Write MD5 hashes to file for all files in a directory tree
Four things:


You should put global constants in CAPITAL_LETTERS, according to PEP8.
I would make the file_name for the checksums a constant
You never close the files you iterate over.
In general, doing all writes at the same time is faster.


So for 2, 3 &amp; 4, maybe use:

for root, subdirs, files in os.walk(SRC_DIR):
    checksums = []
    for file in files:
        with open(os.path.join(root, file), 'rb') as _file:
            checksums.append([root, file, hashlib.md5(_file.read()).hexdigest()])
    writer.writerows(checksums)


This will perform one write per subdir, which should be faster than a write after every file. If you have a lot of files this will take more memory, which should be slightly mitigated by doing a write for each subdir. You can even pull the checksums list outside of the os.walk loop to have only one (possibly giant) write.

Otherwise, clear and readable code!