When I submit the query 

SELECT ra, decl, w1mag, w1mag - w2mag FROM wise_00.allwise_p3as_psd
WHERE qserv_areaspec_circle(283.831250,-30.545278,0.20)=1

through pyvo, the values in the result column "w1mag - w2mag" are all strings instead of floats. Manipulating the results with pandas seems to kind of work anyways, but it becomes excruciatingly slow.

I can check what the XML looks like with:

query_string = ("SELECT ra, decl, w1mag, w1mag - w2mag FROM wise_00.allwise_p3as_psd "
{{ "WHERE qserv_areaspec_circle(283.831250,-30.545278,0.20)=1 ")}}
query = pyvo.dal.TAPQuery(query=query_string, baseurl='http://lsst-lsp-stable.ncsa.illinois.edu/api/tap')
res = query.execute_stream()
res_str = res.read()
res_str[:1500]

and I see: 

<FIELD name="w1mag - w2mag" datatype="char" arraysize="*" />