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An article to guide you through Python anonymous functions

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2023-07-25 15:58:361674browse

1. Preface

When passing in a function, sometimes there is no need to explicitly define the function. It is more convenient to pass in the anonymous function directly.


2. Anonymous functions

In Python, for anonymous functions Limited support is provided. Still taking the map() function as an example, when calculating f(x)=x2, in addition to defining an f(x) In addition to the function, you can also directly pass in the anonymous function:

print(list(map(lambda x: x * x, [1, 2, 3, 4, 5, 6, 7, 8, 9])))

An article to guide you through Python anonymous functions

It can be seen from the comparison that the anonymous function lambda x: x x is actually:

def f(x):
    return x * x

Note:

##key Lambda represents an anonymous function, and the x before the colon represents function parameters.

Anonymous functions have limitations:

就是只能有一个表达式,不用写return,返回值就是该表达式的结果。

匿名函数有个好处:

因为函数没有名字,不必担心函数名冲突。此外,匿名函数也是一个函数对象,也可以把匿名函数赋值给一个变量,再利用变量来调用该函数:

f = lambda x: x * x
print(f)


print(f(5))

An article to guide you through Python anonymous functions

同样,也可以把匿名函数作为返回值返回,比如:

def build(x, y):
    return lambda: x * x + y * y

匿名函数的使用

匿名函数通常被用作高阶函数(higher-order function,参数为函数的函数)的参数。比如,几个内置函数:filter(),map(),reduce()。下面分别看看这几个函数的用法及达到相同效果的python另一种特征的用法

1. filter函数

list = [1, 2, 3]
result = filter(lambda x: x % 2 == 0, list)
print(result)
result = [x for x in list if x % 2 == 0]
print(result)

An article to guide you through Python anonymous functions

2. map函数

result = map(lambda x: x*2, list)
print(result)
result = [x*2 for x in list]
print(result)

An article to guide you through Python anonymous functions

注:

map和filter函数都可以使用列表推导式(list comprehension)代替。

据说,当年lambda是一个Lisp程序员给python加的,而Guido是强烈反对的,最中意的是列表推导式。

3. 跳转表(jump table)

lambda的另一种用法是用来编写跳转表,也就是行为列表或字典,能按需执行特定的动作。

key = "get"
 {"abc":(lambda : 2 + 2),"bcd" : (lambda : 3 + 3), "get" : (lambda : 4 + 4)}[key]()
8

An article to guide you through Python anonymous functions


三、总结

本文基于Python基础,介绍了在实际中匿名函数的使用,介绍了常见的匿名函数, filter函数,map函数,通过一个案例 ( 跳转表) 的分析 ,代码的演示,进行详细的讲解。

Python对匿名函数的支持有限,只有一些简单的情况下可以使用匿名函数。使用Python语言 ,能够让读者更好的理解。

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