The first one is still calling exec on the string form. The second one doesn't work because sympify() can't handle statements like "x = 1".
I recommend avoiding magic to do assignment (and avoiding exec in general), but if you must do it, you're better off injecting the name into the namespace, like var() does. Look at its source to see how it works (https://github.com/sympy/sympy/blob/master/sympy/core/symbol.py#L465). Aaron Meurer On Wed, Sep 11, 2013 at 2:17 PM, Boris Kheyfets <[email protected]> wrote: > 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) ⎬ >> > ⎩ ⎭ >> > >> > -- >> > You received this message because you are subscribed to the Google >> > Groups >> > "sympy" group. >> > To unsubscribe from this group and stop receiving emails from it, send >> > an >> > email to [email protected]. >> > To post to this group, send email to [email protected]. >> > Visit this group at http://groups.google.com/group/sympy. >> > For more options, visit https://groups.google.com/groups/opt_out. >> >> -- >> You received this message because you are subscribed to a topic in the >> Google Groups "sympy" group. >> To unsubscribe from this topic, visit >> https://groups.google.com/d/topic/sympy/D5O51p6OPXE/unsubscribe. >> To unsubscribe from this group and all its topics, send an email to >> [email protected]. >> >> To post to this group, send email to [email protected]. >> Visit this group at http://groups.google.com/group/sympy. >> For more options, visit https://groups.google.com/groups/opt_out. > > > -- > You received this message because you are subscribed to the Google Groups > "sympy" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > To post to this group, send email to [email protected]. > Visit this group at http://groups.google.com/group/sympy. > For more options, visit https://groups.google.com/groups/opt_out. -- You received this message because you are subscribed to the Google Groups "sympy" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at http://groups.google.com/group/sympy. For more options, visit https://groups.google.com/groups/opt_out.
