Provide importlib.util.lazy_import helper function

[ncoghlan]

BPO	32192
Nosy	@warsaw, @brettcannon, @ncoghlan, @ericsnowcurrently, @nanjekyejoannah, @iritkatriel
PRs	bpo-32192: A basic lazy importer example #21330

^{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 = <Date 2020-10-05.18:34:50.201>
created_at = <Date 2017-12-01.08:50:41.820>
labels = ['3.8', 'type-feature', '3.7', 'docs']
title = 'Provide importlib.util.lazy_import helper function'
updated_at = <Date 2020-10-05.18:34:50.200>
user = 'https://github.com/ncoghlan'

bugs.python.org fields:

activity = <Date 2020-10-05.18:34:50.200>
actor = 'brett.cannon'
assignee = 'docs@python'
closed = True
closed_date = <Date 2020-10-05.18:34:50.201>
closer = 'brett.cannon'
components = ['Documentation']
creation = <Date 2017-12-01.08:50:41.820>
creator = 'ncoghlan'
dependencies = []
files = []
hgrepos = []
issue_num = 32192
keywords = ['patch']
message_count = 6.0
messages = ['307370', '307396', '307481', '373620', '377893', '378073']
nosy_count = 7.0
nosy_names = ['barry', 'brett.cannon', 'ncoghlan', 'docs@python', 'eric.snow', 'nanjekyejoannah', 'iritkatriel']
pr_nums = ['21330']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue32192'
versions = ['Python 3.7', 'Python 3.8']

[ncoghlan]

While importlib provides all the *pieces* to implement lazy imports, we don't actually provide a clear way of chaining them together into a lazy import operation. Without any error checking, that looks like:

    import sys
    import importlib.util
    def lazy_import(name):
        spec = importlib.util.find_spec(name)
        loader = importlib.util.LazyLoader(spec.loader)
        spec.loader = loader
        module = importlib.util.module_from_spec(spec)
        sys.modules[name] = module
        loader.exec_module(module)
        return module

    >>> lazy_typing = lazy_import("typing")
    >>> lazy_typing.TYPE_CHECKING
    False

I'm thinking it may make sense to just provide a robust implementation of that, and accept that it may lead to some bug reports that are closed with "You need to fix the module you're loading to be compatible with lazy imports"

[brettcannon]

Already planning this as a PyPI package (at least to start). See https://notebooks.azure.com/Brett/libraries/di2Btqj7zSI/html/Lazy%20importing.ipynb for the design.

[ncoghlan]

Maintaining the actual implementation as a third party module seems like a good idea to me, so I'm marking this as a documentation issue instead.

The idea would be to add this as an example of a very basic lazy importer under https://docs.python.org/3/library/importlib.html#examples

[nanjekyejoannah]

New changeset 8dd32fe by Joannah Nanjekye in branch 'master':
bpo-32192: A basic lazy importer example (GH-21330)
8dd32fe

[iritkatriel]

This seems complete. Can is be closed?

[brettcannon]

Yep, I think the example is enough to close this. Thanks!