How to get to an item in a python enumerate()

Question

I'm new to python.

I have two files, one that contains symbols (words) and another that is a map file.

Both files are text files. The map file does contain form feeds.

I want to find the line in the map file that is one above the line that contains a symbol in the map file.

I have the following code.

Osymbolfile="alistofsymbols"
mapfile="amapfile"
maplines = (line.rstrip('\n\f') for line in open(mapfile))
for line in Osymbolfile:
    line = (line.rstrip('\n') )
    print "line= ",line
    linecount = 0
    for index, scanline in enumerate(maplines):
        if line in scanline:
            print "scanline=",scanline
            print "index=",index
        else:
            linecount = linecount + 1
    print "= ",linecount

After print "index=",index ,I've tried print maplines[index-1] , but I get an error.

How do I obtain the line above the index 'th line in maplines ?

Answer 1

Your maplines object is a generator expression ; these produce items on demand and are not indexable.

You could use a list comprehension instead:

maplines = [line.rstrip('\n\f') for line in open(mapfile)]

Now you have a indexable list object instead. Even better, you can now loop over this object multiple times; you cannot do that with a generator.

The proper way to handle your case however, is to store the previous line only:

with open(mapfile) as maplines:
    prev = None
    for line in maplines:
        if something in line:
            return prev
        prev = line