Strange behavior

[Virgil Stokes]

On 2012-08-14 17:38, light1quark at gmail.com wrote:
> Hi, I am migrating from PHP to Python and I am slightly confused.
>
> I am making a function that takes a startingList, finds all the strings in the list that begin with 'x', removes those strings and puts them into a xOnlyList.
>
> However if you run the code you will notice only one of the strings beginning with 'x' is removed from the startingList.
> If I comment out 'startingList.remove(str);' the code runs with both strings beginning with 'x' being put in the xOnlyList.
> Using the print statement I noticed that the second string that begins with 'x' isn't even identified by the function. Why does this happen?
>
> def testFunc(startingList):
> 	xOnlyList = [];
> 	for str in startingList:
> 		if (str[0] == 'x'):
> 			print str;
> 			xOnlyList.append(str)
> 			startingList.remove(str) #this seems to be the problem
> 	print xOnlyList;
> 	print startingList
> testFunc(['xasd', 'xjkl', 'sefwr', 'dfsews'])
>
> #Thanks for your help!

You might find the following useful:

def testFunc(startingList):
     xOnlyList = []; j = -1
     for xl in startingList:
         if (xl[0] == 'x'):
             xOnlyList.append(xl)
         else:
             j += 1
             startingList[j] = xl
     if j == -1:
         startingList = []
     else:
         del startingList[j:-1]

     return(xOnlyList)


testList1 = ['xasd', 'xjkl', 'sefwr', 'dfsews']
testList2 = ['xasd', 'xjkl', 'xsefwr', 'xdfsews']
testList3 = ['xasd', 'jkl', 'sefwr', 'dfsews']
testList4 = ['asd', 'jkl', 'sefwr', 'dfsews']

xOnlyList = testFunc(testList1)
print 'xOnlyList = ',xOnlyList
print 'testList = ',testList1
xOnlyList = testFunc(testList2)
print 'xOnlyList = ',xOnlyList
print 'testList = ',testList2
xOnlyList = testFunc(testList3)
print 'xOnlyList = ',xOnlyList
print 'testList = ',testList3
xOnlyList = testFunc(testList4)
print 'xOnlyList = ',xOnlyList
print 'testList = ',testList4

And here is another version using list comprehension that I prefer

testList1 = ['xasd', 'xjkl', 'sefwr', 'dfsews']
testList2 = ['xasd', 'xjkl', 'xsefwr', 'xdfsews']
testList3 = ['xasd', 'jkl', 'sefwr', 'dfsews']
testList4 = ['asd', 'jkl', 'sefwr', 'dfsews']

def testFunc2(startingList):
     return([x for x in startingList if x[0] == 'x'], [x for x in
startingList if x[0] != 'x'])

xOnlyList,testList = testFunc2(testList1)
print xOnlyList
print testList
xOnlyList,testList = testFunc2(testList2)
print xOnlyList
print testList
xOnlyList,testList = testFunc2(testList3)
print xOnlyList
print testList
xOnlyList,testList = testFunc2(testList4)
print xOnlyList
print testList