How good is startswith?

Question

Is

text.startswith(\'a\')  

better than

text[0]==\'a\'  

?

Knowing text is not empty and we a

Answer 1

text[0] can fail but the equivalent text[:1] is safe if the string is empty.

If you want to compare more than one characters, I believe .startswith() is better.

Answer 2

PEP 8 explicitly tells to use startswith, because of readability:

- Use ''.startswith() and ''.endswith() instead of string

slicing to check for prefixes or suffixes.

  startswith() and endswith() are cleaner and less error prone.  For
  example:

    Yes: if foo.startswith('bar'):

    No:  if foo[:3] == 'bar':

Answer 3

I'd agree with the others that startswith is more readable, and you should use that. That said, if performance is a big issue for such a special case, benchmark it:

$ python -m timeit -s 'text="foo"' 'text.startswith("a")'
1000000 loops, best of 3: 0.537 usec per loop

$ python -m timeit -s 'text="foo"' 'text[0]=="a"'
1000000 loops, best of 3: 0.22 usec per loop

So text[0] is amost 2.5 times as fast - but it's a pretty quick operation; you'd save ~0.3 microseconds per compare depending on the system. Unless you're doing millions of comparisons in a time critical situation though, I'd still go with the more readable startswith.

Answer 4

Personally I would say startswith is more readable.

Also, from Python 2.5 startwith can take a tuple of prefixes to look for:

>>> "hello world".startswith(("hello","goodbye"))
True