sympy substitute all assigned variables

Question

I declare a variable, define an expression using the variable

from sympy import *
x = symbols ('x')
f = 2 * x 

and then assign a value to the variable

x = 42

How can I substitute the variable in the expression by its current value? For obvious reasons

f.subs (x, x) 

does not work. I know that I can use a different variable

xx  = 42
f.subs (x, xx)

but ...

Nobody likes to get compared to their new girlfriend's ex, but in Matlab you could simply write subs (f) which would be the equivalent to SymPy's f.subs () without any parameters, to substitute all intermediately assigned variables.

Roderic

Well, sympy works differently. Sympy has been carefully created to fit into an existing Python ecosystem. In your case, you could try f.subs ('x', x). Note that if you write x = symbols('x'), and then x = ...., you remove your direct access to the symbol. Matlab is a dedicated environment; each tool has its strong and weak points. Feel free to choose the one that best fits your workflow. By the way, a function that loops through an expression, looks up the variable names into the globals and does substitutions, wouldn't be that hard to write. – Hailstorm

Does this answer your question? Sympy "global" substitution – Hailstorm

I answered a similar question yesterday, https://mcmap.net/q/1925016/-updating-variables-with-sympy – Agrippina

Johan, thank you, f.subs ('x', x) makes my life a bit easier. In aaguirre's GlobalSubs I still have to list all the variables I want to substitute. I am still dreaming of an automatic f.subs() that substitutes all currently assigned variables. But as you said, as soon as I understand a bit more about sympy, I will try to write that function ... – Porush

Answer 1

per above,

>>> f = x**2
>>> x = 42
>>> def update(e):
...     d = globals()
...     return e.xreplace({x: eval(x.name, d) for x in e.free_symbols})
>>> Eq(f, update(f))
x**2 = 1764

Riorsson

Wow, great, thanks a lot, Chris! Now give me a few days to understand what you did there ... – Porush

First question: what does d = dir() do? It seems to work without it. – Porush

Is the use of globals() as the second eval parameter necessary? With my limited knowledge, I cannot imagine any situation in this use case where omitting the reference to the current module namespace could be dangerous or critical. – Porush

I updated the code so d is used (and assigned globals(). Since you are passing the value to function it is safe. but if someone else had access to it I suppose they could name a symbol with some nefarious name which, when evaluated, would to bad things. – Riorsson