saving and reloading variables for interactive work

[Christopher T King]

On Wed, 28 Jul 2004, Darren Dale wrote:

> Could you determine the picklable types using something like this? Its 
> kinda ugly, but maybe something related would work:
> 
> myvars = {}
> for key in vars().keys():
> 	myvar = vars()[key]
> 	if str(type(myvar)) == "<type 'module'>": pass
> 	elif: str(type(myvar)) == "<type 'function'>": pass
> 	elif: str(type(myvar)) == "<type 'unpicklable'>": pass
> 	else: myvars.update({key:myvar})

You can do this a bit more directly using the types module to check the 
types (with a little bit of code restructuring):

 from types import *
 myvars = {}
 for key,myvar in vars().items():
 	if not isinstance(myvar,(ModuleType,FunctionType)):
 	    myvars[key]=myvar

A list comprehension would get you a nice, unreadable one-liner if it's so 
desired:

 from types import *
 myvars = dict([(key,myvar) for key,myvar in vars().items() \
	if not isinstance(myvar,(ModuleType,FunctionType))])

The problem with these types is there's no real guarantee what can be 
pickled and what can't, and of those things that can be pickled, what 
makes sense (functions can be pickled, but their code isn't stored).  
There's currently no ispicklable() function

> Maybe a try/except would be appropriate, but the pickle docs ay that an 
> unspecified number of bytes may have been written to the file before the 
> PicklingError exception is raised.

You could use dumps() instead of dump() for testing picklability before 
picklation, though it smells of hackishness:

 def ispicklable(item):
     try:
         pickle.dumps(item)
     except PicklingError:
         return False
     return True

Then you can just use ispicklable() in place of the 'not isinstance()' 
mess I wrote.

I've done some poking though, and there a better way than that.  It seems
all picklable objects define a __reduce__ method, so you could generalize
this check like this:

 def ispicklable(item):
     try:
        item.__reduce__()
     except KeyError,TypeError:
        return False
     return True

Dunno if any of this helps; sorry if parts make no sense, I've retyped 
parts of this a couple times :P