SymPy: Swap two variables

Question

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 1

There is a simultaneous argument you can pass to the substitution, which will ensure that all 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

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From the docs for subs:

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