Thanks for a reply. I tried it, but:
exec("{Name} = sympify({Expr})".format(Name=Name, Expr=Expr))
gives the same, and
exec(sympify("{Name} = {Expr}".format(Name=Name, Expr=sympify(Expr))))
gives an error...
On Wed, Sep 11, 2013 at 9:29 PM, Aaron Meurer <[email protected]> wrote:
> It's because the string form of the expression contains division of
> numeric literals, which exec evaluates to floats. If you want to avoid
> this, you should use sympify(), or use the srepr() form of the
> expression instead of the str() form.
>
> Aaron Meurer
>
> On Wed, Sep 11, 2013 at 7:51 AM, Boris Kheyfets <[email protected]>
> wrote:
> > Why this
> >
> > d2bdy2 = b_i.diff(y_i, 2)
> > exec("pprint({{'{0}': {0}}})".format("d2bdy2"))
> >
> > is not equivalent to
> >
> > def assign_and_pprint(Name, Expr):
> > exec("{Name} = {Expr}".format(Name=Name, Expr=Expr))
> > exec("pprint({{'{Name}': {Name}}})".format(Name=Name))
> >
> > assign_and_pprint("d2bdy2", b_i.diff(y_i, 2))
> >
> > Compelete example:
> >
> > #!/usr/bin/python2.7
> >
> > from sympy import *
> >
> > def assign_and_pprint(Name, Expr):
> > exec("{Name} = {Expr}".format(Name=Name, Expr=Expr))
> > exec("pprint({{'{Name}': {Name}}})".format(Name=Name))
> >
> >
> > var("""
> > b_i y_i nu
> > """, positive=True)
> >
> > b_i = 2**(S(2)/3) / (nu**(S(4)/3) * (y_i - 1)**(S(8)/3))
> >
> > d2bdy2 = b_i.diff(y_i, 2)
> > exec("pprint({{'{0}': {0}}})".format("d2bdy2"))
> >
> > assign_and_pprint("d2bdy2", b_i.diff(y_i, 2))
> >
> > Which gives:
> >
> > ⎧ 2/3 ⎫
> > ⎪ 88⋅2 ⎪
> > ⎨d2bdy2: ───────────────────⎬
> > ⎪ 4/3 14/3⎪
> > ⎩ 9⋅ν ⋅(yᵢ - 1) ⎭
> > ⎧ -1.33333333333333 -4.66666666666667⎫
> > ⎨d2bdy2: 15.5212547303557⋅ν ⋅(yᵢ - 1) ⎬
> > ⎩ ⎭
> >
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