Merging content of several sorted files

#!/bin/env python
# This script merges metrics output from different runs
# Does a merge sort of the files and groups together equal keys

import sys, os, time
from heapq import heappush, heappop

DELIM = ", "
END_LINE = "\n"
BLANK = "-"

# Override this method for splitting input time to key and value
def parse_input(line):
    parts = map(lambda x : x.strip(), line.split("\t"))
    key = " ".join(parts[0:8])
    value = parts[8]
    return (key, value)
    
def pop_record(f, split_func):
    if f:
        try:
            line = f.readline()
            return split_func(line)
        except:
            print 'Error:', line, parts, f
            return None
    else:
        return None

def generate_header(files):
    
    fout = sys.stdout
    header = []
    for fname in files:
        header.append(fname.split("/")[-1])
        
    fout.write("KEY" + DELIM)
    index = 0
    for s in header:
        if index != 0:
            fout.write(",")
        fout.write(s)
        index += 1
    fout.write(END_LINE)        

def merge_datasets(files):
    n_count = 0
    flist = []
    index = 0
    for fname in files:
        f = open(fname)
        flist.append(f)
        index += 1
        
    fout = sys.stdout
    #Initialize heap
    index = 0; heap = []
    for f in flist:
        (key, value) = pop_record(f, parse_input)
        heappush(heap, (n_count, key, index, value, f))
        n_count += 1
        index += 1
    nfiles = index
    #Iterate through values
    while heap:
        tup = []
        for i in xrange(nfiles):
            if heap:
                (_, key, index, value, f) = heap[0]
                if index == i:
                    out_key = key
                    tup.append(value)
                    heappop(heap)
                    (key, value) = pop_record(f, parse_input)
                    if f:
                        heappush(heap, (n_count, key, index, value, f))
                        n_count += 1
                else:
                    tup.append(BLANK)
            else:
                tup.append(BLANK)
        fout.write(out_key)
        for t in tup:
            fout.write(DELIM + t)
        fout.write(END_LINE)
    for f in flist:
        f.close()
    
if __name__ == "__main__":
    files = sys.argv[1:]
    if files:
        generate_header(files)
        merge_datasets(files)
    else:
        print "Usage: " + sys.argv[0] + " {list of file names to compare}"