ENH: do not populate defaultdict on lookup

Looking up keys in a default dict adds these keys to the dictionary
with their default value when they are not present. This is an issue
if the lookup happens while the dictionary is being iterated. Avoid it.

Author: dnicolodi

mesonpy/__init__.py

@@ -272,12 +272,12 @@ def _wheel_files(self) -> DefaultDict[str, List[Tuple[pathlib.Path, str]]]:
 
     @property
     def _has_internal_libs(self) -> bool:
-        return bool(self._wheel_files['mesonpy-libs'])
+        return bool(self._wheel_files.get('mesonpy-libs'))
 
     @property
     def _has_extension_modules(self) -> bool:
         # Assume that all code installed in {platlib} is Python ABI dependent.
-        return bool(self._wheel_files['platlib'])
+        return bool(self._wheel_files.get('platlib'))
 
     @property
     def normalized_name(self) -> str: