Nils,
Yes, that works quite well both in the example and in my original cases.
Thanks so much for the prompt assist.
John
On Monday, November 17, 2025 at 10:28:28 PM UTC-5 Nils Bruin wrote:
> 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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