There is a distribute() context manager which lets you disable
automatic distribution, though it's not pretty:
>>> from sympy.core.parameters import distribute
>>> with distribute(False):
... print(expr.diff(t))
3*a*(t - t0)**2 + 2*b*(t - t0)
While this is less dangerous than the similar evaluate() context
manager, it is possible this could break something if you put too much
under the context.
As Chris said, we do want to eventually remove this automatic
behavior, but it hasn't been easy to do as a lot of things depend on
it currently. Rearranging things after the fact as Chris suggests is
probably the better solution. There's really no guarantees about what
the form of an expression from diff() will look like.
Aaron Meurer
On Sun, Feb 18, 2024 at 12:10 PM Chris Smith <smi...@gmail.com> wrote:
>
> Autodistribution of Number into an Add is how SymPy works and there is no
> flag for differentiation (or for many functions) that would prevent it.
> Simply pass the expression to `factor_terms` to get it cleaned up. (But that
> will extract a factor of `t-t0`, too, which you might not want so you could
> use `Add(*[factor_terms(i) for i in expr.diff(t).args])` in this case.)
>
> Some day autodistribution will go away and I expect that we will then ask how
> to get constants to distribute into simple expressions.
>
> /c
>
> On Sunday, February 18, 2024 at 4:52:12 AM UTC-6 matthia...@gmail.com wrote:
>>
>> Hi all.
>>
>> I have a simple expression:
>>
>> >>> import sympy as sp
>> >>> a, b, t, t0 = sp.symbols('a b t t0')
>> >>> expr = a*(t - t0)**3 + b*(t - t0)**2
>>
>> And I would like to differentiate it with respect to t:
>>
>> >>> expr.diff(t)
>> 3*a*(t - t0)**2 + b*(2*t - 2*t0)
>>
>> Why is the constant "2" distributed in the second term?
>> It seems like an additional step that SymPy does, which doesn't really
>> "improve" the situation in this case.
>> Maybe there is a more general advantage that's just not visible in
>> this simple case?
>> But if that is so, would it be possible to tell SymPy to skip the
>> distributing?
>>
>> To be clear, this is the result I was expecting:
>>
>> >>> expr.diff(t)
>> 3*a*(t - t0)**2 + 2*b*(t - t0)
>>
>> For context, this question came up in a slightly more complicated
>> situation: https://github.com/AudioSceneDescriptionFormat/splines/issues/31
>>
>> cheers,
>> Matthias
>
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