I never knew about this pattern argument to rewrite. My guess is that
there's a bug in there. According to the docstring, you're supposed to
call it as f.rewrite([sin,], exp), but for me that does the same
thing.

I would suggest using f.replace if you want targeted rewriting.

Aaron Meurer

On Tue, Nov 1, 2016 at 2:06 PM, Nathan Thern <[email protected]> wrote:
> I'm new to sympy and pouring through the documentation. I don't understand
> the "pattern" part of the argument to rewrite. The following portion of a
> session illustrates my question:
>
>>>> f=sin(x)+cos(y)/gamma(z)
>>>> f.rewrite(exp)
> -I*(exp(I*x) - exp(-I*x))/2 + (exp(I*y)/2 + exp(-I*y)/2)/gamma(z)
>>>> f.rewrite(sin,exp)
> -I*(exp(I*x) - exp(-I*x))/2 + (exp(I*y)/2 + exp(-I*y)/2)/gamma(z)
>
> Why does rewrite(sin,exp) also rewrite the cos(...) part of the expression
> when it has been explicitly given "sin" as the pattern to rewrite?
>
> NT
>
>
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