I've got a big expression I need to dissect a certain way

edit

If you are willing to work with polynomials, you can do this. There may be ways to speed up the following. First define the polynomial ring and the indeterminates:

sage: R.<A_123, A_12_34, A_124> = QQ[]

Now define the expression:

sage: a = -A_123^5*A_124^3*A_12_34^4 - 2*A_123^4*A_124^4*A_12_34^4 - A_123^3*A_124^5*A_12_34^4

Note that "for m in a" prints out useful information: pairs (coefficient, monomial):

sage: for m in a: print(m)
(-1, A_123^5*A_12_34^4*A_124^3)
(-2, A_123^4*A_12_34^4*A_124^4)
(-1, A_123^3*A_12_34^4*A_124^5)
sage: sum(m[0]*m[1] for m in a)
-A_123^5*A_12_34^4*A_124^3 - 2*A_123^4*A_12_34^4*A_124^4 - A_123^3*A_12_34^4*A_124^5
sage: sum(m[0]*m[1] for m in a) == a
True

Now pick out the terms where all of the exponents are even:

sage: sum(m[0]*m[1] for m in a if all(x%2 == 0 for x in m[1].exponents()[0]))
-2*A_123^4*A_12_34^4*A_124^4