有 return 语句而不返回任何值的原因

Question

我知道关于这个话题有很多问题（有不好的反馈）。然而下面的代码来自于Fundamentals of Python这本书，这让我觉得作者使用return而不返回任何东西

一定是有一些原因的

def save(self, fileName = None):
        """Saves pickled accounts to a file. The parameter
        allows the user to change filenames."""
        if fileName != None:
            self.fileName = fileName
        elif self.fileName == None:
            return # WHAT ARE THE REASONS FOR HAVING THIS LINE? CAN WE DELETE/REMOVE IT?
        fileObj = open(self.fileName, "wb")
        for account in self.accounts.values():
            pickle.dump(account, fileObj)
        fileObj.close()

完整代码如下

import pickle

class SavingsAccount(object):
    RATE = 0.02 # Single rate for all accounts
    
    def __init__(self, name, pin, balance = 0.0):
        self.name = name
        self.pin = pin
        self.balance = balance
    
    def __str__(self) :
        """Returns the string rep."""
        result = 'Name: ' + self.name + '\n'
        result += 'PIN: ' + self.pin + '\n'
        result += 'Balance: ' + str(self.balance)
        return result
    
    def getBalance(self):
        """Returns the current balance."""
        return self.balance
    
    def getName(self):
        """Returns the current name."""
        return self.name
    
    def getPin(self):
        """Returns the current pin."""
        return self.pin
    
    def deposit(self, amount):
        """Deposits the given amount and returns None."""
        self.balance += amount
        return None
    
    def withdraw(self, amount):
        """Withdraws the given amount.
        Returns None if successful, or an
        error message if unsuccessful."""
        if amount < 0:
            return "Amount must be >= 0"
        elif self.balance < amount:
            return "Insufficient funds"
        else:
            self.balance -= amount
            return None
    
    def computeInterest(self):
        """Computes, deposits, and returns the interest."""
        interest = self.balance * SavingsAccount.RATE
        self.deposit(interest)
        return interest
    

class Bank(object):
    def __init__(self, fileName):
        self.accounts = {}
        self.fileName = fileName
        
    def __str__(self) :
        """Return the string rep of the entire bank."""
        return '\n'.join(map(str, self.accounts.values()))

    def makeKey(self, name, pin):
        """Makes and returns a key from name and pin."""
        return name + "/" + pin

    def add(self, account):
        """Inserts an account with name and pin as a key."""
        key = self.makeKey(account.getName(),  account.getPin())
        self.accounts[key] = account
        
    def remove(self, name, pin):
        """Removes an account with name and pin as a key."""
        key = self.makeKey(name, pin)
        return self.accounts.pop(key, None)
    
    def get(self, name, pin):
        """Returns an account with name and pin as a key
        or None if not found."""
        key = self.makeKey(name, pin)
        return self.accounts.get(key, None)
    
    def computeInterest(self):
        """Computes interest for each account and
        returns the total."""
        total = 0.0
        for account in self.accounts.values():
            total += account.computeInterest()
        return total
    
    def save(self, fileName = None):
        """Saves pickled accounts to a file. The parameter
        allows the user to change filenames."""
        if fileName != None:
            self.fileName = fileName
        elif self.fileName == None:
            return
        fileObj = open(self.fileName, "wb")
        for account in self.accounts.values():
            pickle.dump(account, fileObj)
        fileObj.close()

如何修改

def save(self, fileName=None)

，使程序会要求用户提供文件名，即：

bank = Bank(None)
bank.add(SavingsAccount("Wilma", "1001", 4000.00))
bank.add(SavingsAccount("Fred", "1002", 1000.00))
bank.save() # a prompt would appear: 'Please provide a name for the file to be saved'

Answer 1

请注意，

return

 后面不跟任何值与写 

return None

 相同。在您显示的代码中，

return

语句用于在未传递文件名的情况下终止函数，而不是尝试执行

if

和

elif

后面的代码。