Python saving an eval function

问题:

Say I have a function fun(f, x, y) where x and y are numbers and f is a string specifying a function such as "1 / x ** 2 + y".

I wish to use this function f a lot, say a few million times, and the values of x and y change between each use.
Therefore calling eval(f) takes a significant amount of time as opposed to just calculating the value of the function each time. (About 50x, in my measured case.)

Is there any way to save this function f so that I would only have to call eval once?

PS. Please do not discuss the (un)safety of using eval here, I am aware of it, but this code isn't going anywhere where a 3rd party will run it, nor is it relevant to my question.

回答1:

You could eval the lambda, so you just evaluate it once, and after that it's a function that you can use:

s = "1 / x ** 2 + y"

s = "lambda x,y: "+s
f = eval(s)
x = 2
y = 3
print(f(x,y))

I get 3.25, but I can change x and y as many times I need without evaluating the expression again.

回答2:

If Jean_Francois' solution still isn't fast enough, you can take a look at numba. f_numba = jit(f), and then probably also @jit the function that calls f_numba so that f_numba is inlined into the caller. Depends on your application.