CODEWITHSUNDEEP
pythonsympysubstitution
Hans Wurst
Hans Wurst

Reputation: 227

Why does replacing a variable in a expression by variable with same name but different symbol name not work

How do I replace a variable in a function with a new definition of the initial variable ?

import sympy as sy
g, a, x = sy.symbols("g a x")
g = 5*a*x

Entering g in the interpreter at this point prints:
5*a*x.

Now I would like to replace the printed a with an alpha. Why does the following not work?

a = sy.symbols("alpha")
g = g.subs(a,a)

I am well aware that i could use the following: 

import sympy as sy
g, a, x = sy.symbols("g a x")
g = 5*a*x
alpha = sy.symbols("alpha")
g = g.subs(a,alpha)

But I would like to understand why the former approach is not working. Generally I am interested how the assigned name a= and the string in symbols("a") are connected. Must they be the same for sympy to work correctly ?

Upvotes: 1

Views: 275

Answers (1)

user6655984
user6655984

Reputation:

g = g.subs(a, a) can never do anything, as it's replacing a thing with itself.

To replace a with something else

g.subs(a, sy.symbols('alpha'))

should be used. The relevant topic in SymPy docs is Assignment does not create a relation. A simpler example: 

a = sy.symbols('a')
b = a + 3      # b is now a+3 
a = 5          # b is still a+3 and it will not become 8

At the end of this code, the Python variable a has no relation to the SymPy symbol named "a". The variable is 5; the symbol is still Symbol("a") and will forever be, as SymPy expressions are immutable. Instead of a = 5 one should have done b.subs(a, 5) to perform the substitution.

I am interested how the assigned name a= and the string in symbols("a") are connected.

They are not connected at all. It's convenient to use Python variable a to point to a symbol named "a", that is the only reason people do that. Unfortunately the pattern then misleads them into thinking that if they put a = something new, that will have an effect on Symbol("a") -- it will not. The only effect is that Python variable now points to something else; the expressions containing Symbol("a") are unaffected.

Upvotes: 1

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