Read files and count lengths for dictionary variable in python

I am making something like a random sentence generator. I want to make a random sentence from words taken randomly from 10 .csv files, which change size frequently so I have to count their size before I select a random line. I have already made it but I'm using way too much code... it currently does something like this:

def file_len(fname):
    f = open(fname)
    try:
        for i, l in enumerate(f):
            pass
    finally:
        f.close()
    return i + 1

then selecting random lines...

for all of them I do this:

file1_size = file_len('items/file1.csv')
file1_line = randrange(file1_size) + 1
file1_output = linecache.getline('items/file1.csv', file1_line)

and when it is done for all of them, I just print the output...

print file1_out + file2_out + file3_out ...

Also, sometimes I only want to use some files and not others, so I'd just print the ones I want... e.g. if I just want files number 3, 4 and 7 then I do:

print file3_out + file4_out + file7_out

Obviously there's 30 lines in the line counting, selecting random and assigning that to a variable - 3 lines of code for each file. But things are getting more complex and I thought a dictionary variable might be able to do what I want more quickly and with less code.

I thought it would be good to generate a variable whereby we end up with

random_lines = {'file1.csv': 24, 'file2.csv': 13, 'file3.csv': 3, 'file4.csv': 22, 'file5.csv': 8, 'file6.csv': 97, 'file7.csv': 34, 'file8.csv': 67, 'file9.csv': 31, 'file10.csv': 86}

(The key is the filename and the integer is a random line within the file, re-assigned each time the code is run)

Then, some kind of process that picks the required items (let's say sometimes we only want to use lines from files 1, 6, 8, and 10) and outputs the random line

output = file1.csv random line + file6.csv random line + file8.csv random line + file10.csv random line

then

print output

If anyone can see the obvious quick way to do this (I don't think it's rocket science but I am a beginner at python!) then I'd be very grateful!