Python, as far as know, does not allow modification of an archived file. That is why I want to:

1. Unpack the zip in memory (zip_in).
2. Go over each file in the zip_in, and change it if needed, then copy it to zip_out. For now I'm happy with just making a copy of a file.
3. Save zip_out.

I was experimenting with zipfile and io but no luck. Partially because I'm not sure how all that works and which object requires which output.

Working Code

import os
import io
import codecs
import zipfile

# Make in-memory copy of a zip file
# by iterating over each file in zip_in
# archive.
#
# Check if a file is text, and in that case
# open it with codecs.

zip_in = zipfile.ZipFile(f, mode='a')
zip_out = zipfile.ZipFile(fn, mode='w')
for i in zip_in.filelist:
    if os.path.splitext(i.filename)[1] in ('.xml', '.txt'):
        c = zip_in.open(i.filename)
        c = codecs.EncodedFile(c, 'utf-8', 'utf-8').read()
        c = c.decode('utf-8')
    else:
        c = zip_in.read(i.filename)
    zip_out.writestr(i.filename, c)
zip_out.close()

Old Example, With a Problem

# Make in-memory copy of a zip file
# by iterating over each file in zip_in
# archive.
#
# This code below does not work properly.

zip_in = zipfile.ZipFile(f, mode='a')
zip_out = zipfile.ZipFile(fn, mode='w')
for i in zip_in.filelist:
    bc = io.StringIO() # what about binary files?
    zip_in.extract(i.filename, bc)
    zip_out.writestr(i.filename, bc.read())
zip_out.close()

The error is TypeError: '_io.StringIO' object is not subscriptable