Make imports lazy (to speed up asdf extension)

[braingram]

The general idea is to speed up the asdf entry point (which is loaded when the asdf extension manager is created). The entrypoint lists gwcs.extensions:

gwcs/pyproject.toml

Line 35 in 53025a9

gwcs = "gwcs.extension:get_extensions"

This will trigger the * imports in gwcs.__init__.py:

gwcs/gwcs/__init__.py

Lines 69 to 72 in 53025a9

	from .wcs import * # noqa
	from .wcstools import * # noqa
	from .coordinate_frames import * # noqa
	from .selector import * # noqa

which end up importing lots of "heavy hitters" (like scipy, big parts of astropy etc).

Minimal code for testing the entry point is:

from importlib_metadata import entry_points
list(entry_points(group="asdf.extensions", name="gwcs"))[0].load()

To check which modules this import (and ignoring asdf as that's unavoidable):

import asdf
import sys
from importlib_metadata import entry_points
pre_modules = set(sys.modules.keys())
list(entry_points(group="asdf.extensions", name="gwcs"))[0].load()
imported = pre_modules ^ set(sys.modules.keys())

With the current gwcs this imports 747 modules including many parts of scipy and astropy and on my system the entry point loading takes ~600ms.

With this PR the same code imports 90 modules with none from scipy and takes ~250ms. The remaining imports are as follows:

Click to expand

{'asdf._core',
 'asdf._core._integration',
 'asdf_astropy',
 'asdf_astropy._astropy_init',
 'asdf_astropy._version',
 'asdf_astropy.converters',
 'asdf_astropy.converters.coordinates',
 'asdf_astropy.converters.coordinates.angle',
 'asdf_astropy.converters.coordinates.earth_location',
 'asdf_astropy.converters.coordinates.frame',
 'asdf_astropy.converters.coordinates.representation',
 'asdf_astropy.converters.coordinates.sky_coord',
 'asdf_astropy.converters.coordinates.spectral_coord',
 'asdf_astropy.converters.fits',
 'asdf_astropy.converters.fits.fits',
 'asdf_astropy.converters.helpers',
 'asdf_astropy.converters.table',
 'asdf_astropy.converters.table.table',
 'asdf_astropy.converters.time',
 'asdf_astropy.converters.time.time',
 'asdf_astropy.converters.time.time_delta',
 'asdf_astropy.converters.transform',
 'asdf_astropy.converters.transform.compound',
 'asdf_astropy.converters.transform.core',
 'asdf_astropy.converters.transform.functional_models',
 'asdf_astropy.converters.transform.mappings',
 'asdf_astropy.converters.transform.math_functions',
 'asdf_astropy.converters.transform.polynomial',
 'asdf_astropy.converters.transform.projections',
 'asdf_astropy.converters.transform.properties',
 'asdf_astropy.converters.transform.rotations',
 'asdf_astropy.converters.transform.spline',
 'asdf_astropy.converters.transform.tabular',
 'asdf_astropy.converters.unit',
 'asdf_astropy.converters.unit.equivalency',
 'asdf_astropy.converters.unit.magunit',
 'asdf_astropy.converters.unit.quantity',
 'asdf_astropy.converters.unit.unit',
 'asdf_astropy.converters.utils',
 'asdf_astropy.integration',
 'asdf_astropy.resources',
 'asdf_coordinates_schemas',
 'asdf_coordinates_schemas._version',
 'asdf_coordinates_schemas.integration',
 'asdf_standard.integration',
 'asdf_transform_schemas',
 'asdf_transform_schemas._version',
 'asdf_transform_schemas.integration',
 'asdf_unit_schemas',
 'asdf_unit_schemas._version',
 'asdf_unit_schemas.integration',
 'asdf_wcs_schemas',
 'asdf_wcs_schemas._version',
 'asdf_wcs_schemas.integration',
 'asdf_wcs_schemas.resources',
 'astropy',
 'astropy._version',
 'astropy.config',
 'astropy.config.configuration',
 'astropy.config.paths',
 'astropy.extern',
 'astropy.extern.configobj',
 'astropy.extern.configobj.configobj',
 'astropy.extern.configobj.validate',
 'astropy.logger',
 'astropy.tests',
 'astropy.tests.runner',
 'astropy.utils',
 'astropy.utils._compiler',
 'astropy.utils.codegen',
 'astropy.utils.compat',
 'astropy.utils.compat.misc',
 'astropy.utils.compat.numpycompat',
 'astropy.utils.decorators',
 'astropy.utils.exceptions',
 'astropy.utils.introspection',
 'astropy.utils.misc',
 'astropy.utils.shapes',
 'astropy.utils.state',
 'astropy.version',
 'difflib',
 'glob',
 'gwcs',
 'gwcs.converters',
 'gwcs.converters.geometry',
 'gwcs.converters.selector',
 'gwcs.converters.spectroscopy',
 'gwcs.converters.wcs',
 'gwcs.extension',
 'shlex'}

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.27%. Comparing base (eb9d316) to head (6b5c599).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #502      +/-   ##
==========================================
- Coverage   87.28%   87.27%   -0.01%     
==========================================
  Files          22       21       -1     
  Lines        3821     3820       -1     
==========================================
- Hits         3335     3334       -1     
  Misses        486      486              

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