Uncompressing a .Z file with Python

Question

I'm trying to uncompress a *.Z file using Python. I downloaded it via FTP (binary mode). The file successfully uncompresses with 7zip (whose "info" on the file says it's of type "Z"). The original file can be found at ftp://cddis.gsfc.nasa.gov/gps/products/1860/igr18600.sp3.Z.

I've read up on the use of the zlib module in Python and have some test code I'm using:

import zlib

comp_data = open('C:\Temp\igr18600.sp3.Z', 'rb').read()

print(comp_data[0:10])

uncomp_data = zlib.decompress(comp_data)
with open('c:\temp\igr18600.sp3', 'wb') as f:
    f.write(uncomp_data)
    f.close()

When I execute this I get the following output:

b'\x1f\x9d\x90#\xc6@\x91\x01#F'
Traceback (most recent call last):
  File "test.py", line 7, in <module>
    uncomp_data = zlib.decompress(comp_data)
zlib.error: Error -3 while decompressing data: incorrect header check

zlib clearly doesn't like the header. The first couple of bytes appear to match the proper magic number sequence 0x1F9d for a compressed file (per https://en.wikipedia.org/wiki/List_of_file_signatures).

In a pinch I can work around this by shelling out to 7zip directly. But I was hoping to find a pure Python type of answer. Despite spending most of the day googling for an answer (or this error message) I haven't had much luck. Perhaps my search skills are atrophying?

Answer 1

Python does not have the equivalent of Unix uncompress available in a module, which is what you'd need to decompress a .Z file. You would either need to a) shell out to the Unix compress command, b) shell out to gzip, c) shell out to 7-zip (both gzip and 7-zip have the ability to decompress .Z files), d) modify the original uncompress code in C and link that to Python (the code is available online), or e) write your own LZW decompressor in native Python.

For d), you can find some C code I wrote to do this job in this answer on mathematica.stackexchange.com. See the unlzw() function.