Defer import error for packaging.licenses in environments with packagin<24.2

Author: abravalheri

setuptools/_normalization.py

@@ -148,3 +148,21 @@ def safer_best_effort_version(value: str) -> str:
     # See bdist_wheel.safer_verion
     # TODO: Replace with only safe_version in the future (no need for best effort)
     return filename_component(best_effort_version(value))
+
+
+try:
+    from packaging.licenses import (
+        canonicalize_license_expression as _canonicalize_license_expression,
+    )
+except ImportError:
+
+    def _canonicalize_license_expression(expression: str) -> str:
+        # Defer import error to affect only users that actually use it
+        # https://github.com/pypa/setuptools/issues/4894
+        raise ImportError(
+            "Cannot import `packaging.licenses`."
+            """
+            Setuptools>=77.0.0 requires "packaging>=24.2" to work properly.
+            Please make sure you have a suitable version installed.
+            """
+        )

setuptools/dist.py

@@ -12,7 +12,6 @@
 from typing import TYPE_CHECKING, Any, Union
 
 from more_itertools import partition, unique_everseen
-from packaging.licenses import canonicalize_license_expression
 from packaging.markers import InvalidMarker, Marker
 from packaging.specifiers import InvalidSpecifier, SpecifierSet
 from packaging.version import Version
@@ -24,6 +23,7 @@
     command as _,  # noqa: F401 # imported for side-effects
 )
 from ._importlib import metadata
+from ._normalization import _canonicalize_license_expression
 from ._path import StrPath
 from ._reqs import _StrOrIter
 from .config import pyprojecttoml, setupcfg
@@ -423,7 +423,7 @@ def _finalize_license_expression(self) -> None:
         license_expr = self.metadata.license_expression
         if license_expr:
             str_ = _static.Str if _static.is_static(license_expr) else str
-            normalized = str_(canonicalize_license_expression(license_expr))
+            normalized = str_(_canonicalize_license_expression(license_expr))
             if license_expr != normalized:
                 InformationOnly.emit(f"Normalizing '{license_expr}' to '{normalized}'")
                 self.metadata.license_expression = normalized