anonymous function of Python

Let me introduce you to the anonymous function in Python today.

anonymous functions

1. lambdadefined by keywords
2. Parameter list does not require parentheses
3. The colon is not the opening of a new statement block.
4. Can only be written on one line
5. Without a return statement, the value of the last expression is the return value

A simple example:

In [1]: lambda x: x+1Out[1]: <function __main__.<lambda>># 第一种用法(lambda x: x + 1)(3) # 第一个括号用来改变优先级，第二对括号表示函数调用In [2]: (lambda x: x+1)(3)Out[2]: 4# 第二种用法，把匿名函数赋值给变量f = lambda x: x + 1f(5)In [3]: f = lambda x: x+1In [4]: f(5)Out[4]: 6lambda x, y: x + y# 也可以这样调用(lambda x, y: x + y)(3, 5)  # 第一个括号是用来改变优先级的，第二个括号是调用函数的add = lambda x, y: x + yadd(3, 5)

You can also set default parameters:

lambda x, y=1: x + yadd = lambda x, y=1: x + yadd(3)# 调用的时候也可以用关键字参数add(x=5)lambda x, y=1: x + y<function <lambda> at 0x1019e10d0>add = lambda x, y=1: x + yadd(3)4# 调用的时候也可以用关键字参数add(x=5)6

Can I have variable parameters?

In [5]: f = lambda *x: xIn [6]: f(1, 2, 3)Out[6]: (1, 2, 3)In [7]: lst = [1, 2, 3]In [8]: f(*lst)Out[8]: (1, 2, 3)

Are the keyword parameters OK?

In [9]: f = lambda **kw: kwIn [10]: f(a=0)Out[10]: {‘a‘: 0}In [11]: dict01 = {‘name‘: ‘lavenliu.cn‘, ‘age‘: 23}In [12]: f(**dict01)Out[12]: {‘age‘: 23, ‘name‘: ‘lavenliu.cn‘}

Variable parameters are used in conjunction with variable keyword parameters?

In [13]: (lambda *args, **kwargs: print(args, kwargs))(*range(3), **{str(x): x for x in range(3)})(0, 1, 2) {‘0‘: 0, ‘1‘: 1, ‘2‘: 2}In [14]: (lambda *args, **kwargs: print(args, kwargs))(*range(3), **{str(x): x for x in range(3)})(0, 1, 2) {‘0‘: 0, ‘1‘: 1, ‘2‘: 2}

Can I have a parameter slot?

In [15]: f = lambda x, *, y: x+yIn [16]: f(1, y=3)Out[16]: 4

The parameter changes supported by the normal function are supported by the anonymous function.

One more example:

In [17]: concat = lambda *args: ‘‘.join(args)In [18]: concat(‘a‘, ‘b‘, ‘c‘)Out[18]: ‘abc‘In [19]: lower = (lambda x, y: x if x < y else y)In [20]: lower(7, 3)Out[20]: 3In [21]: lower(‘a‘, ‘b‘)Out[21]: ‘a‘In [22]: lower(2.3, 4.5)Out[22]: 2.3

Anonymous functions are usually used in conjunction with higher-order functions, passed in as arguments, or returned as return values.

Anonymous functions are best not defined as recursive functions.

In [29]: fib = lambda n: 1 if n == 0 or n == 1 else fib(n-1) + fib(n-2)In [30]: fib(5)Out[30]: 8

An example:

from collections import namedtupleUser = namedtuple(‘User‘, [‘name‘, ‘age‘])users = [User(‘lavenliu‘, 23), User(‘taoqi‘, 20)]def get_age(user):    return user.agesorted(users, key=get_age)[User(name=‘taoqi‘, age=20), User(name=‘lavenliu‘, age=23)]# 或者使用匿名函数sorted(users, key=lambda x: x.age)[User(name=‘taoqi‘, age=20), User(name=‘lavenliu‘, age=23)]list(map(lambda x: x.age, users))list(filter(lambda x: x.age < 30, users))

Implement a map function yourself,

In [31]: def map_(fn, it):    ...:     return [fn(x) for x in it]    ...: In [32]: def square(n):    ...:     return n ** 2    ...: In [33]: lst1 = [1, 2, 3, 4, 5]# 使用自定义的map函数In [34]: map_(square, lst1)Out[34]: [1, 4, 9, 16, 25]

Implement a filter function yourself,

def filter_(fn, it):    return [x for x in it if fn(x)]

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anonymous function of Python