Can no longer import some modules without having ASE installed

[ml-evs]

Python version

3.11.7

Pymatgen version

2024.2.20

Operating system version

Any

Current behavior

I observed this in a fresh virtual environment when trying to install and use mp_api. Before #3619 (or related follow-up commits, can't tell) you were able to import the AseAtomsAdaptor without an error, even if you didn't have ASE installed. This was changed by the creation of the MSONAtoms class which uses Atoms via inheritance and thus is loaded at import time.

Example as posted in #3619 (comment)

>>> from mp_api.client.mprester import MPRester
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/mevans/repos/re2fractive/re2fractive/experiments/2_featurizing_refractive_index_data/.venv-mp-api/lib/python3.11/site-packages/mp_api/client/__init__.py", line 8, in <module>
    from .mprester import MPRester
  File "/home/mevans/repos/re2fractive/re2fractive/experiments/2_featurizing_refractive_index_data/.venv-mp-api/lib/python3.11/site-packages/mp_api/client/mprester.py", line 10, in <module>
    from emmet.core.electronic_structure import BSPathType
  File "/home/mevans/repos/re2fractive/re2fractive/experiments/2_featurizing_refractive_index_data/.venv-mp-api/lib/python3.11/site-packages/emmet/core/electronic_structure.py", line 11, in <module>
    from pymatgen.analysis.magnetism.analyzer import (
  File "/home/mevans/repos/re2fractive/re2fractive/experiments/2_featurizing_refractive_index_data/.venv-mp-api/lib/python3.11/site-packages/pymatgen/analysis/magnetism/__init__.py", line 5, in <module>
    from pymatgen.analysis.magnetism.analyzer import (
  File "/home/mevans/repos/re2fractive/re2fractive/experiments/2_featurizing_refractive_index_data/.venv-mp-api/lib/python3.11/site-packages/pymatgen/analysis/magnetism/analyzer.py", line 24, in <module>
    from pymatgen.transformations.advanced_transformations import MagOrderingTransformation, MagOrderParameterConstraint
  File "/home/mevans/repos/re2fractive/re2fractive/experiments/2_featurizing_refractive_index_data/.venv-mp-api/lib/python3.11/site-packages/pymatgen/transformations/advanced_transformations.py", line 33, in <module>
    from pymatgen.io.ase import AseAtomsAdaptor
  File "/home/mevans/repos/re2fractive/re2fractive/experiments/2_featurizing_refractive_index_data/.venv-mp-api/lib/python3.11/site-packages/pymatgen/io/ase.py", line 44, in <module>
    class MSONAtoms(Atoms, MSONable):
                    ^^^^^
NameError: name 'Atoms' is not defined

Expected Behavior

The module should still be importable in cases where ase is not available, as it was before.

Minimal example

python -m venv .venv
. .venv/bin/activate
pip install pymatgen
$ python -c "from pymatgen.analysis.magnetism.analyzer import *"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/mevans/repos/re2fractive/re2fractive/experiments/2_featurizing_refractive_index_data/.venv-pymatgen/lib/python3.11/site-packages/pymatgen/analysis/magnetism/__init__.py", line 5, in <module>
    from pymatgen.analysis.magnetism.analyzer import (
  File "/home/mevans/repos/re2fractive/re2fractive/experiments/2_featurizing_refractive_index_data/.venv-pymatgen/lib/python3.11/site-packages/pymatgen/analysis/magnetism/analyzer.py", line 24, in <module>
    from pymatgen.transformations.advanced_transformations import MagOrderingTransformation, MagOrderParameterConstraint
  File "/home/mevans/repos/re2fractive/re2fractive/experiments/2_featurizing_refractive_index_data/.venv-pymatgen/lib/python3.11/site-packages/pymatgen/transformations/advanced_transformations.py", line 33, in <module>
    from pymatgen.io.ase import AseAtomsAdaptor
  File "/home/mevans/repos/re2fractive/re2fractive/experiments/2_featurizing_refractive_index_data/.venv-pymatgen/lib/python3.11/site-packages/pymatgen/io/ase.py", line 44, in <module>
    class MSONAtoms(Atoms, MSONable):
                    ^^^^^
NameError: name 'Atoms' is not defined

which runs correctly if you pip install pymatgen==2024.2.8.

Relevant files to reproduce this bug

No response

[ml-evs]

Also just a comment that I would've expected this to be picked up by a linter; for me Atoms is possible unbound is flagged up by pyright but perhaps mypy is less picky as configured?

[DanielYang59]

I noticed the same issue while trying to export XRD pattern for example here with mp_api with the following demo code:

from mp_api.client import MPRester
from pymatgen.analysis.diffraction.xrd import XRDCalculator
from pymatgen.symmetry.analyzer import SpacegroupAnalyzer

with MPRester(api_key="<enter your api key>") as mpr:
    # first retrieve the relevant structure
    structure = mpr.get_structure_by_material_id("mp-728693")

# important to use the conventional structure to ensure
# that peaks are labelled with the conventional Miller indices
sga = SpacegroupAnalyzer(structure)
conventional_structure = sga.get_conventional_standard_structure()

# this example shows how to obtain an XRD diffraction pattern
# these patterns are calculated on-the-fly from the structure
calculator = XRDCalculator(wavelength="CuKa")
pattern = calculator.get_pattern(conventional_structure)

The ImportError is captured in

pymatgen/pymatgen/io/ase.py

Lines 25 to 34 in 57842a7

	try:
	from ase.atoms import Atoms
	from ase.calculators.singlepoint import SinglePointDFTCalculator
	from ase.constraints import FixAtoms
	from ase.io.jsonio import decode, encode
	from ase.spacegroup import Spacegroup
	ase_loaded = True
	except ImportError:
	ase_loaded = False

So no explicit error like no module named ase is raised.