"x"

[Little Grey Men]

Would like some advice on use of string "x" as expression source.

"x**1" can work ok with sympify

"1x" can work with parse_expr

We try "x" with sympify and can see it return a type that is not sympy.Expr

Apparently "x" can never work by design <- will that change?,

is some option combo to parse "x"?

thanks for any comments

Daniel.

[Raghav Bhola]

sympify("x") --> always returns a Symbol("x") not an Expr 

sympify("x**1")--> work fine because it become a proper expression ( pow object).

parse_expr("x") --> is more flexible, and with (local_dict) or transformation you can treat "x" as an expression.

here is the solution: use parse_expr instead of sympify.

      from sympy import Symbol
      from sympy.parsing.sympy_parser import parse_expr

      x = Symbol("x")
      expr = parse_expr("x", local_dict={"x": x})
       print(expr)   # outputs: x

[Oscar Benjamin]

It is not clear what you mean here:

In [15]: isinstance(sympify("x"), Expr)
Out[15]: True

--
Oscar

[Little Grey Men]

Thank you both for your helpful answers...

Certain blogs/observations had led me to the way I asked the question here.

like cannot use "x" and parse_expr has symbolize built in etc

Anyhow I will assimilate your info into the problem and it seems like it quite possible...

thanks

Daniel.

[Little Grey Men]

OK I am able to see working the version with parse_expr with a local_dict

the previous sympify syntax used which failed with "x" was slightly more complex in that it also used a dict(subs).

Should i be able to call parse_expr with the local_dict to declare x AND the dict(subs) to evaluate.

In such a case if it were valid, it would make sense the evaluate flag would be set to True.

Does any of the above make sense ?

Thanks for any comments,

Daniel.

[Oscar Benjamin]

On Tue, 6 Jan 2026 at 14:15, Little Grey Men <daniel...@gmail.com> wrote:
>
> OK I am able to see working the version with parse_expr with a local_dict
> the previous sympify syntax used which failed with "x" was slightly more complex in that it also used a dict(subs).
>
> Should i be able to call parse_expr with the local_dict to declare x AND the dict(subs) to evaluate.
> In such a case if it were valid, it would make sense the evaluate flag would be set to True.
>
> Does any of the above make sense ?

It is too imprecise to understand. Show the actual code and explain
very clearly what happens when you run the code and what it is that
you want to happen.

--
Oscar

[Little Grey Men]

no problem i think what i asking like :

input_str="2x"

transformations = ... implicit_mult ...

expr = parse_expr (input_str, local_dict={"x":x}, transformations=transformations).subs("x", 4.0)

I think above is the approx replacement i looking for.

cheers

Daniel.

[Little Grey Men]

ok so I had to switch back to simpify since of the tokenize approach with parse_expr too different.

"x" also started working without a local_dict defined ?

I think "2x" cannot be turned into an expression <- if you have an answer for that ?!

The overall approach seems create the expression first and then use it's subs method if required.

cheers!

[Little Grey Men]

parse_expr with the Symbol and local_dict allows for "2x" so no need to reply further thanks !