Python 基础学习 partial() 用法

partial :

你可以使用partial函数(其实是个class)从原函数中派生出固定某些参数的新函数,使函数所需的参数减少(因为相当于设定好了某些参数的值),从而简化代码.

partial的定义代码片段:

class partial:
    """New function with partial application of the given arguments
    and keywords.
    """

    __slots__ = "func", "args", "keywords", "__dict__", "__weakref__"

    def __new__(*args, **keywords):
        if not args:
            raise TypeError("descriptor '__new__' of partial needs an argument")
        if len(args) < 2:
            raise TypeError("type 'partial' takes at least one argument")
        cls, func, *args = args
        if not callable(func):
            raise TypeError("the first argument must be callable")
        args = tuple(args)

        if hasattr(func, "func"):
            args = func.args + args
            tmpkw = func.keywords.copy()
            tmpkw.update(keywords)
            keywords = tmpkw
            del tmpkw
            func = func.func

        self = super(partial, cls).__new__(cls)

        self.func = func
        self.args = args
        self.keywords = keywords
        return self

    def __call__(*args, **keywords):
        if not args:
            raise TypeError("descriptor '__call__' of partial needs an argument")
        self, *args = args
        newkeywords = self.keywords.copy()
        newkeywords.update(keywords)
        return self.func(*self.args, *args, **newkeywords)
        ...
           

例子:

from functools import  partial

def multimath(a,b,mode):
    if mode == "add":
        return a+b
    elif mode =="multi":
        return a*b
    elif mode =="sub":
        return a-b
    elif mode == "divide":
        return a/b


multimath = partial(multimath,mode ="multi")
print(multimath(3,4))

multimath = partial(multimath,4,mode ="divide")
print(multimath(3))
           

outcome:

12
1.3333333333333333
           

思考:

• partial() 可以给输入函数指定默认参数，简化之后的函数参数量
• partial() 既可以设定args 参数，又可以设定关键词参数

  **kwargs

  .当设定args参数时，输入的数值将会按照待处理函数中args参数的顺序依次指定．如

  partial(multimath,4,mode ="divide")

  中４作为

  multimath

  中第一个参数ａ来被指定，这样转化后的

  multimath

  默认首先输入了参数４，然后才输入第二个参数３(也就是ｂ的值)