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

Each Answer to this Q is separated by one/two green lines.

For example, if I run

``````import sympy
x, y, z = sympy.symbols('x:z')
f = sympy.exp(x + y) - sympy.sqrt(z)
``````

is there any method of `f` that I can use to get a list or tuple of `sympy.Symbol` objects that the expression contains? I’d rather not have to parse `srepr(f)` or parse downward through `f.args`.

In this case, `g.args.args.args` gives me `Symbol("z")`, while `g.args.args.args` gives me the tuple `(Symbol("x"), Symbol("y"))`, but obviously these are expression-specific.

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])
``````

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))

{?,?,?}
{?,?,?}
``````

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))
``````

(it’s worth checking the output).

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

{?}
[?]
{?,?}
``````

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 : (x, n) = sympy.symbols("x n")

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

In : f.free_symbols
Out: {x}

In : f.variables
Out: [n]
`````` The answers/resolutions are collected from stackoverflow, are licensed under cc by-sa 2.5 , cc by-sa 3.0 and cc by-sa 4.0 .