Python: How to evaluate a function which is string?

Question

I get some models from database as

f(t)=(2.128795454425367)+(208.54359721863273)*t+(26.098128487929266)*t^2+(3.34369909584111)*t^3+(-0.3450228278737971)*t^4+         


        

Answer 1

If performance isn't a major concern -- and if you're only evaluating it at 12 points, I suspect it's not -- then you can leverage the handy sympy library to do a lot of the work for you. For example:

>>> import sympy
>>> sympy.sympify("t**5 - t + 3")
t**5 - t + 3
>>> sympy.sympify("t**5 - t + 3").subs({"t": 10})
99993

We can wrap this up in a function which returns a function:

import sympy

def definition_to_function(s):
    lhs, rhs = s.split("=", 1)
    rhs = rhs.rstrip('; ')
    args = sympy.sympify(lhs).args
    f = sympy.sympify(rhs)
    def f_func(*passed_args):
        argdict = dict(zip(args, passed_args))
        result = f.subs(argdict)
        return float(result)
    return f_func

which we can then apply, even to more complex cases beyond the easy reach of regex:

>>> s = "f(t)=(2.128795454425367)+(208.54359721863273)*t+(26.098128487929266)*t^2+(3.34369909584111)*t^3+(-0.3450228278737971)*t^4+(-0.018630757967458885)*t^5+(0.0015029038553239819)*t^6;"
>>> f = definition_to_function(s)
>>> f(0)
2.128795454425367
>>> f(10)
4230.6764921149115
>>> f = definition_to_function("f(a,b,c) = sin(a)+3*b-4*c")
>>> f(1,2,3)
-5.158529015192103
>>> import math
>>> math.sin(1)+3*2-4*3
-5.158529015192103