英文:
How to use print function with for in an interactive session
问题
以下是翻译好的部分：
简单问题关于打印函数（Python 3.9.13）是什么导致了这些行为（链接到一些信息页面将不胜感激）。我有一个字符串列表，我想在交互式会话中将它们每个打印在单独的行上。
问题：您能解释这些行为，特别是是什么导致了那些"None"出现以及如何避免它？
英文:
Simple question about print function (python 3.9.13) what causes these (a link to some informative page will be appreciated). I have a list of strings and I want to print them each on separate line in an interactive session.
>>> aa = ['word 1','test 2', 'blah 3', 'ding 4']
>>> [print(x) for x in aa]
word 1
test 2
blah 3
ding 4
[None, None, None, None]
>>> (print(x) for x in aa)
<generator object <genexpr> at 0x000001D8AC975DD0>
>>> print(x) for x in aa
SyntaxError: invalid syntax
>>> {print(x) for x in aa}
word 1
test 2
blah 3
ding 4
{None}
>>>
Question: could you explain these behaviours, especially what causes those None to appear and how to avoid it?
答案1
得分: 3
None 出现在这些地方是因为 print 返回 None（我可以尝试解释这个问题，但是StackOverflow上的其他人已经更好地解释了，比如这个答案。
>>> [print(x) for x in aa]
因为 print("foo") 的输出是 None，所以 [print(x) for x in aa] 的输出将是 [None, None, None None]（每个 aa 元素对应一个 None）。
>>> (print(x) for x in aa)
这创建了一个 生成器对象，稍后可以对其进行迭代：
o = (print(x) for x in aa)
for i in o:
i
# 输出是
1
2
3
4
5
>>> {print(x) for x in aa}
这与示例＃1类似，但不是创建一个列表，而是创建一个集合（基本上是一个只包含唯一值的无序列表）。由于列表 [None, None, None None] 中只有一个唯一值，整个集合被简化为{None}。
如何避免
避免这个问题的最简单方法是在类似于列表推导式的情况下将 print 放在外面。而不是 [print(x) for x in aa]，可以使用 print([x for x in aa])。此外，如果创建函数，请让它们返回值，然后使用 print 调用函数，而不是让函数以打印值的方式结束。
英文:
None appears in these areas because print returns None (I could try to explain this, but others on StackOverflow have explained this better, such as this answer.
>>> [print(x) for x in aa]
Because the output of print("foo") is None, the output of [print(x) for x in aa] is going to be [None, None, None None], (one None for each element of aa.
>>> (print(x) for x in aa)
This creates a generator object, something which can later be iterated over:
o = (print(x) for x in aa)
for i in o:
i
# output is
1
2
3
4
5
>>> {print(x) for x in aa}
This is similar to example #1, except instead of creating a list, it creates a set (basically an unordered list with only unique values). Since there is only one unique value in the list [None, None, None None], the whole thing is reduced down to {None}
How to avoid it
The simplest way of avoiding it is to use print outside of things similar to list comprehensions. Instead of [print(x) for x in aa], do print([x for x in aa]). Also, if you create functions, have them return values, then call the function with print, instead of having the function end by printing a value.
答案2
得分: 2
[print(x) for x in aa], {print(x) for x in aa} 是列表和字典推导式。(print(x) for x in aa) 是生成器推导式。语法可以理解为：
对于可迭代对象 aa 中的每个元素 x，将 (print(x) 的返回值放入列表/集合中。因此，对于每个元素 x，执行 print(x)，打印结果并返回 None。你看到的是 print 导致每个元素被打印，但也看到结果列表，其中只包含 None。
请参考 https://docs.python.org/3.9/whatsnew/2.0.html?highlight=comprehension#list-comprehensions 了解列表推导式。
你可以这样写：
for x in aa: print(x)
将其放在一行上，然后按两次回车。
英文:
[print(x) for x in aa], {print(x) for x in aa} are list and dictionary comprehensions. (print(x) for x in aa) is a generator comprehension. The syntax can be read as:
For each element x in the iterable aa, emplace the return value of (print(x) into a list/set. So for each element x, print(x) is executed, prints the result and returns None. You are seeing that print resulted in each element being printed, but also see the resulting list, that only contains None.
See https://docs.python.org/3.9/whatsnew/2.0.html?highlight=comprehension#list-comprehensions for list comprehensions
What you can do is to write
for x in aa: print(x)
on a single line and then hit enter twice
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