Nico Schlömer Nico Schlömer - 5 months ago 25
Python Question

SymPy: Swap two variables

In an expression like



import sympy

a = sympy.Symbol('a')
b = sympy.Symbol('b')

x = a + 2*b


I'd like to swap
a
and
b
to retrieve
b + 2*a
. I tried



y = x.subs([(a, b), (b, a)])
y = x.subs({a: b, b: a})


but neither works; the result is
3*a
in both cases as
b
, for some reason, gets replaced first.

Any hints?

Answer

There is a simultaneous argument you can pass to the substitution, which will ensure the substitutions happen simultaneously and don't interfere with one another as they are doing now.

y = x.subs({a:b, b:a}, simultaneous=True)

Outputs:

2*a + b

From the docs for subs:

If the keyword simultaneous is True, the subexpressions will not be evaluated until all the substitutions have been made.