Oh, hold on a minute. There may be something wrong with my dxf file.
I have a dxf file with two layers, "0", and "gear". I tried:
>>> len(msp.query('*[layer=="0"]'))
40
>>> len(msp.query('*[layer=="gear"]'))
0
Then I created a dxf file using librecad with a circle in, "0", and a rectangle in, "gear", I get:
>>> len(msp.query('*[layer=="0"]'))
1
>>> len(msp.query('*[layer=="gear"]'))
1
Hmmm. May have answered my own question!