问题
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?
回答1:
When you're doing advanced expression manipulation like this (okay, your example is still simple), the replace method is very useful:
test.replace(f, lambda arg: 2*arg) # returns: 2*x + 2*y
From the docs:
[replace] Traverses an expression tree and performs replacement of matching subexpressions from the bottom to the top of the tree.
The 2nd example of the docs show that any function can be replaced by another that works on its arguments, as shown above for your example as well.
来源：https://stackoverflow.com/questions/27953715/substitute-sympy-function-with-arbitrary-arguments