qspace_light: explain the CellConstructor requirement instead of a TypeError

The light mode calls DiagonalizeSupercell(q_only=True), which only newer
CellConstructor releases provide. Against an older one the flag failed with
'DiagonalizeSupercell() got an unexpected keyword argument q_only', raised
from inside __setattr__ and pointing at neither the flag nor the requirement.

A module-level probe now reports the capability, so the constructor raises an
explanatory error, and tests/test_qspace_light skips instead of failing when
the installed CellConstructor cannot support it. Nothing else changes: the
default path never touches q_only.

Author: SorBalda

Modules/Ensemble.py

@@ -107,6 +107,17 @@ def Main(self):
 # Deprecated alias kept for backward compatibility: it only tells whether a
 # Julia backend is installed, the runtime is not initialized at import time.
 __JULIA_EXT__ = JuliaExt.available()
+
+# The qspace_light path is built on DiagonalizeSupercell(q_only=True), which
+# only newer CellConstructor releases provide. Probe it once, so that asking
+# for the flag against an older one gives an explanatory error instead of a
+# TypeError raised from inside __setattr__.
+import inspect as _inspect
+try:
+    _CC_HAS_Q_ONLY = "q_only" in _inspect.signature(
+        CC.Phonons.Phonons.DiagonalizeSupercell).parameters
+except (AttributeError, TypeError, ValueError):
+    _CC_HAS_Q_ONLY = False
 __JULIA_ERROR__ = ""
 
 
@@ -351,6 +362,14 @@ def __setattr__(self, name, value):
 
         if name == "dyn_0":
             if self.__dict__.get("qspace_light", False):
+                if not _CC_HAS_Q_ONLY:
+                    raise RuntimeError(
+                        "Ensemble(qspace_light=True) requires a CellConstructor "
+                        "providing DiagonalizeSupercell(q_only=True); the "
+                        "installed one does not have it, so the light mode "
+                        "cannot avoid the dense (3N,3N) allocation it exists "
+                        "to avoid. Use qspace_light=False (the default), or "
+                        "install the CellConstructor that provides q_only.")
                 # q-space light mode: q_only=True never assembles the (3N,3N)
                 # supercell polarization matrix. Only the O(N) frequencies,
                 # the q-space pols and the sorted-mode -> (iq, band) maps are

tests/test_qspace_light/test_qspace_light.py

@@ -24,6 +24,16 @@
 import cellconstructor.Phonons
 import sscha, sscha.Ensemble
 
+# The light mode is built on DiagonalizeSupercell(q_only=True), which only
+# newer CellConstructor releases provide. Skip the whole module rather than
+# fail when it is missing: nothing here can run, and the failure would say
+# nothing about this package.
+if not bool(getattr(sscha.Ensemble, "_CC_HAS_Q_ONLY", False)):
+    pytest.skip(
+        "the installed CellConstructor has no "
+        "DiagonalizeSupercell(q_only=True), which qspace_light requires",
+        allow_module_level=True)
+
 _JULIA = bool(getattr(sscha.Ensemble, "__JULIA_EXT__", False))
 needs_julia = pytest.mark.skipif(
     not _JULIA, reason="Julia Fourier backend not available")