Python3中二维列表换行打印出现[None]的问题解决

问题描述 #

作为编程新手，我想实现二维列表的换行打印，目前已经能打出换行效果，但输出末尾会出现一个由多个None组成的列表。以下是我的代码和当前输出：

原代码 #

# All Toppings
toppings = ["cheese", "olives", "pepperoni", "mushrooms", "sausage", "pineapple", "anchovies"]

# The Prices
prices = ["$2", "$6", "$1", "$3", "$2", "$7", "$2"]
prices.sort()

# Number of Toppings
num_pizzas = len(toppings)

# Combing Pizza Toppings and Prices
pizza_and_prices = list(map(list,zip(prices,toppings)))

#After the Guy Bought the Last Anchovies Slice
pizza_and_prices.pop()
pizza_and_prices.append(["$2.5", "peppers"])

#ACTUAL MENU
print("***TOOZIE'S PIZZARIA***")
print("We sell " + str(num_pizzas) + " different kinds of Pizza!")
print(list(map(print, pizza_and_prices)))

当前输出 #

***TOOZIE'S PIZZARIA***
We sell 7 different kinds of Pizza!
['$1', 'cheese']
['$2', 'olives']
['$2', 'pepperoni']
['$2', 'mushrooms']
['$3', 'sausage']
['$6', 'pineapple']
['$2.5', 'peppers']
[None, None, None, None, None, None, None]

问题原因 #

问题出在最后一行代码print(list(map(print, pizza_and_prices)))：

• map(print, pizza_and_prices)会遍历pizza_and_prices的每个元素，对每个元素调用print()函数，这一步已经完成了换行打印的操作；
• 但print()函数本身没有返回值，默认返回None，所以map会把每个print()调用后的None收集成一个迭代器；
• 接着list()把这个迭代器转换成包含多个None的列表，最后外层的print()又把这个列表打印出来，就出现了末尾的[None, None, ...]。

解决方法 #

只需要去掉外层的print(list(...))，换成直接遍历打印每个元素即可，有两种常用方式：

方法一：for循环遍历（最直观） #

把最后一行代码替换成：

for item in pizza_and_prices:
    print(item)

方法二：遍历map对象（不生成列表） #

如果想保留map的用法，直接遍历map对象即可，不需要转成列表再打印：

for _ in map(print, pizza_and_prices):
    pass

修改后的完整代码示例 #

# All Toppings
toppings = ["cheese", "olives", "pepperoni", "mushrooms", "sausage", "pineapple", "anchovies"]

# The Prices
prices = ["$2", "$6", "$1", "$3", "$2", "$7", "$2"]
prices.sort()

# Number of Toppings
num_pizzas = len(toppings)

# Combing Pizza Toppings and Prices
pizza_and_prices = list(map(list,zip(prices,toppings)))

#After the Guy Bought the Last Anchovies Slice
pizza_and_prices.pop()
pizza_and_prices.append(["$2.5", "peppers"])

#ACTUAL MENU
print("***TOOZIE'S PIZZARIA***")
print("We sell " + str(num_pizzas) + " different kinds of Pizza!")
# 替换成for循环打印
for item in pizza_and_prices:
    print(item)

修改后输出 #

***TOOZIE'S PIZZARIA***
We sell 7 different kinds of Pizza!
['$1', 'cheese']
['$2', 'olives']
['$2', 'pepperoni']
['$2', 'mushrooms']
['$3', 'sausage']
['$6', 'pineapple']
['$2.5', 'peppers']

内容的提问来源于stack exchange，提问作者Toozie