Inserting a character in a list if the element contains '>"

Question

I am trying to insert 'a' into filesContent list whenever there is an '>' in the element. The code below succeeds in insert an 'a' element after '>' except for the last 2 '>' elements. I don't understand why.

def specialparseFile(fname):
    filesContent = list()
    with open(fname) as f:
        lines = [i.strip() for i in f]
        lines = [line for line in lines if line]
        for i in lines:
            filesContent.append(i)

        for x in range(len(filesContent)):

            if '>' in filesContent[x]:
                filesContent.insert(x + 1, 'a')
    print(filesContent)
    return filesContent

Answer 1

Say for example your filesContent is l:

l = ['>', 'b', '>']

It has length 3, so your for loop would loop through indices 0, 1 and 2. After index 0 the array would be:

l = ['>', 'a', 'b', '>']

Now you can see your issue, the last > will never be reached as it is at index 3.

You can loop backwards instead to resolve this:

range(len(filesContent) - 1, -1, -1)

Or if you prefer:

reversed(range(len(filesContent)))

Now looping through 2, 1, 0, after the first iteration you will have:

l = ['>', 'b', '>', 'a']

With the next index being 1, it is clear that the previous addition wont effect the list that still has to be looped through.

Answer 2

The root cause of your bug has been well-explained in @Nick A's answer, another way is to remove for loop and forget the index of list at all, by using the following code:

def specialparseFile(fname):
    with open(fname) as f:
        lines = filter(None, map(lambda x: x.strip(), f))
        result = [[l, 'a'] if '<' in l else [l] for l in lines]
        return sum(result, []) # flatten the nested list generated by previous line of code

Answer 3

you can also do both in the same loop

...
for item in lines:
    filesContent.append(item)
    if ">" in item:
        filesContent.append( "a" )