Python 偏函数 partial

partial 固定函数的部分参数，生成新的简化函数。

基本用法

from functools import partial

# 固定第一个参数
def power(base, exp):
    return base ** exp

square = partial(power, exp=2)
cube = partial(power, exp=3)

print(square(4))  # 16
print(cube(3))    # 27

固定多个参数

def greet(greeting, name, punctuation='!'):
    return f"{greeting}, {name}{punctuation}"

# 固定 greeting
say_hello = partial(greet, 'Hello')
print(say_hello('Alice'))        # Hello, Alice!
print(say_hello('Bob', '?'))     # Hello, Bob?

# 固定多个参数
hi_alice = partial(greet, 'Hi', 'Alice')
print(hi_alice())                # Hi, Alice!
print(hi_alice(punctuation='.')) # Hi, Alice.

与内置函数配合

from functools import partial

# 固定 int 的 base 参数
binary_to_int = partial(int, base=2)
hex_to_int = partial(int, base=16)

print(binary_to_int('1010'))  # 10
print(hex_to_int('FF'))       # 255

# 固定 print 的 sep 参数
space_print = partial(print, sep=' ')
space_print('a', 'b', 'c')  # a b c

简化回调函数

from functools import partial

def process_data(data, transform, validate=True):
    if validate:
        data = validate_data(data)
    return transform(data)

# 创建预处理函数
uppercase_processor = partial(process_data, transform=str.upper)
lowercase_processor = partial(process_data, transform=str.lower)

data = "Hello World"
print(uppercase_processor(data))  # HELLO WORLD
print(lowercase_processor(data))  # hello world

作为函数参数

from functools import partial

def apply_operation(numbers, operation):
    return [operation(n) for n in numbers]

def multiply(x, y):
    return x * y

numbers = [1, 2, 3, 4, 5]

# 固定乘数
double = partial(multiply, 2)
triple = partial(multiply, 3)

print(apply_operation(numbers, double))  # [2, 4, 6, 8, 10]
print(apply_operation(numbers, triple))  # [3, 6, 9, 12, 15]

partial 属性

from functools import partial

def func(a, b, c):
    return a + b + c

p = partial(func, 1, b=2)

print(p.func)     # 原函数 <function func at ...>
print(p.args)     # 固定位置参数 (1,)
print(p.keywords) # 固定关键字参数 {'b': 2}

partialmethod

用于绑定类方法的偏函数。

from functools import partialmethod

class Cell:
    def __init__(self):
        self._alive = False

    def set_state(self, state):
        self._alive = bool(state)

    set_alive = partialmethod(set_state, True)
    set_dead = partialmethod(set_state, False)

cell = Cell()
cell.set_alive()
print(cell._alive)  # True

cell.set_dead()
print(cell._alive)  # False

partial vs lambda

# partial 方式
from functools import partial
double = partial(lambda x, y: x * y, 2)

# lambda 方式
double_lambda = lambda x: x * 2

# partial 保留原函数信息
print(double.func)      # 存在
print(double_lambda)    # 只是匿名函数

要点总结

• partial 固定部分参数，创建简化函数
• partial(func, *args, **keywords) 语法
• 可固定位置参数和关键字参数
• 适用于重复调用相同参数的场景
• partialmethod 用于类方法绑定
• partial 保留原函数信息，便于调试