为什么 map(print, a_list) 不起作用? [英] Why map(print, a_list) doesn't work?

问题描述

对于普通函数，map 效果很好:

For a normal function, map works well:

def increment(n):
    return n+1
l = [1, 2, 3, 4, 5]
l = map(increment, l)
print l
>>> [2, 3, 4, 5, 6]

但是，如果将 print 放在 map 函数中:

However, if it's print being put inside the map function:

l = [1, 2, 3, 4, 5]
l = map(print, l)
print l

python 会抱怨:

python will complain:

l = map(print, l)
            ^
SyntaxError: invalid syntax

是什么让 print 与众不同?print(x) 难道不是一个有效的函数调用吗?以上代码在python 2.7下测试.

What makes print special? Doesn't print(x) also a valid function call? The above code are tested under python 2.7.

推荐答案

在 Python 2.x 中，print 是一个语句，而不是一个函数.如果您在 Python 3.x 中尝试此操作，它将起作用.

In Python 2.x, print is a statement, not a function. If you try this in Python 3.x it will work.

在 Python 2.x 中，您可以说 print(x)，这不是语法错误，但实际上不是函数调用.正如 1 + (3) 与 1 + 3 相同，print(x) 与 print x<相同/code> 在 Python 2.x 中.

In Python 2.x, you can say print(x) and it is not a syntax error, but it isn't actually a function call. Just as 1 + (3) is the same as 1 + 3, print(x) is the same as print x in Python 2.x.

在 Python 2.x 中，您可以这样做:

In Python 2.x you can do this:

def prn(x):
    print x

然后你可以这样做:

map(prn, lst)

它会起作用.请注意，您可能不想做 lst = map(prn, lst) 因为 prn() 返回 None，因此您将替换您的值列表具有相同长度的值列表 None.

and it will work. Note that you probably don't want to do lst = map(prn, lst) because prn() returns None, so you will replace your list of values with a same-length list of just the value None.

Python 2.x 的另外两个解决方案

Two other solutions for Python 2.x.

如果你想彻底改变print的行为，你可以这样做:

If you want to completely change the behavior of print, you can do this:

from __future__ import print_function

map(print, lst)

这使得 print 成为一个函数，就像在 Python 3.x 中一样，因此它可以与 map() 一起使用.

This makes print into a function just as it is in Python 3.x, so it works with map().

或者，您可以这样做:

from pprint import pprint

map(pprint, lst)

pprint() 是一个打印东西的函数，它可以作为内置函数使用.我不确定它与默认的 print 有什么不同(它说这是一个漂亮的打印"功能，但我不确定它到底有什么不同).

pprint() is a function that prints things and it is available as a built-in. I'm not exactly sure how it is different from the default print (it says it is a "pretty-print" function but I'm not sure how exactly that makes it different).

此外，根据 PEP 8 标准，不建议使用 l 作为变量名，因此我在示例中使用 lst 代替.

Also, according to the PEP 8 standard, it is non-recommended to use l as a variable name, so I am using lst instead in my examples.

http://www.python.org/dev/peps/pep-0008/

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