That's sound great because that's the way I do.
Best regards.
Christophe.
2009/12/8 Aaron S. Meurer <asmeu...@gmail.com>
> The problem is using strings directly in functions. The proper way to do
> this is to convert the string to a SymPy object first with the sympify()
> function. So for example, this would be perfectly legitimate:
>
> from sympy import sympify, diff, S
> from sympy.core.sympify import SympifyError # Note to fellow developers:
> why isn't this being imported with the above
> expr = "2*x**3+5"
> variable = "x" # You will want to get the variable of differentiation
> try:
> sexier = sympify(expr)
> svariable = S(var) # S is the same as sympify
> except SympifyError:
> print "Could not parse expression."
> print diff(sexpr, svariable)
>
> Using strings directly in functions is not a good idea among other things
> because not all functions support it. For example:
>
> >>> simplify("x + 1")
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> File "sympy/simplify/simplify.py", line 1318, in simplify
> expr = Poly.cancel(powsimp(expr))
> File "sympy/simplify/simplify.py", line 1121, in powsimp
> if expr.is_Pow:
> AttributeError: 'str' object has no attribute 'is_Pow'
>
> Aaron Meurer
> On Dec 8, 2009, at 5:51 AM, Christophe BAL wrote:
>
> Hello,
> I would like to use sympy in the following way :
>
> 1. The user types some formula like for example "2*x**3+5".
> 2. My little program named test.py must have to differentiate this
> formula.
>
> The problem is that the formula is known as a string.
> Is there a way to not used string so as to work with the user's formula ?
>
> Best regards.
> Christophe.
>
> PS : I send this message because Ondrej said to not use string in some
> messages.
>
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