You could use pattern matching in the replace method:
In [12]: var('a b')
Out[12]: (a, b)
In [13]: c = Wild('c')
In [14]: f = Function('f')
In [15]: f(a + b).replace(f(c), 2*c)
Out[15]: 2⋅a + 2⋅b
On Thursday, January 15, 2015 at 1:50:21 AM UTC+3, Craig Stringham wrote:
>
> This should be an easy task, but I'm having a hard time getting it to work
> in Sympy. I want to substitute an undefined function with arbitrary
> arguments with a specific formula for example:
>
> from sympy import *
> var('a b c')
> f= Function('f')
> test= f(a+b)
> lin= test.subs({f(c):2*(c)})print(lin)
>
> I want this to print out
>
> 2*(a+b)
>
> However, for that I have to use
>
> lin= test.subs({f(a+b):2*(a+b)})
>
> Do I have to define f as a class in order to do this substitution?
>
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