Python dict fromkeys()用法及代碼示例

有時需要根據給定的鍵生成字典。蠻力地執行它會花費時間，並且會變得更加乏味。因此，fromkeys()僅使用一種方法即可幫助我們輕鬆完成此任務。本文介紹了與此函數相關的工作和其他方麵。

用法：fromkeys(seq, val)

參數：
seq:要轉換為字典的序列。
val:需要分配給生成的鍵的初始值。默認為無。

返回：如果沒有提供值，則字典的鍵映射到“無”，否則映射到字段中提供的值。

代碼1：演示fromkeys()的工作

# Python 3 code to demonstrate  
# working of fromkeys() 
  
# initializing sequence  
seq = { 'a', 'b', 'c', 'd', 'e' } 
  
# using fromkeys() to convert sequence to dict  
# initializing with None 
res_dict = dict.fromkeys(seq) 
  
# Printing created dict 
print ("The newly created dict with None values:" + str(res_dict)) 
  
  
# using fromkeys() to convert sequence to dict  
# initializing with 1 
res_dict2 = dict.fromkeys(seq, 1) 
  
# Printing created dict 
print ("The newly created dict with 1 as value:" + str(res_dict2))

輸出：

The newly created dict with None values:{‘d’:None, ‘a’:None, ‘b’:None, ‘c’:None, ‘e’:None}
The newly created dict with 1 as value:{‘d’:1, ‘a’:1, ‘b’:1, ‘c’:1, ‘e’:1}

fromdict()以可變對象作為值的行為：

fromdict()也可以隨默認對象一起提供。但是在這種情況下，字典會構成一個深層副本，即如果我們在原始列表中附加值，則附加會在鍵的所有值中進行。

預防措施：某些詞典理解技術可用於創建新列表作為鍵值，而不將原始列表指向鍵值。

代碼2：演示可變對象的行為。

# Python 3 code to demonstrate  
# behaviour with mutable objects 
  
# initializing sequence and list 
seq = { 'a', 'b', 'c', 'd', 'e' } 
lis1 = [ 2, 3 ] 
  
# using fromkeys() to convert sequence to dict 
# using conventional method 
res_dict = dict.fromkeys(seq, lis1) 
  
# Printing created dict 
print ("The newly created dict with list values:"
                                    + str(res_dict)) 
  
# appending to lis1 
lis1.append(4) 
  
# Printing dict after appending 
# Notice that append takes place in all values 
print ("The dict with list values after appending:"
                                    + str(res_dict)) 
  
lis1 = [ 2, 3 ] 
print ('\n') 
  
# using fromkeys() to convert sequence to dict 
# using dict. comprehension 
res_dict2 = { key:list(lis1) for key in seq } 
  
# Printing created dict 
print ("The newly created dict with list values:" 
                                  + str(res_dict2)) 
  
# appending to lis1 
lis1.append(4) 
  
# Printing dict after appending 
# Notice that append doesnt take place now. 
print ("The dict with list values after appending (no change):"
                                               + str(res_dict2))

輸出：

The newly created dict with list values:{‘d’:[2, 3], ‘e’:[2, 3], ‘c’:[2, 3], ‘a’:[2, 3], ‘b’:[2, 3]}
The dict with list values after appending:{‘d’:[2, 3, 4], ‘e’:[2, 3, 4], ‘c’:[2, 3, 4], ‘a’:[2, 3, 4], ‘b’:[2, 3, 4]}

The newly created dict with list values:{‘d’:[2, 3], ‘e’:[2, 3], ‘c’:[2, 3], ‘a’:[2, 3], ‘b’:[2, 3]}
The dict with list values after appending (no change):{‘d’:[2, 3], ‘e’:[2, 3], ‘c’:[2, 3], ‘a’:[2, 3], ‘b’:[2, 3]}

相關用法

注：本文由純淨天空篩選整理自manjeet_04大神的英文原創作品 Python Dictionary | fromkeys() method。非經特殊聲明，原始代碼版權歸原作者所有，本譯文未經允許或授權，請勿轉載或複製。