CODEWITHSUNDEEP
python
grieve
grieve

Reputation: 13508

What is the pythonic way to count the leading spaces in a string?

I know I can count the leading spaces in a string with this:

>>> a = "   foo bar baz qua   \n"
>>> print "Leading spaces", len(a) - len(a.lstrip())
Leading spaces 3
>>>

But is there a more pythonic way?

Upvotes: 81

Views: 60473

Answers (8)

kriss
kriss

Reputation: 24177

Yet another way to do it for the sake of completeness. Probably useless as unlikely faster or shorter than other answers.

import re
a = "   foo bar baz qua   \n"
print(len(re.split("\S", a, 1)[0]))

A good property of that syntax is that it literally gives you the prefix.

Upvotes: 0

zenpoy
zenpoy

Reputation: 20126

Your way is pythonic but incorrect, it will also count other whitespace chars, to count only spaces be explicit a.lstrip(' '). Compare

a = "   \r\t\n\tfoo bar baz qua   \n"
print("Leading spaces", len(a) - len(a.lstrip()))

>>> Leading spaces 7

and

print("Leading spaces", len(a) - len(a.lstrip(' '))

>>> Leading spaces 3

Upvotes: 118

Student
Student

Reputation: 719

You can use a regular expression:

def count_leading_space(s): 
    match = re.search(r"^\s*", s) 
    return 0 if not match else match.end()

In [17]: count_leading_space("    asd fjk gl")                                  
Out[17]: 4

In [18]: count_leading_space(" asd fjk gl")                                     
Out[18]: 1

In [19]: count_leading_space("asd fjk gl")                                      
Out[19]: 0

Upvotes: 2

jedi5218
jedi5218

Reputation: 61

I recently had a similar task of counting indents, because of which I wanted to count tab as four spaces:

def indent(string: str):
    return sum(4 if char is '\t' else 1 for char in string[:-len(string.lstrip())])

Upvotes: 6

mgilson
mgilson

Reputation: 309969

You could use itertools.takewhile

sum( 1 for _ in itertools.takewhile(str.isspace,a) )

And demonstrating that it gives the same result as your code:

>>> import itertools
>>> a = "    leading spaces"
>>> print sum( 1 for _ in itertools.takewhile(str.isspace,a) )
4
>>> print "Leading spaces", len(a) - len(a.lstrip())
Leading spaces 4

I'm not sure whether this code is actually better than your original solution. It has the advantage that it doesn't create more temporary strings, but that's pretty minor (unless the strings are really big). I don't find either version to be immediately clear about that line of code does, so I would definitely wrap it in a nicely named function if you plan on using it more than once (with appropriate comments in either case).

Upvotes: 26

Junuxx
Junuxx

Reputation: 14261

Just for variety, you could theoretically use regex. It's a little shorter, and looks nicer than the double call to len().

>>> import re
>>> a = "   foo bar baz qua   \n"
>>> re.search('\S', a).start() # index of the first non-whitespace char
3

Or alternatively:

>>> re.search('[^ ]', a).start() # index of the first non-space char
3

But I don't recommend this; according to a quick test I did, it's much less efficient than len(a)-len(lstrip(a)).

Upvotes: 18

ecatmur
ecatmur

Reputation: 157374

Using next and enumerate:

next((i for i, c in enumerate(a) if c != ' '), len(a))

For any whitespace:

next((i for i, c in enumerate(a) if not c.isspace()), len(a))

Upvotes: 4

Matt Luongo
Matt Luongo

Reputation: 14829

That looks... great to me. Usually I answer "Is X Pythonic?" questions with some functional magic, but I don't feel that approach is appropriate for string manipulation.

If there were a built-in to only return the leading spaces, and the take the len() of that, I'd say go for it- but AFAIK there isn't, and re and other solutions are absolutely overkill.

Upvotes: 2

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