Python 基礎學習 partial() 用法

partial :

你可以使用partial函數(其實是個class)從原函數中派生出固定某些參數的新函數,使函數所需的參數減少(因為相當于設定好了某些參數的值),進而簡化代碼.

partial的定義代碼片段:

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() 可以給輸入函數指定預設參數，簡化之後的函數參數量
• partial() 既可以設定args 參數，又可以設定關鍵詞參數

  **kwargs

  .當設定args參數時，輸入的數值将會按照待處理函數中args參數的順序依次指定．如

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

  中４作為

  multimath

  中第一個參數ａ來被指定，這樣轉化後的

  multimath

  預設首先輸入了參數４，然後才輸入第二個參數３(也就是ｂ的值)