[Python-Dev] Does Zip Importer have to be Special?

Brett Cannon brett at python.org
Thu Jul 24 19:48:59 CEST 2014

On Thu Jul 24 2014 at 1:07:12 PM, Phil Thompson <phil at riverbankcomputing.com>

> I have an importer for use in applications that embed an interpreter
> that does a similar job to the Zip importer (except that the storage is
> a C data structure rather than a .zip file). Just like the Zip importer
> I need to import my importer and add it to sys.path_hooks. However the
> earliest opportunity I have to do this is after the Py_Initialize() call
> returns - but this is too late because some parts of the standard
> library have already needed to be imported.
> My current workaround is to include a modified version of _bootstrap.py
> as a frozen module that has the necessary steps added to the end of its
> _install() function.
> The Zip importer doesn't have this problem because it gets special
> treatment - the call to its equivalent code is hard-coded and happens
> exactly when needed.
> What would help is a table of functions that were called where
> _PyImportZip_Init() is currently called. By default the only entry in
> the table would be _PyImportZip_Init. There would be a way of modifying
> the table, either like how PyImport_FrozenModules is handled or how
> Inittab is handled.
> ...or if there is a better solution that I have missed that doesn't
> require a modified _bootstrap.py.

Basically you want a way to specify arguments into
importlib._bootstrap._install() so that sys.path_hooks and sys.meta_path
were configurable instead of hard-coded (it could also be done just past
importlib being installed, but that's a minor detail). Either way there is
technically no reason not to allow for it, just lack of motivation since
this would only come up for people who embed the interpreter AND have a
custom importer which affects loading the stdlib as well (any reason you
can't freeze the stdblib as a solution?).

We could go the route of some static array that people could modify.
Another option would be to allow for the specification of a single function
which is called just prior to importing the rest of the stdlib,

The problem with all of this is you are essentially asking for a hook to
let you have code have access to the interpreter state before it is fully
initialized. Zipimport and the various bits of code that get loaded during
startup are special since they are coded to avoid touching anything that
isn't ready to be used. So if we expose something that allows access prior
to full initialization it would have to be documented as having no
guarantees of interpreter state, etc. so we are not held to some API that
makes future improvements difficult.

IOW allowing for easy patching of Python is probably the best option I can
think of. Would tweaking importlib._bootstrap._install() to accept
specified values for sys.meta_path and sys.path_hooks be enough so that you
can change the call site for those functions?
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