python lambda function

lambda用于创建匿名函数, 不需要函数名, 只需要指定参数名. 返回值.

匿名函数的用法举例

1. 返回匿名函数, 赋予给变量

例如没有参数的用法

>>> def make_incrementor(n):

...   return lambda : 1 + n 

... 

>>> f = make_incrementor(1)

>>> f()

2

2. 用于lists的sort方法.

参考

https://docs.python.org/3/library/stdtypes.html#lists

4.7.5. Lambda Expressions

Small anonymous functions can be created with the lambda keyword. This function returns the sum of its two arguments: lambda a, b: a+b. Lambda functions can be used wherever function objects are required. They are syntactically restricted to a single expression. Semantically, they are just syntactic sugar for a normal function definition. Like nested function definitions, lambda functions can reference variables from the containing scope:

>>>

>>> def make_incrementor(n):
...     return lambda x: x + n
...
>>> f = make_incrementor(42)
>>> f(0)
42
>>> f(1)
43

The above example uses a lambda expression to return a function. Another use is to pass a small function as an argument:

>>>

>>> pairs = [(1, 'one'), (2, 'two'), (3, 'three'), (4, 'four')]
>>> pairs.sort(key=lambda pair: pair[1])
>>> pairs
[(4, 'four'), (1, 'one'), (3, 'three'), (2, 'two')]