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

问题

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[0].args[1].args[0] gives me Symbol("z"), while g.args[1].args[0].args gives me the tuple (Symbol("x"), Symbol("y")), but obviously these are expression-specific.

回答1:

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])

回答2:

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]

来源：https://stackoverflow.com/questions/30018977/how-can-i-get-a-list-of-the-symbols-in-a-sympy-expression