Remove empty strings from list

Dave Angel davea at ieee.org
Tue Sep 15 05:06:57 CEST 2009


Helvin wrote:
> Hi,
>
> Sorry I did not want to bother the group, but I really do not
> understand this seeming trivial problem.
> I am reading from a textfile, where each line has 2 values, with
> spaces before and between the values.
> I would like to read in these values, but of course, I don't want the
> whitespaces between them.
> I have looked at documentation, and how strings and lists work, but I
> cannot understand the behaviour of the following:
>                         line = f.readline()
> 			line = line.lstrip() # take away whitespace at the beginning of the
> readline.
> 			list = line.split(' ') # split the str line into a list
>
>                         # the list has empty strings in it, so now,
> remove these empty strings
> 			for item in list:
> 				if item is ' ':
> 					print 'discard these: ',item
> 					index = list.index(item)
> 					del list[index]         # remove this item from the list
> 				else:
> 					print 'keep this: ',item
> The problem is, when my list is :  ['44', '', '', '', '', '',
> '0.000000000\n']
> The output is:
>     len of list:  7
>     keep this:  44
>     discard these:
>     discard these:
>     discard these:
> So finally the list is:   ['44', '', '', '0.000000000\n']
> The code above removes all the empty strings in the middle, all except
> two. My code seems to miss two of the empty strings.
>
> Would you know why this is occuring?
>
> Regards,
> Helvin
>
>   
(list already is a defined name, so you really should call it something 
else.


As Chris says, you're modifying the list while you're iterating through 
it, and that's undefined behavior.  Why not do the following?

mylist = line.strip().split(' ')
mylist = [item for item in mylist if item]

DaveA



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