Working on a python assignment and was curious as to what [:-1] means in the context of the following code: instructions = f.readline()[:-1]
Have searched on here on S.O. and on Google but to no avail. Would love an explanation!
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Sometimes -1 is used to express the end of an array of things. My guess is this means to read from beginning to the end of the line (but just a guess, hence not an official answer).Daedalus– Daedalus2013-03-20 21:37:27 +00:00Commented Mar 20, 2013 at 21:37
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I'm not sure it's a dup, because of that "in the context of…" part—which is the part that you and, especially, Pavel Anossov answered.abarnert– abarnert2013-03-20 21:40:38 +00:00Commented Mar 20, 2013 at 21:40
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4 Answers
It slices the string to omit the last character, in this case a newline character:
>>> 'test\n'[:-1]
'test'
Since this works even on empty strings, it's a pretty safe way of removing that last character, if present:
>>> ''[:-1]
''
This works on any sequence, not just strings.
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.
answered Mar 20, 2013 at 21:37
Martijn Pieters
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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".
2 Comments
abarnert
Although Martijn Pieters's answer is a bit more detailed and explanatory, I like that this one directly describes the idiom of using
[:-1] with readline, answering the OP's "in the context of…" part. (So, +1 to both of you.)
DSM
It might be worth noting that making this assumption can be a dangerous thing to do, even on a system where you expect the end-of-line marker to be one character long-- it's not uncommon to wind up with a missing newline on the last line of a file. This is particularly true when Python is involved: I've seen
'\n'.join(lines) used before, which will result in exactly this happening, and then you lose a data character..It selects all but the last element of a sequence.
Example below using a list:
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]
1 Comment
Gareth Latty
It slices the sequence - it just might happen to be a list.
It gets all the elements from the list (or characters from a string) but the last element.
: represents going through the list
-1 implies the last element of the list
1 Comment
abarnert
This is written ambiguously enough to be confusing. It's all of the elements up to but not including the last element. (If you already know how slices/ranges/etc. work in Python, then knowing that -1 is the last element tells you everything you need… but if you don't, you still won't after this answer.)