why symbol of the key is not defined in formula

Question

I try to define the key of a dictionary as symbol and then I make an expression with those keys. but in the def, there is always show error that said symbol of the key is not defined. but I do debug and it said those symbol of the key is already there.

import sympy as sm

def try_sym(dic):
    for k, v in dic.items():
        exec(k+"=sm.Symbol(k)")

    f = sm.Symbol("f")
    #a=sm.Symbol("a")
    f = a+b
    return f

dic = {"a":1, "b":10}

f =sm.Symbol("f")
f=try_sym(dic)
print(f)

why the two conflicting `f=` lines? We don't 'initial' variables in python. — hpaulj, Jun 27 '23 at 00:20
Be careful - strings, variable names, and `sympy symbols` are different things. What's the purpose of this `dict` and its keys? — hpaulj, Jun 27 '23 at 00:54

score 0 · Answer 1 · answered Jun 26 '23 at 23:44

0

import numpy as np
import sympy as sm

def try_sym(dic):
    for k, v in dic.items():
        exec(k+"=sm.Symbol(k)")

    f = sm.Symbol("f")
    #a=sm.Symbol("a")
    f = a+b
    return f

dic = {"a":1, "b":10}

f =sm.Symbol("f")
f=try_sym(dic)
print(f)

answered Jun 26 '23 at 23:44

jmin

15
3

it is not the answer.https://stackoverflow.com/questions/1463306/how-to-get-local-variables-updated-when-using-the-exec-call – jmin Jun 26 '23 at 23:47
It does, though, explain why `exec` is not working as you want. – hpaulj Jun 27 '23 at 03:04

score 0 · Accepted Answer · answered Jun 27 '23 at 06:13

You could stick with a local dict:

In [66]: def try_sym(dic):
    ...:     dd = {k:Symbol(k) for k in dic}
    ...:     f = sum(v for v in dd.values())
    ...:     return f
    ...: 

In [67]: try_sym(dic)
Out[67]: a + b

Using exec as well as creating local and global variables 'dynamically' is generally discouraged. While possible, Python doesn't make it easily.

why symbol of the key is not defined in formula

2 Answers2