How to batch substitute lines in many files in python?

Question

I have several folders (with several sub-folders) of .md files, which share the same basic structure:

###### v1
Blah blah 
###### v2
Blah blah 
...

And I want to add the base folder's name e.g., NET, ESV, to the end of each header:

###### v1 NET 
Blah blah 
###### v2 NET 
Blah blah 
...

This is my amateur attempt at Python, which currently fails:

import os, re, fileinput 

for root, dirs, files in os.walk("/NET"): 
    for file in files: 
        if file.endswith(".md"): 
            for line in fileinput(files=file, inplace=1, backup='.bak'):
                line = re.sub(r'(#+ v\d{1,3})( )', r'\1\2NET')

Suggestions or tips for efficient python regex sub?

Answer 1

import os, re

for root, dirs, files in os.walk("/NET"): 
    for file in files: 
        if not file.endswith(".md"):
            continue
        finame = os.path.join( root, file )
        foname = finame[:-3]+'.tmp'
        fout = open(foname,'w')
        for line in open(finame):
            line = re.sub(r'(#+ v\d{1,3}) ', r'\1  NET', line)
            fout.write( line )
        fout.close()
        os.remove( finame )
        os.rename( foname, finame )