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In python one can use parse_expr to convert a string which contains valid sympy expression to an sympy expression.

I can't find how to do the same in sage. I tried eval but this works sometimes and not other times. Here is an example

   var('x')
   sage: expr=sin(x)
   sage: expr=str(expr)
   sage: expr=eval(expr)
         sin(x)

But not on this one

sage: expr=-1/2*x/(x^2 + 1)
sage: expr=str(expr)
sage: expr=eval(expr)
---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
<ipython-input-16-599d1fa18625> in <module>()
----> 1 expr=eval(expr)

<string> in <module>()

/usr/lib/python2.7/site-packages/sage/structure/element.pyx in sage.structure.element.Element.__xor__ (build/cythonized/sage/structure/element.c:8986)()
    951 
    952     def __xor__(self, right):
--> 953         raise RuntimeError("Use ** for exponentiation, not '^', which means xor\n"+\
    954               "in Python, and has the wrong precedence.")
    955 

RuntimeError: Use ** for exponentiation, not '^', which means xor
in Python, and has the wrong precedence.

I do not know before hand the "type" of the expression, other than it is valid sage expression, but in a string.

I am looking for something similar to Mathematica's ToExpression described in http://reference.wolfram.com/language/ref/ToExpression.html or python parse_expr

I found answer here https://ask.sagemath.org/question/41135/converting-strings-into-expressions/ using something called SR which I still do not understand, but it does not work for me.

sage: expr=integrate(x^2*(sqrt(x^2 + 1) - 2)/((x^3 - (x^2 + 1)^(3/2) - 1)*sqrt(x^2 + 1)), x)
sage: expr=str(expr)
sage: expr=SR(expr)

TypeError                                 Traceback (most recent call last)

So, how would one convert string that contains arbitrary sage valid expression to sage expression?

In python one can use parse_expr to convert a string which contains valid sympy expression to an sympy expression.

I can't find how to do the same in sage. I tried eval but this works sometimes and not other times. Here is an example

   var('x')
   sage: expr=sin(x)
   sage: expr=str(expr)
   sage: expr=eval(expr)
         sin(x)

But not on this one

sage: expr=-1/2*x/(x^2 + 1)
sage: expr=str(expr)
sage: expr=eval(expr)
---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
<ipython-input-16-599d1fa18625> in <module>()
----> 1 expr=eval(expr)

<string> in <module>()

/usr/lib/python2.7/site-packages/sage/structure/element.pyx in sage.structure.element.Element.__xor__ (build/cythonized/sage/structure/element.c:8986)()
    951 
    952     def __xor__(self, right):
--> 953         raise RuntimeError("Use ** for exponentiation, not '^', which means xor\n"+\
    954               "in Python, and has the wrong precedence.")
    955 

RuntimeError: Use ** for exponentiation, not '^', which means xor
in Python, and has the wrong precedence.

I do not know before hand the "type" of the expression, other than it is valid sage expression, but in a string.

I found I could do this

sage: var('x')
sage: expr=-1/2*x/(x^2 + 1)
sage: expr=str(expr)
sage: expr=sage_eval(expr,locals={'x':x})
sage: expr
-1/2*x/(x^2 + 1)

and it works. But this means I need to know before hand that 'x' symbol was there. Which is not possible for me.

I am looking for something similar to Mathematica's ToExpression described in http://reference.wolfram.com/language/ref/ToExpression.html or python parse_expr

I found answer here https://ask.sagemath.org/question/41135/converting-strings-into-expressions/ using something called SR which I still do not understand, but it does not work for me.

sage: expr=integrate(x^2*(sqrt(x^2 + 1) - 2)/((x^3 - (x^2 + 1)^(3/2) - 1)*sqrt(x^2 + 1)), x)
sage: expr=str(expr)
sage: expr=SR(expr)

TypeError                                 Traceback (most recent call last)

So, how would one convert string that contains arbitrary sage valid expression to sage expression?