Defer import error for packaging.licenses in environments with packaging<24.2 (#4898)

Author: abravalheri

newsfragments/4898.bugfix.rst

@@ -0,0 +1,4 @@
+Better error messages for ``packaging.licenses`` import errors in environments with ``packaging<24.2``\.
+The import statement was also deferred to spare users that are not using
+license expressions.
+

pyproject.toml

@@ -102,7 +102,6 @@ core = [
 
 	# for distutils
 	"jaraco.functools >= 4",
-	"packaging",
 	"more_itertools",
 ]
 

setuptools/_normalization.py

@@ -4,6 +4,7 @@
 """
 
 import re
+from typing import TYPE_CHECKING
 
 import packaging
 
@@ -148,3 +149,31 @@ 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))
+
+
+def _missing_canonicalize_license_expression(expression: str) -> str:
+    """
+    Defer import error to affect only users that actually use it
+    https://github.com/pypa/setuptools/issues/4894
+    >>> _missing_canonicalize_license_expression("a OR b")
+    Traceback (most recent call last):
+    ...
+    ImportError: ...Cannot import `packaging.licenses`...
+    """
+    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.
+        """
+    )
+
+
+try:
+    from packaging.licenses import (
+        canonicalize_license_expression as _canonicalize_license_expression,
+    )
+except ImportError:  # pragma: nocover
+    if not TYPE_CHECKING:
+        # XXX: pyright is still upset even with # pyright: ignore[reportAssignmentType]
+        _canonicalize_license_expression = _missing_canonicalize_license_expression

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