fix cmp= in mv.py (brombo/galgebra#34)

[eric-wieser]

@chaowenguo commented on Jan 19, 2017

example:

import ga,sympy
bases=ga.Ga('e_t|x',g=[1,-1],coords=sympy.symbols('t,x',real=True))

print bases.grad

If I use python -3 to run the source code, it will show:
mv.py:1485: DeprecationWarning: the cmp argument is not supported in 3.x
self.terms.sort(key=operator.itemgetter(1), cmp=Pdop.compare)
that means you can not use cmp in sort in python3.

I look at all the program, just line 1485 in mv.py use Pdop.compare, so just change line 1485 in mv.py into
self.terms.sort(key=Pdop.compare)

change the whole definition of Pdop.compare in line 1765 in mv.py into:

def compare(pdop): # compare two Pdops
keys = list(pdop[1].pdiffs.keys())
return (pdop[1].order,-len(keys),''.join([str(pdop[1].Ga.coords.index(x)) for x in keys]))

Now use order to sort first aka to
if pdop1.order > pdop2.order:
return 1
if pdop1.order < pdop2.order:
return -1

-len(keys) to sort second aka to
if lkeys1 < lkeys2:
return 1
else:
return -1

''.join([str(pdop[1].Ga.coords.index(x)) for x in keys]) to sort third aka to
if lkeys1 == lkeys2:
s1 = ''.join([str(pdop1.Ga.coords.index(x)) for x in keys1])
s2 = ''.join([str(pdop1.Ga.coords.index(x)) for x in keys2])
if s1 < s2:
return -1
else:
return 1
reference https://docs.python.org/3/howto/sorting.html

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[eric-wieser]

This was fixed in e727e9e, although we could do some more cleanup here if we wanted.