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

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_symbolsset([x, z, y])

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_symbolsOut[218]: {x}In [219]: f.variablesOut[219]: [n]