[:-1] 在 python 中是什么意思/做什么？
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What does [:-1] mean/do in python?
提问by Matt.
Working on a python assignment and was curious as to what [:-1] means in the context of the following code: instructions = f.readline()[:-1]
正在处理 Python 任务，并且很好奇 [:-1] 在以下代码的上下文中的含义： instructions = f.readline()[:-1]
Have searched on here on S.O. and on Google but to no avail. Would love an explanation!
在 SO 和 Google 上搜索过这里，但无济于事。想要一个解释！
采纳答案by Martijn Pieters
It slices the string to omit the last character, in this case a newline character:
它对字符串进行切片以省略最后一个字符，在本例中为换行符：
>>> 'test\n'[:-1]
'test'
Since this works even on empty strings, it's a pretty safe way of removing that last character, if present:
由于这甚至适用于空字符串，因此删除最后一个字符（如果存在）是一种非常安全的方法：
>>> ''[:-1]
''
This works on any sequence, not just strings.
这适用于任何序列，而不仅仅是字符串。
For lines in a text file, I'd actually use line.rstrip('\n')to only remove a newline; sometimes the last line in the file doesn't end in a newline character and using slicing then removes whatever other character is last on that line.
在文本文件中的行，我实际使用line.rstrip('\n')来只删除一个换行符; 有时文件中的最后一行不会以换行符结尾，然后使用切片删除该行最后一行的任何其他字符。
回答by Kartik
It gets all the elements from the list (or characters from a string) but the last element.
它获取列表中的所有元素（或字符串中的字符），但最后一个元素。
:represents going through the list
-1implies the last element of the list
:表示遍历列表-1意味着列表
的最后一个元素
回答by Fredrik Pihl
It selects all but the last element of a sequence.
它选择序列中除最后一个元素之外的所有元素。
Example below using a list:
下面使用列表的示例：
In [15]: a=range(10)
In [16]: a
Out[16]: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
In [17]: a[:-1]
Out[17]: [0, 1, 2, 3, 4, 5, 6, 7, 8]
回答by Pavel Anossov
It means "all elements of the sequence but the last". In the context of f.readline()[:-1]it means "I'm pretty sure that line ends with a newline and I want to strip it".
它的意思是“序列的所有元素，但最后一个”。在f.readline()[:-1]它的上下文中意味着“我很确定该行以换行符结尾，我想删除它”。
