Numerical evaluation in exec?

[Boris Kheyfets]

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)                 ⎬
⎩                                                                     ⎭

[Aaron Meurer]

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

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[Boris Kheyfets]

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...

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[Aaron Meurer]

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