Question

I have two text files in the following format:

The first is this on every line:

Key1:Value1

The second is this:

Key2:Value2

Is there a way I can replace Value1 in file1 by the Value2 obtained from using it as a key in file2?

For example:

file1:

foo:hello
bar:world

file2:

hello:adam
bar:eve

I would like to get:

foo:adam
bar:eve

There isn’t necessarily a match between the two files on every line. Can this be done neatly in awk or something, or should I do it naively in Python?

Answer 1

Create two dictionaries, one for each file. For example:

file1 = {}
for line in open('file1', 'r'):
    k, v = line.strip().split(':')
    file1[k] = v

Or if you prefer a one-liner:

file1 = dict(l.strip().split(':') for l in open('file1', 'r'))

Then you could do something like:

result = {}
for key, value in file1.iteritems():
    if value in file2:
        result[key] = file2[value]

Another way is you could generate the key-value pairs in reverse for file1 and use sets. For example, if your file1 contains foo:bar, your file1 dict is {bar: foo}.

for key in set(file1) & set(file2):
    result[file1[key]] = file2[key]

Basically, you can quickly find common elements using set intersection, so those elements are guaranteed to be in file2 and you don’t waste time checking for their existence.

Edit: As pointed out by @pepr You can use collections.OrderedDict for the first method if order is important to you.