lazy loaded clib

[PeterPetrik]

• introduced pygeodiff.shutdown() to correctly unload on windows
• pulled out context from geodifflib to geodiff
• geodifflib is now shared between all instances of pygeodiff.GeoDiff()

latest python had some problems with comments with \ so I replaced it with :

fix #205

[dvdkon]

I rebased this PR on current master.

[wonder-sk]

actually...

• is it correct that GeoDiff class calls shutdown() upon its destruction? I think that may cause a series of loads and unloads of the geodiff C library when GeoDiff is created and deleted in sequence - or not?
• and do I understand it correctly that each GeoDiff has its own GeoDiffLib instance? (the PR description says the geodifflib should be shared now?)

[dvdkon]

• is it correct that GeoDiff class calls shutdown() upon its destruction? I think that may cause a series of loads and unloads of the geodiff C library when GeoDiff is created and deleted in sequence - or not?

Hm, I missed that. I think it doesn't make sense, since GeoDiffLib.shutdown() is also called by GeoDiffLib.__del__. But loading/unloading the library when multiple GeoDiff objects are created with distinct lifetimes is probably unavoidable, if the goal is to unload the library when it's no longer needed.

• and do I understand it correctly that each GeoDiff has its own GeoDiffLib instance? (the PR description says the geodifflib should be shared now?)

Yes, this way we'll end up with multiple GeoDiffLib instances (that will clean up after themselves on Windows, but still). I'll commit a fix.

[volcan01010]

An unsolicited comment from me.

You could use Python's @property decorator here to simplify the lazy loading.

https://realpython.com/python-property/

It would look something like this.

def __init__(self, libname=None):
    ...
    self._clib = None
    self._context = None

@property
def clib(self):
    if self._clib is None:
        self._clib = GeoDiffLib(self.libname)
    return self._clib

@property
def context(self):
    if self._context is None:
        self._context = self.clib.create_context()
        context.callbackLogger = None
    return self._context

def shutdown(self):
    self.clib.destroy_context(self.context)
    self._clib = None
    self._context = None

This means that you don't have to call _lazy_load all the time, as the propery creates the clib and context when they are first accessed. It does mean that the shutdown function will create new clib and context just to destroy them if you call it before you have used them elsewhere.