Move desimodel footprint imports to just-in-time in io.py

[Copilot]

Importing desitarget.io unconditionally triggered a desimodel data-not-installed warning even when no tile-related functions were called, because from desimodel.footprint import is_point_in_desi, tiles2pix sat at module level.

Changes

• Removed module-level import of is_point_in_desi and tiles2pix from desimodel.footprint
• Added just-in-time imports inside the three functions that actually use these symbols:
  • read_targets_in_tiles_quick — imported unconditionally at function entry (both symbols always used)
  • read_targets_in_quick — imported inside if shape == 'tiles': branch (only needed for that shape)
  • read_targets_in_tiles — imported before first use (both symbols used on all non-early-return paths)

# Before: triggers desimodel warning on every `import desitarget.io`
from desimodel.footprint import is_point_in_desi, tiles2pix  # line 34, module level

# After: desimodel only loaded when tile-reading functions are actually called
def read_targets_in_tiles_quick(...):
    ...
    from desimodel.footprint import is_point_in_desi, tiles2pix
    pixlist = tiles2pix(nside, tiles=tiles)

Python's import cache (sys.modules) means the deferred import has no repeated overhead after the first call.

• Fixes just-in-time imports in io.py #893

[Copilot]

Pull request overview

This PR prevents import desitarget.io from unconditionally importing desimodel (and triggering desimodel data-not-installed warnings) by moving desimodel.footprint imports to just-in-time imports inside the tile-reading functions that actually use them.

Changes:

• Removed the module-level from desimodel.footprint import is_point_in_desi, tiles2pix import.
• Added deferred (function-scope) imports of is_point_in_desi / tiles2pix within the three tile-based readers that use them.
• Kept non-tile call paths free of desimodel imports, matching the intent of issue #893.

Comments suppressed due to low confidence (2)

py/desitarget/io.py:4069

• These comments suggest desimodel is a "required dependency", but this code is actually deferring an optional dependency import until the shape == 'tiles' path is used. Reword (and consider dropping the redundant note) to clarify the intent is avoiding desimodel import at module import time.

        # import desimodel only when needed to avoid required dependency
        # Note: `is_point_in_desi` will be used below

py/desitarget/io.py:4253

• Comment wording is misleading: desimodel is still required at runtime for tile-based filtering, but the intent is to avoid importing desimodel when desitarget.io is imported. Reword to clarify this is a deferred import of an optional dependency.

    # import only when needed to avoid desimodel required dependency

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[sbailey]

@moustakas please confirm that this branch fixed your fastspecfit case reported in #894 .

Tests are failing due to inability to download DESI_SURVEYOPS shapshot from data.desi.lbl.gov .

[moustakas]

Yes, looks good, thanks.

[coveralls]

coverage: 40.674% (+5.2%) from 35.469% — copilot/just-in-time-imports-io into main