How to tell if a file is gzip compressed?

Question

I have a Python program which is going to take text files as input. However, some of these files may be gzip compressed.

Is there a cross-platform, usable from Py

Answer 1

gzip itself will raise an OSError if it's not a gzipped file.

>>> with gzip.open('README.md', 'rb') as f:
...     f.read()
...
Traceback (most recent call last):
  File "", line 2, in 
  File "/Users/dennis/.asdf/installs/python/3.6.6/lib/python3.6/gzip.py", line 276, in read
    return self._buffer.read(size)
  File "/Users/dennis/.asdf/installs/python/3.6.6/lib/python3.6/gzip.py", line 463, in read
    if not self._read_gzip_header():
  File "/Users/dennis/.asdf/installs/python/3.6.6/lib/python3.6/gzip.py", line 411, in _read_gzip_header
    raise OSError('Not a gzipped file (%r)' % magic)
OSError: Not a gzipped file (b'# ')

Can combine this approach with some others to increase confidence, such as checking the mimetype or looking for a magic number in the file header (see other answers for an example) and checking the extension.

import pathlib

if '.gz' in pathlib.Path(filepath).suffixes:
   # some more inexpensive checks until confident we can attempt to decompress
   # ...
   try ...
     ...
   except OSError as e:
     ...