Supposed improvement in Python file-reading speed, submitted by one of my students

gistfile1.txt

    inputfile = open(filename)

    #...

    # I swear enumerate() makes this faster.
    for i, line in enumerate(inputfile):
        if readnext == True: # Previous line started with an '@'
            # linescounted += 1 # For line count check.
            for bp in line.rstrip():
                if bp == 'G':
                    total_count += 1 # Increment total count on valid counts
                    g_count += 1
                elif bp == 'C':
                    total_count += 1
                    c_count += 1
                elif bp == 'A':
                    total_count += 1
                    a_count += 1
                elif bp == 'T':
                    total_count += 1
                    t_count += 1
                elif count_invalid == True:
                    total_count += 1 # Increment total count on invalid counts
                seq_count += 1
            readnext = False
        elif readnext == False and line[0] == '@': # Read first character of line.
            readnext = True # Count pairs in next line