Understanding Monkey Patching in Python: Concepts, Example, and Performance Boost

Monkey patch (also called monkey patching) is a technique that allows dynamic replacement of attributes or functions in a running program, effectively adding or modifying functionality at runtime.

In Python, monkey patching enables you to modify a class or module while the program is executing.

Example: a game developer originally imported the built‑in json module many times and later discovered that ujson is much faster. Instead of changing every import, a single monkey patch at program start can replace json.dumps with ujson.dumps .

Below is a helper module that defines a timing decorator and a json_dumps function using the standard json library:

"""
file:json_serialize.py
"""
import time
import json

# time test decorator
def run_time(func):
    def inner(*args, **kwargs):
        start_time = time.time()
        result = func(*args, **kwargs)
        end_time = time.time()
        print(f'程序用时：{end_time - start_time}')
        return result
    return inner

@run_time
def json_dumps(obj):
    return json.dumps(obj)

# generate test dict
test_dict = {i: 1 for i in range(1, 10000001)}

Running the original program:

"""
file:run.py
"""
from json_serialize import json_dumps, test_dict

print(f'json.dumps编码用时:', end='')
r1 = json_dumps(test_dict)

The original implementation is slow because the standard json encoder is not optimized for large data.

After applying a monkey patch that swaps the json.dumps function with ujson.dumps :

"""
file:run.py
"""
import json
import ujson
from json_serialize import json_dumps, test_dict

def monkey_patch_json():
    json.dumps = ujson.dumps

monkey_patch_json()
print(f'使用猴子补丁之后json.dumps编码用时:', end='')
json_dumps(test_dict)

Now the json module in the whole process has been replaced by the faster ujson implementation, resulting in a noticeable performance improvement.