how to find the index of the first non-whitespace character in a string in python?

Question

Scenario:

>>> a='   Hello world'
index = 3

In this case the "H" index is '3'. But I need a more general method such that for any string variable 'a' takes I need to know the index of the first character?

Alternative scenario:

>>> a='\tHello world'
index = 1

Answer 1

If you mean the first non-whitespace character, I'd use something like this ...

>>> a='   Hello world'
>>> len(a) - len(a.lstrip())
3

Another one which is a little fun:

>>> sum(1 for _ in itertools.takewhile(str.isspace,a))
3

But I'm willing to bet that the first version is faster as it does essentially this exact loop, only in C -- Of course, it needs to construct a new string when it's done, but that's essentially free.

---

For completeness, if the string is empty or composed of entirely whitespace, both of these will return len(a) (which is invalid if you try to index with it...)

>>> a = "foobar"
>>> a[len(a)]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: string index out of range

Answer 2

Using regex :

>>> import re
>>> a='   Hello world'
>>> re.search(r'\S',a).start()
3
>>> a='\tHello world'
>>> re.search(r'\S',a).start()
1
>>>

Function to handle the cases when the string is empty or contains only white spaces:

>>> def func(strs):
...     match = re.search(r'\S',strs)
...     if match:
...         return match.start()
...     else:
...         return 'No character found!'
...     
>>> func('\t\tfoo')
2
>>> func('   foo')
3
>>> func('     ')
'No character found!'
>>> func('')
'No character found!'

Answer 3

You can also try:

a = '   Hello world'
a.index(a.lstrip()[0])
=> 3

It'll work as long as the string contains at least one non-space character. We can be a bit more careful and check this before:

a = '    '
-1 if not a or a.isspace() else a.index(a.lstrip()[0])
=> -1

Answer 4

Another method, just for fun... Using a special function!

>>> def first_non_space_index(s):
    for idx, c in enumerate(s):
        if not c.isspace():
            return idx


>>> a = '   Hello world'        
>>> first_non_space_index(a)
3

Answer 5

Following mgilson's answer, you can use lstrip to strip any characters you'd like -

unwanted = ':!@#$%^&*()_+ \t\n'
a= '  _Hello world'
res = len(a) - len(a.lstrip(unwanted))