In [1]: import sympy

In [2]: (x, y, z) = sympy.symbols("x y z")

In [3]: x+y**z
Out[3]: x + y**z

In [4]: (x+y**z).free_symbols
Out[4]: {z, y, x}

In [5]: sympy.init_printing()

In [6]: x+y**z
Out[6]:
     z
x + y

In [7]: (x+y**z).free_symbols
Out[7]: set([x, y, z])

In [8]: {1, 2, 3}
Out[8]: set([1, 2, 3])