在 Python 中向现有对象实例添加方法

Add a method to an existing object instance in Python

使用该types.MethodType函数将方法添加到现有对象实例，例如
my_inst.increase_salary = types.MethodType(increase_salary, my_inst). 该
MethodType函数会将方法绑定到实例。

主程序


import types


class Employee():
    def __init__(self, name, salary):
        self.name = name
        self.salary = salary


alice = Employee('Alice', 100)


def increase_salary(self):
    self.salary += 100

    return self.salary


alice.increase_salary = types.MethodType(increase_salary, alice)


print(alice.increase_salary())  # 👉️ 200
print(alice.increase_salary())  # 👉️ 300

向现有对象添加方法时，我们必须注意将其绑定到对象，以便在调用方法时将实例作为第一个参数传递

主程序


class Employee():
    def __init__(self, name, salary):
        self.name = name
        self.salary = salary


alice = Employee('Alice', 100)


def increase_salary(self):
    self.salary += 100

    return self.salary


print(increase_salary) # <function increase_salary at 0x7f6b939ffd90>

print(alice.__init__) # <bound method Employee.__init__ of <__main__.Employee object at 0x7f6b938f7100>>

然而，如果我们在类上添加方法，我们就不必关心绑定它，它将是一个未绑定的方法，可用于所有先前创建的实例。

主程序


class Employee():
    def __init__(self, name, salary):
        self.name = name
        self.salary = salary


alice = Employee('Alice', 100)
bob = Employee('Bob', 100)


def increase_salary(self):
    self.salary += 100

    return self.salary


Employee.increase_salary = increase_salary

print(Employee.increase_salary)  # <function increase_salary at 0x7fb813bf7d90>


print(alice.increase_salary())  # 👉️ 200

print(bob.increase_salary())  # 👉️ 200

When the method is added directly on the existing object instance, it is only
available on the specific instance.

main.py


import types


class Employee():
    def __init__(self, name, salary):
        self.name = name
        self.salary = salary


alice = Employee('Alice', 100)
bob = Employee('Bob', 100)


def increase_salary(self):
    self.salary += 100

    return self.salary


alice.increase_salary = types.MethodType(increase_salary, alice)

# <bound method increase_salary of <__main__.Employee object at 0x7f6edb29f4c0>>
print(alice.increase_salary)

print(alice.increase_salary())  # 👉️ 200

# ⛔️ AttributeError: 'Employee' object has no attribute 'increase_salary'
print(bob.increase_salary())

An alternative to using the MethodType function is to use a descriptor to bind
the method to the class instance.

main.py


class Employee():
    def __init__(self, name, salary):
        self.name = name
        self.salary = salary


alice = Employee('Alice', 100)
bob = Employee('Bob', 100)


def increase_salary(self):
    self.salary += 100

    return self.salary


alice.increase_salary = increase_salary.__get__(alice)


print(alice.increase_salary())  # 👉️ 200
print(alice.increase_salary())  # 👉️ 300

# <bound method increase_salary of <__main__.Employee object at 0x7f31cc8174c0>>
print(increase_salary.__get__(alice))

We used the
__get__()
method on the function.

Functions have a __get__() method so they can be converted to a method when accessed as attributes.

This helps us convert the increase_salary function to a method that is bound
to the specific class instance.

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