Problem dumping sympy expressions that contain dynamicsymbols

[mitkof6]

Hi,

I verified that dill is able to dump sympy symbolic expressions. Unfortunately, there are expressions that contain symbols that may be a function of time (e.g. dynamicsymbols).

import sympy as sp
import dill
from sympy.physics.mechanics import dynamicsymbols

q = dynamicsymbols('q')
t = sp.symbols('t')
dump = dill.dumps(q, -1)

Is there a way to export these kind of expressions.

Best

[mmckerns]

It seems that there's a simple workaround that will allow these objects to be pickled:

import sympy as sp
import dill
from sympy.physics.mechanics import dynamicsymbols

q = dynamicsymbols('q')
t = sp.symbols('t')
q.__class__.__module__ = '__main__'
_q = dill.dumps(q)
q_ = dill.loads(_q)
assert q_ == q

The question is now: is this a general fix (i.e. it should be applied to all kinds of objects missing the module information) and thus should go into dill -- or is it a sympy-specific workaround, and thus should be applied in sympy? (FYI @asmeurer)

That will take a little further investigation.

[asmeurer]

Also fixes it for sympy.Function('f')(sympy.Symbol('x')). It is a dynamically created class from a metaclass. It looks like __module__ is set to None before you set it to __main__.

[mmckerns]

This succeeds for python 3.x, and fails for python 2.7. dill will drop support for `2.7 after the upcoming release, and then will close this issue at this time. See #413

[mmckerns]

no longer relevant