That's why subs can also accept a dictionary argument, where you don't have
to rely on the python name matching the symbol name:
def example_works_too(fn, var, val):
return fn.subs({var:val})
The "keywords" based version is just a convenience for interactive work. A
substitution dictionary (where the keys can actually be the symbols!) is
the more general interface. It's documented in x.subs. You can see from the
documentation of x.substitute that "subs" is the main routine, since it
refers you there.
On Monday, 17 November 2025 at 13:56:29 UTC-8 [email protected] wrote:
> Hi,
>
> I am writing some Python functions using SageMath (with the SageMath
> Kernel 10.7 on Mac, in Jupyter Lab). I have a question about how to safely
> pass vars into a Python function, if it's possible at all. The intent is
> to make the function usable no matter what variable the caller chooses for
> his function. The issue shows up in the substitute function, but of course
> I'd like the solution to work in other contexts as well. (At least in some
> cases, it seems to). Is there another way to approach this?
>
> Thanks!
>
> Here's a highly simplified example that shows the issue:
>
> def example_reasonable(fn, variable, value):
> """This is a simple example showing the issue; this example doesn't
> work"""
> # print(variable)
> # prints u as expected....
> # but this substitution doesn't work
> return fn.substitute(variable=value)
>
> def example_works(fn, value):
> return fn.substitute(u=value)
>
> u = var('u')
> f = u^2 + 7*u
> print(example_reasonable(f, u, 3)) # Output should be 30, is just function
> print(example_works(f, 3)) # Output should be 30, and is 30
>
> Output:
> u^2 + 7*u 30
>
>
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