How to find and replace multiple lines in text file?

If the file is large, you want to read and write one line at a time, so the whole thing isn't loaded into memory at once.

# create a dict of find keys and replace values
findlines = open('find.txt').read().split('\n')
replacelines = open('replace.txt').read().split('\n')
find_replace = dict(zip(findlines, replacelines))

with open('data.txt') as data:
    with open('new_data.txt', 'w') as new_data:
        for line in data:
            for key in find_replace:
                if key in line:
                    line = line.replace(key, find_replace[key])
            new_data.write(line)

Edit: I changed the code to read().split('\n') instead of readliens() so \n isn't included in the find and replace strings