mercurial: support loading modules from zipimporter The previous refactor to module importing broke module loading when mercurial.* modules were loaded from a zipfile (using a zipimporter). This scenario is likely encountered when using py2exe. Supporting zipimporter and the traditional importer side-by-side turns out to be quite a pain. In Python 2.x, the standard, file-based import mechanism is partially implemented in C. The sys.meta_path and sys.path_hooks hook points exist to allow custom importers in Python/userland. zipimport.zipimporter and our "hgimporter" class from earlier in this patch series are 2 of these. In a standard Python installation (no matter if running in py2exe or similar or not), zipimport.zipimporter appears to be registered in sys.path_hooks. This means that as each sys.path entry is consulted, it will ask zipimporter if it supports that path and zipimporter will be used if that entry is a zip file. In a py2exe environment, sys.path contains an entry with the path to the zip file containing the Python standard library along with Mercurial's Python files. The way the importer mechanism works is the first importer that declares knowledge of a module (via find_module() returning an object) gets to load it. Since our "hgimporter" is registered in sys.meta_path and returns an interest in specific mercurial.* modules, the zipimporter registered on sys.path_hooks never comes into play for these modules. So, we need to be zipimporter aware and call into zipimporter to load modules. This patch teaches "hgimporter" how to call out into zipimporter when necessary. We detect the necessity of zipimporter by looking at the loader for the "mercurial" module. If it is a zipimporter instance, we load via zipimporter. The behavior of zipimporter is a bit wonky. You appear to need separate zipimporter instances for each directory in the zip file. I'm not sure why this is. I suspect it has something to do with the low-level importing mechanism (implemented in C) operating on a per-directory basis. PEP-302 makes some references to this. I was not able to get a zipimporter to import modules outside of its immediate directory no matter how I specified the module name. This is why we use separate zipimporter instances for the ".zip/mercurial" and ".zip/mercurial/pure" locations. The zipimporter documentation for Python 2.7 explicitly states that zipimporter does not import dynamic modules (C extensions). Yet from a py2exe distribution on Windows - where the .pyd files are *not* in the zip archive - zipimporter imported these dynamic modules just fine! I'm not sure if dynamic modules can't be imported from *inside* the zip archive or whether zipimporter looks for dynamic modules outside the zip archive. All I know is zipimporter does manage to import the .pyd files on Windows and this patch makes our new importer compatible with py2exe. In the ideal world, We'd probably reimplement or fall back to parts of the built-in import mechanism instead of handling zipimporter specially. After all, if we're loading Mercurial modules via something that isn't the built-in file-based importer or zipimporter, our custom importer will likely fail because it doesn't know how to call into it. I'd like to think that we'll never encounter this in the wild, but you never know. If we do encounter it, we can come up with another solution. It's worth nothing that Python 3 has moved a lot of the importing code from C to Python. Python 3 gives you near total control over the import mechanism. So in the very distant future when Mercurial drops Python 2 support, it's likely that our custom importer code can be refactored to something a bit saner.