How can I get a list of the symbols in a sympy expression?

A very useful attribute is atoms

x, y, z = sympy.symbols('x:z')
expr1 = sympy.exp(x + y) - sympy.sqrt(z)
display(expr1.free_symbols)
display(expr1.atoms(sympy.Symbol))

{𝑥,𝑦,𝑧} 
{𝑥,𝑦,𝑧}

In addition to symbols, atoms can extract other atoms, e.g.:

display(expr1.atoms(sympy.Function))
display(expr1.atoms(sympy.Number))
display(expr1.atoms(sympy.NumberSymbol))
display(expr1.atoms(sympy.function.AppliedUndef))
display(expr1.atoms(sympy.Mul))
display(expr1.atoms(sympy.Add))

(it's worth checking the output). Regarding the answer by gerrit

n = sympy.Symbol('n')
k2 = sympy.Sum(x, (n, 0, 10))
display(k2.free_symbols)
display(k2.variables)
display(k2.atoms(sympy.Symbol))

{𝑥} 
[𝑛]
{𝑛,𝑥}

Note that JuniorCompressors answer only lists free variables.

If you have a Sum, a Product, an Integral, or something similar, you may or may not want to additionally know the integration/summation variable using the .variables attribute:

In [216]: (x, n) = sympy.symbols("x n")

In [217]: f = sympy.Sum(x, (n, 0, 10))

In [218]: f.free_symbols
Out[218]: {x}

In [219]: f.variables
Out[219]: [n]

You can use:

f.free_symbols

which will return a set of all free symbols.

Example:

>>> import sympy
>>> x, y, z = sympy.symbols('x:z')
>>> f = sympy.exp(x + y) - sympy.sqrt(z)
>>> f.free_symbols
set([x, z, y])