How to append a file to a tar file use python tarfile module?

Question

I want to append a file to the tar file. For example, the files in test.tar.gz are a.png, b.png, c.png. I have a new png file named a.png, I want to append to a.png to test.tar.gz and cover the old file a.png in test.tar.gz. My code:

import tarfile
a = tarfile.open('test.tar.gz', 'w:gz')
a.add('a.png')
a.close()

then, all the files in test.tar.gz disappeard but a.png, if I change my code to this:

import tarfile
a = tarfile.open('test.tar.gz', 'a:')# or a:gz
a.add('a.png')
a.close()

the program is crashed, error log:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/tarfile.py", line 1678, in open
    return func(name, filemode, fileobj, **kwargs)
  File "/usr/lib/python2.7/tarfile.py", line 1705, in taropen
    return cls(name, mode, fileobj, **kwargs)
  File "/usr/lib/python2.7/tarfile.py", line 1588, in __init__
    raise ReadError(str(e))
tarfile.ReadError: invalid header

What are my mistakes and what should I do?

Update. From the documentation, it follows that gz files cannot be open in a mode. If so, what is the best way to add or update files in an existing archive?

Answer 1

From tarfile documentation:

Note that 'a:gz' or 'a:bz2' is not possible. If mode is not suitable to open a certain (compressed) file for reading, ReadError is raised. Use mode 'r' to avoid this. If a compression method is not supported, CompressionError is raised.

So I guess you should decompress it using gzip library, add the files using the a: mode in tarfile, and then compress again using gzip.