1.2.3　實現(xiàn)2：使用函數(shù)的單個賬戶

在代碼清單1-3對應的版本中，將代碼拆分為單獨的函數(shù)，每個函數(shù)對應一種操作。這里仍然只模擬了一個賬戶。

代碼清單1-3：使用函數(shù)的只包含一個賬戶的銀行模擬程序（文件: Bank2_OneAccount WithFunctions.py）

   # Non-OOP
   # Bank 2
   # Single account
　
   accountName = ‘’
   accountBalance = 0
   accountPassword = ‘’
　
? def newAccount(name, balance, password):
       global accountName, accountBalance, accountPassword
       accountName = name
       accountBalance = balance
       accountPassword = password
　
   def show():
       global accountName, accountBalance, accountPassword
       print(‘       Name’, accountName)
       print(‘       Balance:’, accountBalance)
       print(‘       Password:’, accountPassword)
       print()
　
? def getBalance(password):
       global accountName, accountBalance, accountPassword
       if password != accountPassword:
           print(‘Incorrect password’)
           return None
       return accountBalance
　
? def deposit(amountToDeposit, password):
       global accountName, accountBalance, accountPassword
       if amountToDeposit < 0:
           print(‘You cannot deposit a negative amount!’)
           return None
　
       if password != accountPassword:
           print(‘Incorrect password’)
           return None
　
       accountBalance = accountBalance + amountToDeposit
       return accountBalance
　
? def withdraw(amountToWithdraw, password):
    ? global accountName, accountBalance, accountPassword
       if amountToWithdraw < 0:
           print(‘You cannot withdraw a negative amount’)
           return None
       if password != accountPassword:
           print(‘Incorrect password for this account’)
           return None
　
       if amountToWithdraw > accountBalance:
           print(‘You cannot withdraw more than you have in your account’)
           return None
　
     ? accountBalance = accountBalance - amountToWithdraw
       return accountBalance
　
   newAccount(“Joe”, 100, ‘soup’) # create an account
　
   while True:
       print()
       print(‘Press b to get the balance’)
       print(‘Press d to make a deposit’)
       print(‘Press w to make a withdrawal’)
       print(‘Press s to show the account’)
       print(‘Press q to quit’)
       print()
　
       action = input(‘What do you want to do? ‘)
       action = action.lower() # force lowercase
       action = action[0] # just use first letter
       print()
　
       if action == ‘b’:
           print(‘Get Balance:’)
           userPassword = input(‘Please enter the password: ‘)
           theBalance = getBalance(userPassword)
           if theBalance is not None:
               print(‘Your balance is:’, theBalance)
　
     ? elif action == ‘d’:
           print(‘Deposit:’)
           userDepositAmount = input(‘Please enter amount to deposit: ‘)
           userDepositAmount = int(userDepositAmount)
           userPassword = input(‘Please enter the password: ‘)
　
         ? newBalance = deposit(userDepositAmount, userPassword)
           if newBalance is not None:
               print(‘Your new balance is:’, newBalance)
　
   --- snip calls to appropriate functions ---
　
   print(‘Done’)

在這個版本中，為銀行賬戶的每個操作（創(chuàng)建賬戶（?）、查詢余額（?）、存款（?）和取款（?））分別創(chuàng)建了一個函數(shù)，并重新組織代碼，使主代碼調用不同的函數(shù)。

這樣一來，主程序變得易讀了許多。例如，如果用戶輸入d，表示他們想要存款（?），主代碼現(xiàn)在會調用一個名為deposit()的函數(shù)（?），并傳入存款數(shù)目，以及用戶輸入的賬戶密碼。

但是，如果查看這里的任何函數(shù)（如withdraw()函數(shù)）的定義，就會發(fā)現(xiàn)代碼使用了global語句（?）來訪問（獲取或設置）代表賬戶的變量。在Python中，只有當想要在函數(shù)中修改一個全局變量的值時，才需要使用global語句。但是，這里使用它們，只是為了清楚地表明，這些函數(shù)引用了全局變量（盡管只獲取它們的值）。

作為一般編程原則，函數(shù)絕不應該修改全局變量。函數(shù)只應該使用傳遞給它的數(shù)據(jù)，基于這些數(shù)據(jù)進行計算，然后返回結果（當然，不必返回結果）。這個程序中的withdraw()函數(shù)確實可以正常運行，但違反了這個原則，它不僅修改了全局變量accountBalance的值（?），還訪問了全局變量accountPassword的值。