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To: python-list@python.org
From: Peter Otten <__peter__@web.de>
Subject: Re: Strange behavior
Date: Thu, 16 Aug 2012 15:02:40 +0200
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Xref: csiph.com comp.lang.python:27159

Virgil Stokes wrote:

>>> def testFunc(startingList):
>>>xOnlyList = []; j = -1
>>>for xl in startingList:
>>>if (xl[0] == 'x'):
>> That's going to fail in the starting list contains an empty string. Use
>> xl.startswith('x') instead.
> Yes, but this was by design (tacitly assumed that startingList was both a
> list and non-empty).

You missunderstood it will fail if the list contains an empty string, not if 
the list itself is empty: 

>>> words = ["alpha", "", "xgamma"]
>>> [word for word in words if word[0] == "x"]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: string index out of range

The startswith() version:

>>> [word for word in words if word.startswith("x")]
['xgamma']

Also possible:

>>> [word for word in words if word[:1] == "x"]
['xgamma']

> def testFunc1(startingList): 
>      ''' 
>        Algorithm-1 
>        Note: 
>          One should check for an empty startingList before 
>          calling testFunc1 -- If this possibility exists! 
>      ''' 
>      return([x for x in startingList if x[0] == 'x'], 
>             [x for x in startingList if x[0] != 'x']) 
>  
> 
> I would be interested in seeing code that is faster than algorithm-1

In pure Python? Perhaps the messy variant:

def test_func(words):
    nox = []
    append = nox.append
    withx = [x for x in words if x[0] == 'x' or append(x)]
    return withx, nox