返回值

函数并非总是直接显示输出，相反，它可以处理一些数据，并返回一个或一组值。函数的返回值被称为返回值。

1. 简单的返回值

def get_formatted_name(first_name, last_name):
    full_name = first_name + ' ' + last_name
    return full_name.title()


musician = get_formatted_name('jimi', 'hendrix')
print(musician)

调用返回值的函数时，需要提供一个变量存储返回的值。

2. 让实参变成可选的

def get_formatted_name(first_name, middle_name, last_name):
    full_name = first_name + ' ' + middle_name + ' ' + last_name
    return full_name.title()


musician = get_formatted_name('john', 'lee', 'hooker')
print(musician)

然而并非每个人都有中间名，怎样让中间名变成可选呢？

def get_formatted_name(first_name, last_name, middle_name=' '):
    if middle_name:
        full_name = first_name + ' ' + middle_name + ' ' + last_name
    else:
        full_name = first_name + ' ' + last_name
    return full_name.title()


musician = get_formatted_name('john', 'hooker', 'lee')
print(musician)
musician = get_formatted_name('jimi', 'hendrix')
print(musician)

给形参中间名一个空字符为默认值，将其移动至形参列表的末尾；调用函数时确保实参中间名方最后。

3. 返回字典

def build_person(first_name, last_name):
    person = {'first': first_name, 'last': last_name}
    return person


musician = build_person('jimi', 'hendrix')
print(musician)

扩展函数，使其接受可选值

def build_person(first_name, last_name, age=' '):
    person = {'first': first_name, 'last': last_name}
    if age:
        person['age'] = age
    return person


musician = build_person('jimi', 'hendrix', age=27)
print(musician)

4. 结合使用函数和while循环

def get_formatted_name(first_name, last_name):
    full_name = first_name + ' ' + last_name
    return full_name.title()


while True:
    print("\nPlease tell me your name:")
    f_name = input("First name: ")
    l_name = input("Last name: ")

    formatted_name = get_formatted_name(f_name, l_name)
    print("\nHello, " + formatted_name + "!")

无限循环调用定义的函数，say hello everyone!!! 该在什么地方提供推出呢？

def get_formatted_name(first_name, last_name):
    full_name = first_name + ' ' + last_name
    return full_name.title()


while True:
    print("\nPlease tell me your name:")
    print("(enter 'q' at any time to quit)")
    f_name = input("First name: ")
    if f_name == 'q':
        break
    l_name = input("Last name: ")
    if l_name == 'q':
        break

    formatted_name = get_formatted_name(f_name, l_name)
    print("\nHello, " + formatted_name + "!")

每次提示用户输入时均可推出。

传递列表

def greet_users(names):
    for name in names:
        mag = "Hello, " + name.title() + "!"
        print(mag)


user_names = ['hannah', 'bob', 'margot']
greet_users(user_names)

运行结果：

Hello, Hannah!
Hello, Bob!
Hello, Margot!

1. 在函数中修改列表

# 创建一个列表，其中包含一些要打印的设计
unprinted_designs = ['iphone case', 'robot pendant', 'dodecahedron']
completed_models = []

# 模拟打印每个设计，直到没有未打印的设计为止，打印后移至completed_models中
while unprinted_designs:
    current_design = unprinted_designs.pop()

    # 模拟根据设计制作打印模型的过程
    print("Printing model: " + current_design)
    completed_models.append(current_design)

# 显示打印好的模型
print("\nThe following models have been printed:")
print(completed_models)

运行结果：

Printing model: dodecahedron
Printing model: robot pendant
Printing model: iphone case

The following models have been printed:
['dodecahedron', 'robot pendant', 'iphone case']

 用函数如何表达上述代码的意思呢？

def print_models(unprinted_designs, completed_models):
    while unprinted_designs:
        current_design = unprinted_designs.pop()
        print("Printing model: " + current_design)
        completed_models.append(current_design)


def show_completed_models(completed_models):
    print("\nThe following models have been printed:")
    for completed_model in completed_models:
        print(completed_model)


unprinted_designs = ['iphone case', 'robot pendant', 'dodecahedron']
completed_models = []

print_models(unprinted_designs, completed_models)
show_completed_models(completed_models)

        当print_models函数调用之后，列表completed_models已经不是最初定义的空，所有列表unprinted_designs中的元素已转移至列表completed_models，接下来调用show_completed_models函数就将列表completed_models中的元素都打印出来。

2. 禁止函数修改列表

上述的例子中print_models函数调用之后，列表unprinted_designs中的元素均已移除，此时的列表为空。但若想保留列表中的元素呢？

print_models(unprinted_designs[:], completed_models)

 用切片法 [ : ] 创建列表副本，函数调用时使用的是列表的副本，而不是列表本身，此时函数中对列表做的修改不会影响到列表unprinted_designs。