Python_快速生成字典_fromkeys()

一、fromkeys(seq,value)

  1、說明：

  指定key值和value值，快速生成字典

  seq -- 字典鍵值列表。
  value -- 可選參數, 設置鍵序列（seq）的值

>>> sd = {}
>>> d = ["q","w","e"]
>>> sd.fromkeys(d,1)
{'q': 1, 'w': 1, 'e': 1}

 2、細節

  (1)注意：value值為序列的效果

>>> sd = {}
>>> d = ["q","w","e"]
>>> v = [1,2,3]
>>> sd.fromkeys(d,v)#value值為序列的效果
{'q': [1, 2, 3], 'w': [1, 2, 3], 'e': [1, 2, 3]}

 (2)多次賦值時，不會追加，會覆蓋

>>> d = {"s":6}
>>> s = ["q","w","e"]
>>> d = d.fromkeys(s,1)#多次賦值操作
>>> print(d)#會將原來的值覆蓋
{'q': 1, 'w': 1, 'e': 1}

 

  3、實例： 

    一個函數傳入一個List列表，生成一個字典：所有元素作為字典的key

   正確寫入：

>>> def func(L):
...     d_result = {}
...     for i in l:
...         d_result = d_result.fromkeys(L,1)#要有變量賦值，否則結果會為空
...     return d_result
...
>>> s = ["q","w","e"]
>>> func(s)
{'q': 1, 'w': 1, 'e': 1}

 *注意：錯誤寫入

>>> def func(l):
...     d_result = {}
...     for i in l:
...         d_result.fromkeys(i,"1")#沒有進行賦值
...     return d_result
...
>>> s = ["q","w","e"]
>>> func(s)#打印的結果為空
{}