import sympy
a = sympy.Symbol('a')
b = sympy.Symbol('b')
x = sympy.Symbol('x')
y = sympy.Symbol('y')
a
$\displaystyle a$
f = a*x+b*y
f
$\displaystyle a x + b y$
f = a*(x+y)
f
$\displaystyle a \left(x + y\right)$
g = f.expand()
g
$\displaystyle a x + a y$
g.diff(x)
$\displaystyle a$
g.simplify()
$\displaystyle a \left(x + y\right)$
factor, collect are two other useful functions
g.subs({x:b})
$\displaystyle a b + a y$
h = g.subs({x:(3*b+5)})
h
$\displaystyle a y + a \left(3 b + 5\right)$
eq = []
eq.append(x+5*y)
eq.append(x-1)
solution = sympy.solve(eq,[x,y])
solution
{x: 1, y: -1/5}
i = x+5*y
j = i.subs(solution)
j
$\displaystyle 0$
type(j)
sympy.core.numbers.Zero
type(j.evalf(5))
sympy.core.numbers.Zero