code: def is_gzipfile(filename):
      
      assert os.path.exists(filename), 'Input {} does not '.format(filename) + \
          'point to a file.'
      with file(filename, 'rb') as in_f:
          start_of_file = in_f.read(3)
          if start_of_file == '\x1f\x8b\x08':
              # bam files are bgzipped and they share the magic sequence with gzip.  Pysam will error
              # if the input is gzip but not if it is a bam.
              try:
                  _ = Samfile(filename)
              except ValueError:
                  return True
              else:
                  return False
          else:
              return False
docs: This function attempts to ascertain the gzip status of a file based on the "magic signatures" of
      the file. This was taken from the stack overflow
      http://stackoverflow.com/questions/13044562/python-mechanism-to-identify-compressed-file-type\
          -and-uncompress