What does [:-1] mean/do in python?
PythonPython Problem Overview
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!
Python Solutions
Solution 1 - Python
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.
Solution 2 - Python
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".
Solution 3 - Python
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]
Solution 4 - Python
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