[Python-Dev] Undocumented PEP 302 protocol change by need-for-speed sprint (original) (raw)

Phillip J. Eby pje at telecommunity.com
Thu Jul 20 21:37:25 CEST 2006

At 12:28 PM 7/20/2006 -0700, Brett Cannon wrote:

On 7/20/06, Phillip J. Eby <<mailto:pje at telecommunity.com>pje at telecommunity.com> wrote:

While investigating the need to apply <http://python.org/sf/1525766>http://python.org/sf/1525766 I found that there was a modification to pkgutil during the need-for-speed sprint that affects the PEP 302 protocol in a backwards incompatible way.

Specifically, PEP 302 documents that pathimportercache always contains either importer objects or None. Any code written to obtain importer objects is therefore now broken, because import.c is slapping False in for non-existent filesystem paths. The pkgutil module was then hacked to work around this problem, thereby hiding the breakage from at least the standard library, but not any external libraries that follow the PEP 302 protocol to find importers. There are several options as to how to proceed: 1. Revert the change 2. Document the breakage, update PEP 302, and make everybody update their code 3. Make it not break existing code, by using a NonexistentPathImporter or NullImporter type in place of "False" in sys.pathimportercache. Any thoughts? Revert it. Is it really that much of a bonus to use False over None? Both evaluate to false and both are already singleton so you can use 'is' for testing.

The changed code still uses None. PEP 302 defines None as meaning that a sys.path entry does not have an importer. It's just that the need-for-speed patch adds the use of True and False. None still means "no importer", but True now means "no importer, path exists" and False now means "no importer, path does not exist".

The idea is that import.c can then skip checking the existence of the path when it sees True or False, but it then means that code that gets data from path_importer_cache needs to know about these new special values, or else it will get an attribute error when it tries to call True.find_module().

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