The documentation of polynomial module lists plenty of ways to handle coefficients. For example:
>>> import sympy
>>> x,y,z = sympy.symbols('x,y,z')
>>> p = sympy.poly((x+2*y-z)**3)
>>> p.coeffs()
[1, 6, -3, 12, -12, 3, 8, -12, 6, -1]
These are nonzero coefficients in lexicographic order. To see the monomials in matching order, use
>>> p.monoms()
[(3, 0, 0), (2, 1, 0), (2, 0, 1), (1, 2, 0), (1, 1, 1), (1, 0, 2), (0, 3, 0), (0, 2, 1), (0, 1, 2), (0, 0, 3)]
To get the coefficient of a particular monomial, use
>>> p.coeff_monomial(x**2*y)
6