python 定义函数时加个->是个什么意思，有几个用

idi0t

如题, 像这样 -> 是什么操作符，哪有说明
def func(arg) - > string:
    return arg + "what?"

reyleon

你在哪儿看到Python有这样的写法?

idi0t

回复 2# reyleon


    别人python3代码里，搜了一天了，刚提问就在官网翻到了，不过初看不理解做啥的，还有参数后加个:和类型，能解释下有什么意义和用处吗？
4.7.7. Function Annotations

Function annotations are completely optional metadata information about the types used by user-defined functions (see PEP 484 for more information).

Annotations are stored in the __annotations__ attribute of the function as a dictionary and have no effect on any other part of the function. Parameter annotations are defined by a colon after the parameter name, followed by an expression evaluating to the value of the annotation. Return annotations are defined by a literal ->, followed by an expression, between the parameter list and the colon denoting the end of the def statement. The following example has a positional argument, a keyword argument, and the return value annotated:
>>>

>>> def f(ham: str, eggs: str = 'eggs') -> str:
...     print("Annotations:", f.__annotations__)
...     print("Arguments:", ham, eggs)
...     return ham + ' and ' + eggs
...
>>> f('spam')
Annotations: {'ham': <class 'str'>, 'return': <class 'str'>, 'eggs': <class 'str'>}
Arguments: spam eggs
'spam and eggs'


   

idi0t

回复 2# reyleon
看样子就是蛋疼的注释，无语....


   

reyleon

涨姿势了, Python3威武..

MMMIX

idi0t 发表于 2016-06-16 15:26
回复 2# reyleon

能解释下有什么意义和用处吗？

就和 C 里面的函数声明是类似的作用，可以用来检查函数调用的时候参数类型是否正确之类的。只不过，在 Python3 这里，这些信息是可选的，而且其他工具，例如调试器，也能用；至于具体怎么用，就看工具的作者了，没有什么一定之规。