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 :
> The user types some formula like for example "2*x**3+5".
> 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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