Add FrozenImporter.get_filename().

[ericsnowcurrently]

BPO	45659
Nosy	@ericsnowcurrently, @FFY00

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-10-28.18:52:00.285>
labels = ['type-bug', 'library', '3.11']
title = 'Add FrozenImporter.get_filename().'
updated_at = <Date 2021-10-28.19:00:44.652>
user = 'https://github.com/ericsnowcurrently'

bugs.python.org fields:

activity = <Date 2021-10-28.19:00:44.652>
actor = 'eric.snow'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-10-28.18:52:00.285>
creator = 'eric.snow'
dependencies = []
files = []
hgrepos = []
issue_num = 45659
keywords = []
message_count = 2.0
messages = ['405225', '405226']
nosy_count = 2.0
nosy_names = ['eric.snow', 'FFY00']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue45659'
versions = ['Python 3.11']

[ericsnowcurrently]

Now that __file__ is set on frozen (stdlib) modules, we can add an implementation for FrozenImporter.get_filename(). This would make FrozenImporter and implementation of ExecutionLoader. [1]

There is a caveat: in spec_from_loader() [2] we infer the presence of get_filename() to mean the loader is file-based, which FrozenImporter isn't.
In that case we call spec_from_file_location() [3], which should not be used for frozen modules. Most importantly, spec.origin for frozen modules should remain "frozen", spec.cached should remain None, and spec.has_location should remain False. That's because the module was imported frozen and not from a file (even though it originated in a file).

[1] https://docs.python.org/3/library/importlib.html#importlib.abc.ExecutionLoader
[2] in Lib/import/_bootstrap.py
[3] in Lib/import/_bootstrap_external.py

[ericsnowcurrently]

Note that there isn't any intent currently to implement the FileLoader ABC [1], which would require having FrozenImporter instances, adding the "name" and "path" attributes to them, as well as the get_data() method. The same goes for the SourceLoader ABC. [2]

Again, frozen modules don't have files but instead were only derived from files during the build process. For frozen stdlib modules we happen to know where to find the corresponding source files.

[1] https://docs.python.org/3/library/importlib.html#importlib.abc.FileLoader
[2] https://docs.python.org/3/library/importlib.html#importlib.abc.SourceLoader