遍歷字典鍵值

1. 字典的索引並不是序列數字，不能依序列出，要遍歷 (traverse) 字典的鍵及值可用下列 for 迴圈取出所有鍵值配對：

   for key,value in dict_name.items():
   

2. 範例：遍歷字典 uid，並列出其值及鍵。

   pyuser@deyu:~/zzz$ cat dict6.py
   #!/usr/bin/python
   
   uid = { 'dywang':1000, 'david':2346, 'linda':1324 }
   for key,value in uid.items():
   	print("uid = %d" % value)
   	print("key = %s" % key)
   

3. 執行結果：for 迴圈逐一輸出字典所有元素。

   pyuser@deyu:~/zzz$ ./dict6.py 
   uid = 1000
   key = dywang
   uid = 2346
   key = david
   uid = 1324
   key = linda
   

4. 修改 dict6.py 為 dict6a.py，增加 for 迴圈中判斸 key 是 david 時，刪除該元素。

   pyuser@deyu:~/zzz$ cat ./dict6a.py
   #!/usr/bin/python
   
   uid = { 'dywang':1000, 'david':2346, 'linda':1324 }
   for key,value in uid.items():
       print("uid = %d" % value)
       print("key = %s" % key)
       if key == 'david':
           del uid[key]
   

5. 執行結果：出現錯誤，python 無法在for 迴圈改變字典大小。

   pyuser@deyu:~/zzz$ ./dict6a.py
   uid = 1000
   key = dywang
   uid = 2346
   key = david
   Traceback (most recent call last):
     File "./dict6a.py", line 5, in <module>
       for key,value in uid.items():
   RuntimeError: dictionary changed size during iteration
   

6. 為解決上述問題，uid.items()必須轉換成 list。

   pyuser@deyu:~/zzz$ vi dict6b.py
   pyuser@deyu:~/zzz$ cat dict6b.py
   #!/usr/bin/python
   
   uid = { 'dywang':1000, 'david':2346, 'linda':1324 }
   for key,value in list(uid.items()):
       print("uid = %d" % value)
       print("key = %s" % key)
       if key == 'david':
           del uid[key]
   print(uid)
   

7. 執行結果：uid.items() 轉成 list 後，迴圈中成功刪除 devid。

   pyuser@deyu:~/zzz$ ./dict6b.py
   uid = 1000
   key = dywang
   uid = 2346
   key = david
   uid = 1324
   key = linda
   {'dywang': 1000, 'linda': 1324}