驗證元素是否存在

1. 字典的索引並不是序列數字，不能依序列出，要判斷字典中是否存在某鍵可用下列語法：

   key in dict_name
   

2. 範例：輸入一鍵，判斷字典 uid 是否存在此鍵？不存在則輸入其值並新增。

   [dywang@deyu zzz]$ cat dict5.py 
   #!/usr/bin/python3
   # coding: utf-8
   
   uid = { 'dywang':1000, 'david':2346, 'linda':1324 }
   key = input("Enter a key: ") 
   if key in uid:
       print("%s in uid, uid[%s] = %d" % (key, key, uid[key]))
   else:
       value = int(input("Enter a value: "))
       uid[key] = value
       print("add %s to uid" % key)
       print("uid[%s] = %d" % (key, value))
   

3. 執行結果：輸入 dywang 時，程式判斷此鍵已存在於字典 uid；輸入 rita 則要求輸入 value，並新增此元素於字典中。

   [dywang@dywIssd zzz]$ ./dict5.py 
   Enter a key: dywang
   dywang in uid, uid[dywang] = 1000
   [dywang@dywIssd zzz]$ ./dict5.py 
   Enter a key: perter
   Enter a value: 1234
   add perter to uid
   uid[perter] = 1234