python - SymPy: Swap two variables -

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in expression like

import sympy  = sympy.symbol('a') b = sympy.symbol('b')  x = + 2*b

i'd swap a , b retrieve b + 2*a. tried

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

but neither works; result 3*a in both cases b, reason, gets replaced first.

any hints?

there simultaneous argument can pass substitution, ensure substitutions happen simultaneously , don't interfere 1 doing now. 

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

outputs: 

2*a + b

from docs subs:

if keyword simultaneous true, subexpressions not evaluated until substitutions have been made.


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