Apply suggestions from code review

Co-authored-by: Brett Cannon <brett@python.org>

docs/licenses.rst

@@ -25,15 +25,14 @@ Reference
 
 .. function:: canonicalize_license_expression(raw_license_expression)
 
-    This function takes a valid Python package or extra name, and returns the
-    normalized form of it.
+    This function takes a valid License-Expression, and returns the normalized form of it.
 
     The return type is typed as :class:`NormalizedLicenseExpression`. This allows type
     checkers to help require that a string has passed through this function
     before use.
 
     :param str raw_license_expression: The License-Expression to canonicalize.
-    :raises InvalidLicenseExpression: If the License-Expression is invalid due to and
+    :raises InvalidLicenseExpression: If the License-Expression is invalid due to an
         invalid/unknown license identifier or invalid syntax.
 
     .. doctest::

src/packaging/licenses/__init__.py

@@ -48,7 +48,7 @@
 
 
 class InvalidLicenseExpression(ValueError):
-    """Raised when a license-expression string
+    """Raised when a license-expression string is invalid
 
     >>> canonicalize_license_expression("invalid")
     Traceback (most recent call last):
@@ -59,19 +59,19 @@ class InvalidLicenseExpression(ValueError):
 
 def canonicalize_license_expression(
     raw_license_expression: str,
-) -> str | NormalizedLicenseExpression:
-    if raw_license_expression == "":
+) -> NormalizedLicenseExpression:
+    if not raw_license_expression:
         message = f"Invalid license expression: {raw_license_expression!r}"
         raise InvalidLicenseExpression(message)
 
     # Pad any parentheses so tokenization can be achieved by merely splitting on
     # white space.
     license_expression = raw_license_expression.replace("(", " ( ").replace(")", " ) ")
-
+    licenseref_prefix = "LicenseRef-"
     license_refs = {
-        ref.lower(): "LicenseRef-" + ref[11:]
+        ref.lower(): "LicenseRef-" + ref[len(licenseref_prefix):]
         for ref in license_expression.split()
-        if ref.lower().startswith("licenseref-")
+        if ref.lower().startswith(licenseref_prefix.lower())
     }
 
     # Normalize to lower case so we can look up licenses/exceptions
@@ -97,11 +97,11 @@ def canonicalize_license_expression(
 
     python_expression = " ".join(python_tokens)
     try:
-        result = eval(python_expression)
+        invalid = eval(python_expression)
     except Exception:
-        result = True
+        invalid = True
 
-    if result is not False:
+    if invalid is not False:
         message = f"Invalid license expression: {raw_license_expression!r}"
         raise InvalidLicenseExpression(message) from None
 

src/packaging/metadata.py

@@ -644,7 +644,7 @@ def _process_requires_dist(
         else:
             return reqs
 
-    def _process_license_expression(self, value: str) -> str | None:
+    def _process_license_expression(self, value: str) -> NormalizedLicenseExpression | None:
         try:
             return licenses.canonicalize_license_expression(value)
         except ValueError as exc:
@@ -816,7 +816,7 @@ def from_email(cls, data: bytes | str, *, validate: bool = True) -> Metadata:
     """:external:ref:`core-metadata-maintainer-email`"""
     license: _Validator[str | None] = _Validator()
     """:external:ref:`core-metadata-license`"""
-    license_expression: _Validator[str | None] = _Validator(added="2.4")
+    license_expression: _Validator[NormalizedLicenseExpression | None] = _Validator(added="2.4")
     """:external:ref:`core-metadata-license-expression`"""
     license_files: _Validator[list[str] | None] = _Validator(added="2.4")
     """:external:ref:`core-metadata-license-file`"""