将字符串转换为SymPy表达式？

Question

I'm trying to convert a string to a SymPy expression. I've tried sympify() and parse_expr() but both are returning errors. How can I correct these errors or workaround them using an alternative method? I'm using Python 3.4.

from sympy import *
s = 'C+O*2'
expr = sympify(s)

from sympy.parsing.sympy_parser import parse_expr
s = 'C+O*2'
expr = parse_expr(s)

Both of these functions return the following error:

File "C:\...\sympy\core\mul.py", line 182, in flatten
    r, b = b.as_coeff_Mul()

TypeError: as_coeff_Mul() missing 1 required positional argument: 'self'

Answer 1

As the documentation for sympify explains:

[T]he O is interpreted as the Order object (used with series) and it raises an error when used improperly[.]

It proposes some workarounds, and the one I prefer is to use the clash locals:

>>> from sympy.abc import _clash1
>>> sympify("C+O*2", locals=_clash1)
C + 2*O