2.9. Monkey Patching

2.9. Monkey Patching — Python

2.9.1. Recap information about classes and objects

class User:
    def __init__(self):
        self.name = 'Alice'

    def hello(self):
        print(f'Hello {self.name}')

u = User()
u.hello()
# Hello Alice

class User:
    def __init__(self):
        self.name = 'Alice'

    def hello(self):
        print(f'Hello {self.name}')

User.hello()
# Traceback (most recent call last):
# TypeError: hello() missing 1 required positional argument: 'self'

class User:
    def __init__(self):
        self.name = 'Alice'

    @staticmethod
    def hello():
        print(f'Hello')

User.hello()
# Hello

2.9.2. Injecting fields

class User:
    def __init__(self):
        self.name = 'Alice'

    def hello(self):
        print(f'Hello {self.name}')


User.group = 'users'    # Injecting field

print(User.group)
# users

User.__dict__
# mappingproxy({
#   '__module__': '__main__',
#   '__init__': <function User.__init__ at 0x10fb9f5e0>,
#   'hello': <function User.hello at 0x10fb9f4c0>,
#   '__dict__': <attribute '__dict__' of 'User' objects>,
#   '__weakref__': <attribute '__weakref__' of 'User' objects>,
#   '__doc__': None,
#   'group': 'NASA',
# })

dir(User)
# ['__class__', '__delattr__', '__dict__', '__dir__', '__doc__',
#  '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__',
#  '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__',
#  '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__',
#  '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__',
#  '__weakref__', 'group', 'hello']

2.9.3. Injecting static methods

Note, that there is no self as a first argument to function:

class User:
    def hello(self):
        print('Old version')


def myfunction():
    print('New version')


User.hello = myfunction
User.hello()
# New version

class User:
    def hello(self):
        print('Old version')


def myfunction(self):
    print('New version')


u1 = User()
u2 = User()

u1.hello()      # Old version
u2.hello()      # Old version

User.hello = myfunction

u1.hello()      # New version
u2.hello()      # New version

class User:
    def hello(self):
        print('Old version')


def myfunction():
    print('New version')


u1 = User()
u2 = User()

u1.hello()      # Old version
u2.hello()      # Old version

u1.hello = myfunction

u1.hello()      # New version
u2.hello()      # Old version

Injecting static and dynamic methods to class:

class User:
    pass


def my_staticmethod():
    print('My Static Method')

def my_dynamicmethod(self):
    print('My Dynamic Method')

User.my_staticmethod = my_staticmethod
User.my_dynamicmethod = my_dynamicmethod
User.my_staticlambda = lambda: print('My Static Lambda')
User.my_dynamiclambda = lambda self: print('My Dynamic Lambda')


User.my_staticmethod()      # My Static Method
User.my_dynamicmethod()     # TypeError: my_dynamicmethod() missing 1 required positional argument: 'self'
User.my_staticlambda()      # My Static Lambda
User.my_dynamiclambda()     # TypeError: <lambda>() missing 1 required positional argument: 'self'

u = User()

u.my_staticmethod()         # TypeError: my_staticmethod() takes 0 positional arguments but 1 was given
u.my_dynamicmethod()        # My Dynamic Method
u.my_staticlambda()         # TypeError: <lambda>() takes 0 positional arguments but 1 was given
u.my_dynamiclambda()        # My Dynamic Lambda

Injecting static methods with parameters:

class User:
    pass


def my_staticmethod(*args, **kwargs):
    print(f'My Static Method, args: {args}, kwargs: {kwargs}')

User.my_staticmethod = my_staticmethod
User.my_staticlambda = lambda *args, **kwargs: print(f'My Static Lambda, args: {args}, kwargs: {kwargs}')


User.my_staticmethod()
# My Static Method, args: (), kwargs: {}

User.my_staticmethod(1, 2, 3, d=4, e=5, f=6)
# My Static Method, args: (1, 2, 3), kwargs: {'d': 4, 'e': 5, 'f': 6}

User.my_staticlambda()
# My Static Lambda, args: (), kwargs: {}

User.my_staticlambda(1, 2, 3, d=4, e=5, f=6)
# My Static Lambda, args: (1, 2, 3), kwargs: {'d': 4, 'e': 5, 'f': 6}

2.9.4. Injecting Dynamic Methods

Note, that there is no self as a first argument to lambda:

class User:
    pass


u = User()
u.hello = lambda name: print(f'Hello {name}')

u.hello('Alice')
# Hello Alice

Note, although there is self in lambda, it is not passed as an argument:

class User:
    def __init__(self):
        self.name = 'Alice'


u = User()
u.hello = lambda self: print(f'Hello {self.name}')

u.hello()
# Traceback (most recent call last):
# TypeError: <lambda>() missing 1 required positional argument: 'self'

u.hello('Alice')
# Traceback (most recent call last):
# AttributeError: 'str' object has no attribute 'name'

Note, the self argument to lambda:

class User:
    pass


User.hello = lambda self: print(f'Hello {self.name}')

u = User()
u.name = 'Alice'

u.hello()
# Hello Alice

2.9.5. Backups

class User:
    def hello(self):
        print('Old version')


def my_function():
    print('New version')


User._old_hello = User.hello
User.hello = my_function
User.hello()
# New version

User.hello = User._old_hello
User.hello()
# Traceback (most recent call last):
# TypeError: hello() missing 1 required positional argument: 'self'

2.9.6. Create Missing Method

class User:
    def hello(self):
        print('hello')

    def __getattr__(self, name):
        print(f'Attribute "{name}" not found')
        setattr(self, name, lambda: print(f'Now I have "{name}"'))
        return super().__getattribute__(name)

    def __getattribute__(self, name):
        print(f'Accessing: "{name}"')
        return super().__getattribute__(name)


u = User()
u.x()
u.x()

# Accessing: x
# Attribute "x" not found
# Now I have "x"

2.9.7. Object Initialization

class User:
    def __init__(self):
        self.name = 'Mark Watney'

    def hello(self):
        print('hello')

u = User()
u.hello()
# hello

class User:
    pass


u = User()
u.name = 'Mark Watney'
u.hello = lambda: print('hello')

u.hello()
# hello

2.9.8. User Cases

from datetime import datetime
import json

json.dumps({'gagarin': datetime(1961, 4, 12, 6, 7)})
# Traceback (most recent call last):
# TypeError: Object of type datetime is not JSON serializable

json.JSONEncoder.default = lambda self, dt: dt.isoformat()
json.dumps({'gagarin': datetime(1961, 4, 12, 6, 7)})
# '{"gagarin": "1961-04-12T06:07:00"}'

import numpy as np

numpy.array = debug(numpy.array)
numpy.array = trace_usage(numpy.array)
numpy.array = profiling(numpy.array)

from datetime import date, datetime
import json


def datetime_encoder(self, value):
    if type(value) is date:
        return f'{value:%Y-%m-%d}'
    if type(value) is datetime:
        return value.isoformat()
    else:
        return str(value)

json.JSONEncoder.default = datetime_encoder

json.dumps({'gagarin': date(1961, 4, 12)})
# {"gagarin": "1961-04-12"}