply(self, n):
        return self._multiplier * n

def increment(n):
    return n + 1

def decrement(n):
    return n - 1

calculator = Calculator(2)
assert calculator.multiply(3) == 6
assert increment(3) == 4
assert decrement(3) == 2


class Machine(object):
    '''A larger machine that has a calculator.'''
    def __init__(self, calculator):
        self._calculator = calculator
    def mult(self, n):
        return self._calculator.multiply(n)
    def incr(self, n):
        return increment(n)
    def decr(self, n):
        return decrement(n)

machine = Machine(Calculator(3))
assert machine.mult(3) == 9
assert machine.incr(3) == 4
assert machine.decr(3) == 2

添加函数increment_mock()和decrement_mock()来模拟increment()和decrement()：

from mock import Mock
import machines

def MockCalculator(multiplier):
    mock = Mock(spec=machines.Calculator, name='MockCalculator')

    def multiply_proxy(n):
        '''Multiply by 2*multiplier instead of multiplier so we can see the
        difference.
        '''
        return 2 * multiplier * n
    mock.multiply = multiply_proxy

    return mock

def increment_mock(n):
    '''Increment by 2 instead of 1 so we can see the difference.'''
    return n + 2

def decrement_mock(n):
    '''Decrement by 2 instead of 1 so we can see the difference.'''
    return n - 2

现在是好的部分。修补 increment() 和 decrement() 以用他们的模拟替换它们：

import unittest
from mock import patch
import machines

@patch('machines.increment', increment_mock)
@patch('machines.decrement', decrement_mock)
class TestMachine(unittest.TestCase):
    def test_mult(self):
        '''The bound method of Calculator is replaced with MockCalculator'''
        machine = machines.Machine(MockCalculator(3))
        self.assertEqual(machine.mult(3), 18)

    def test_incr(self):
        '''increment() is replaced with increment_mock()'''
        machine = machines.Machine(MockCalculator(3))
        self.assertEqual(machine.incr(3), 5)

    def test_decr(self):
        '''decrement() is replaced with decrement_mock()'''
        machine = machines.Machine(MockCalculator(3))
        self.assertEqual(machine.decr(3), 1)