Saving sympy expressions to disk

[Mani Chandra]

Hi,

Is it possible to save sympy expressions to disk and then load them later? For ex:

filename = "equation"

save(filename, expression)

datafile = load(filename)

expression = datafile.expression

or something like that?

Thanks,

Mani Chandra

[Tom Bachmann]

Hi,

sympy should in theory support pickling (the standard python way to save
objects on disk), but I'm not sure how good the support is.

Regards,
Tom

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[Chris Smith]

Just like that (almost):

    >>> eq
    x + y**2
    >>> pickle.dump(eq, file('eq.dat','w'))
    >>> pickle.load(file('eq.dat'))
    x + y**2

The nice thing about this is that assumptions on your variables are preserved, too, so if x and y were designated as real, the x and y in the equation you load will be real, too. Note, however, that if you start a new session and load a saved expression this does not automatically recreate python variables having the same name as the Symbols in the equation. To save on the tedium of redefining those you could do something like this if you last issued `eq = pickle.load(file('eq.dat'))` :

    >>> [var(str(i), **i.assumptions0) for i in eq.atoms(Symbol)]

/Chris

[Mani Chandra]

There is a problem with that procedure when I'm trying to save a function.

In [70]: a = Function('a')

In [71]: pickle.dump(a, file('a.dat', 'w'))

---------------------------------------------------------------------------

PicklingError                             Traceback (most recent call last)

/home/mc/test.py in <module>()

----> 1 

      2 

      3 

      4 

      5 

/usr/lib/python2.7/pickle.pyc in dump(obj, file, protocol)

   1368 

   1369 def dump(obj, file, protocol=None):

-> 1370     Pickler(file, protocol).dump(obj)

   1371 

   1372 def dumps(obj, protocol=None):

/usr/lib/python2.7/pickle.pyc in dump(self, obj)

    222         if self.proto >= 2:

    223             self.write(PROTO + chr(self.proto))

--> 224         self.save(obj)

    225         self.write(STOP)

    226 

/usr/lib/python2.7/pickle.pyc in save(self, obj)

    293             issc = 0

    294         if issc:

--> 295             self.save_global(obj)

    296             return

    297 

/usr/lib/python2.7/pickle.pyc in save_global(self, obj, name, pack)

    746             raise PicklingError(

    747                 "Can't pickle %r: it's not found as %s.%s" %

--> 748                 (obj, module, name))

    749         else:

    750             if klass is not obj:

PicklingError: Can't pickle a: it's not found as sympy.core.function.a

[Aaron Meurer]

This is a known issue. Unfortunately, it seems the pickle does not
work for Function. See
http://code.google.com/p/sympy/issues/detail?id=1198.

Any ideas on how to fix this?

Aaron Meurer

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[Christophe BAL]

Hello.

I don't know if this will hepl you but I've been in trouble when I've tried to pickle class using lambda functions in one attribut.

Best regards.
Christophe.

[Ondrej Certik]

On Fri, Jul 1, 2011 at 8:07 PM, Aaron Meurer <asme...@gmail.com> wrote:
> This is a known issue.  Unfortunately, it seems the pickle does not
> work for Function.  See
> http://code.google.com/p/sympy/issues/detail?id=1198.
>
> Any ideas on how to fix this?

I wonder how this works at live.sympy.org, because there the
expressions are somehow stored in a database. And it works for a
Function.

Ondrej

[Mateusz Paprocki]

Hi,

SymPy Live uses a hack to overcome this. If an expression results in unpicklables, it stores the expression in the database and reevaluates it before every future evaluation.

 

Ondrej

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