Python (SymPy, SciPy), create a lambda character from a string

Question

I struggle to take the text inputs of the equation and evaluate it as a definite integral. I need the called function to go to scipy.integrate.

eq = "x**2"
func = lambda x: eq
func(2)

# outputs:
# x**2


# but if I:

func = lambda x: x**2
func(2)

# outputs:
# 4

+3

Nona urbiz May 03 '11 at 15:47

5 answers

Sven marnach · Answer 1 · 2011-05-03T15:58:15+0000

Not sure, but maybe you're looking

eq = "x**2"
func = eval("lambda x: " + eq)

Please note that use eval()is dangerous if it eqis from an untrusted source (e.g. user input).

rzetterberg · Answer 2 · 2011-05-03T15:59:26+0000

You need to use eval to run eq as code and not treat it as a string.

eq = "x**2"
func = lambda x: eval(eq)
func(2)

# outputs:
# 4

yangle · Answer 3 · 2011-08-12T19:55:20+0000

sympify , . sympy.

:

import sympy as sp
f=sp.sympify('x**2+sin(y)')

Mark mikofski · Answer 4 · 2013-08-07T04:26:34+0000

Try asteval or numexpr for the more secure alternatives to eval () and Sympy.sympify (). evalf ().

bmc · Answer 5 · 2017-07-16T12:28:17+0000

I kept getting a syntax error for the following and relentlessly tried to figure out why:

E="2x+1"
F=lambda x: (x, eval(E)) # to get an xy coordinate

But the problem worked as expected when I changed E to:

E="2*x+1"

Newbie mistake. :)