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[Python-Dev] requirements for moving __import__ over toimportlib?
Nick Coghlanncoghlan at gmail.com
Thu Feb 9 02:26:01 CET 2012
On Thu, Feb 9, 2012 at 2:09 AM, Antoine Pitrou <solipsis at pitrou.net> wrote:> I guess my point was: why is there a function call in that case? The> "import" statement could look up sys.modules directly.> Or the built-in __import__ could still be written in C, and only defer> to importlib when the module isn't found in sys.modules.> Practicality beats purity.I quite like the idea of having builtin __import__ be a *very* thinveneer around importlib that just does the "is this in sys.modulesalready so we can just return it from there?" checks and delegatesother more complex cases to Python code in importlib.Poking around in importlib.__import__ [1] (as well asimportlib._gcd_import), I'm thinking what we may want to do is breakup the logic a bit so that there are multiple helper functions that aC version can call back into so that we can optimise certain simplecode paths to not call back into Python at all, and others to only doso selectively.Step 1: separate out the "fromlist" processing from __import__ into aseparate helper function def _process_fromlist(module, fromlist): # Perform any required imports as per existing code: #http://hg.python.org/cpython/file/aba513307f78/Lib/importlib/_bootstrap.py#l987Step 2: separate out the relative import resolution from _gcd_importinto a separate helper function. def _resolve_relative_name(name, package, level): assert hasattr(name, 'rpartition') assert hasattr(package, 'rpartition') assert level > 0 name = # Recalculate as per the existing code: #http://hg.python.org/cpython/file/aba513307f78/Lib/importlib/_bootstrap.py#l889 return nameStep 3: Implement builtin __import__ in C (pseudo-code below): def __import__(name, globals={}, locals={}, fromlist=[], level=0): if level > 0: name = importlib._resolve_relative_import(name) try: module = sys.modules[name] except KeyError: # Not cached yet, need to invoke the full import machinery # We already resolved any relative imports though, so # treat it as an absolute import return importlib.__import__(name, globals, locals, fromlist, 0) # Got a hit in the cache, see if there's any more work to do if not fromlist: # Duplicate relevant importlib.__import__ logic as C code # to find the right module to return from sys.modules elif hasattr(module, "__path__"): importlib._process_fromlist(module, fromlist) return moduleThis would then be similar to the way main.c already works when itinteracts with runpy - simple cases are handled directly in C, morecomplex cases get handed over to the Python module.Cheers,Nick.[1]http://hg.python.org/cpython/file/default/Lib/importlib/_bootstrap.py#l950-- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia
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