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"""
The compression level must be Z_DEFAULT_COMPRESSION, or between 0 and 9: 
1 gives best speed, 9 gives best compression, 0 gives no compression at 
all (the input data is simply copied a block at a time). 
Z_DEFAULT_COMPRESSION requests a default compromise between speed and 
compression (currently equivalent to level 6).
"""

Z_DEFAULT_COMPRESSION is defined as (-1), and appears to be the value 
used if --compress is specified without --compress-level, making the 
default functionally equivalent to 6.

-- 
Best,
Ben