Grab a line's whitespace/indention with Python

Question

Basically, if I have a line of text which starts with indention, what's the best way to grab that indention and put it into a variable in Python? For example, if the line is:

\t\tthis line has two tabs of indention

Then it would return '\t\t'. Or, if the line was:

    this line has four spaces of indention

Then it would return four spaces.

So I guess you could say that I just need to strip everything from a string from first non-whitespace character to the end. Thoughts?

Answer 1

A sneaky way: abuse lstrip!

fullstr = "\t\tthis line has two tabs of indentation"
startwhites = fullstr[:len(fullstr)-len(fullstr.lstrip())]

This way you don't have to work through all the details of whitespace!

(Thanks Adam for the correction)

Answer 2

import re
s = "\t\tthis line has two tabs of indention"
re.match(r"\s*", s).group()
// "\t\t"
s = "    this line has four spaces of indention"
re.match(r"\s*", s).group()
// "    "

And to strip leading spaces, use lstrip.

---

As there are down votes probably questioning the efficiency of regex, I've done some profiling to check the efficiency of each cases.

Very long string, very short leading space

RegEx > Itertools >> lstrip

>>> timeit.timeit('r.match(s).group()', 'import re;r=re.compile(r"\s*")s="          hello world!"*10000', number=100000)
0.10037684440612793
>>> timeit.timeit('"".join(itertools.takewhile(lambda x:x.isspace(),s))', 'import itertools;s="          hello world!"*10000', number=100000)
0.7092740535736084
>>> timeit.timeit('"".join(itertools.takewhile(str.isspace,s))', 'import itertools;s="          hello world!"*10000', number=100000)
0.51730513572692871
>>> timeit.timeit('s[:-len(s.lstrip())]', 's="          hello world!"*10000', number=100000)
2.6478431224822998

Very short string, very short leading space

lstrip > RegEx > Itertools

If you can limit the string's length to thousounds of chars or less, the lstrip trick maybe better.

>>> timeit.timeit('r.match(s).group()', 'import re;r=re.compile(r"\s*");s="          hello world!"*100', number=100000)
0.099548101425170898
>>> timeit.timeit('"".join(itertools.takewhile(str.isspace,s))', 'import itertools;s="          hello world!"*100', number=100000)
0.53602385520935059
>>> timeit.timeit('s[:-len(s.lstrip())]', 's="          hello world!"*100', number=100000)
0.064291000366210938

This shows the lstrip trick scales roughly as O(√n) and the RegEx and itertool methods are O(1) if the number of leading spaces is not a lot.

Very short string, very long leading space

lstrip >> RegEx >>> Itertools

If there are a lot of leading spaces, don't use RegEx.

>>> timeit.timeit('s[:-len(s.lstrip())]', 's=" "*2000', number=10000)
0.047424077987670898
>>> timeit.timeit('r.match(s).group()', 'import re;r=re.compile(r"\s*");s=" "*2000', number=10000)
0.2433168888092041
>>> timeit.timeit('"".join(itertools.takewhile(str.isspace,s))', 'import itertools;s=" "*2000', number=10000)
3.9949162006378174

Very long string, very long leading space

lstrip >>> RegEx >>>>>>>> Itertools

>>> timeit.timeit('s[:-len(s.lstrip())]', 's=" "*200000', number=10000)
4.2374031543731689
>>> timeit.timeit('r.match(s).group()', 'import re;r=re.compile(r"\s*");s=" "*200000', number=10000)
23.877214908599854
>>> timeit.timeit('"".join(itertools.takewhile(str.isspace,s))', 'import itertools;s=" "*200000', number=100)*100
415.72158336639404

This shows all methods scales roughly as O(m) if the non-space part is not a lot.

Answer 3

def whites(a):
return a[0:a.find(a.strip())]

Basically, the my idea is:

1. Find a strip of starting line
2. Find a difference between starting line and stripped one

Answer 4

This can also be done with str.isspace and itertools.takewhile instead of regex.

import itertools

tests=['\t\tthis line has two tabs of indention',
       '    this line has four spaces of indention']

def indention(astr):
    # Using itertools.takewhile is efficient -- the looping stops immediately after the first
    # non-space character.
    return ''.join(itertools.takewhile(str.isspace,astr))

for test_string in tests:
    print(indention(test_string))

Answer 5

How about using the regex \s* which matches any whitespace characters. You only want the whitespace at the beginning of the line so either search with the regex ^\s* or simply match with \s*.

Answer 6

If you're interested in using regular expressions you can use that. /\s/ usually matches one whitespace character, so /^\s+/ would match the whitespace starting a line.