Issues with substitution

edit

I have a list L = [a13 == a10, a14 == a11, a15 == a12, a16 == a7, a17 == a8, a18 == a9]

I then have a running through a loop giving it these values

a = 1

a = 2a15a16 + 2a13a17 + 2a13a18 +1849

etc

I have

print(a)

a.subs(L)

print(a)

and it indicates no change, but I would of thought/ expected substitution to of taken place. Maybe I am being idiot, but please tell me where. I also posted this on stack overflow, sorry if I am not meant to post on both.

Thanks.