Show License-Expression if present in package metadata

With Core Metadata 2.4 a new field, License-Expression, has been added.
If it's present, favor it over the deprecated (with PEP 639) legacy
unstructured License field.

Closes: #13112

Author: befeleme

news/13112.feature.rst

@@ -0,0 +1 @@
+Prefer to display ``License-Expression`` in ``pip show`` if metadata version is at least 2.4.

src/pip/_internal/commands/show.py

@@ -66,6 +66,7 @@ class _PackageInfo(NamedTuple):
     author: str
     author_email: str
     license: str
+    license_expression: str
     entry_points: List[str]
     files: Optional[List[str]]
 
@@ -161,6 +162,7 @@ def _get_requiring_packages(current_dist: BaseDistribution) -> Iterator[str]:
             author=metadata.get("Author", ""),
             author_email=metadata.get("Author-email", ""),
             license=metadata.get("License", ""),
+            license_expression=metadata.get("License-Expression", ""),
             entry_points=entry_points,
             files=files,
         )
@@ -180,13 +182,18 @@ def print_results(
         if i > 0:
             write_output("---")
 
+        metadata_version_tuple = tuple(map(int, dist.metadata_version.split(".")))
+
         write_output("Name: %s", dist.name)
         write_output("Version: %s", dist.version)
         write_output("Summary: %s", dist.summary)
         write_output("Home-page: %s", dist.homepage)
         write_output("Author: %s", dist.author)
         write_output("Author-email: %s", dist.author_email)
-        write_output("License: %s", dist.license)
+        if metadata_version_tuple >= (2, 4) and dist.license_expression:
+            write_output("License-Expression: %s", dist.license_expression)
+        else:
+            write_output("License: %s", dist.license)
         write_output("Location: %s", dist.location)
         if dist.editable_project_location is not None:
             write_output(

tests/data/packages/license.dist-0.1-py2.py3-none-any.whl

@@ -217,6 +217,16 @@ def test_all_fields(script: PipTestEnvironment) -> None:
     """
     Test that all the fields are present
     """
+    # future-compat: once pip adopts PEP 639 in pyproject.toml and
+    # its build backend produces metadata 2.4 or greater,
+    # it will display "License-Expression" rather than License
+    verbose = script.pip("show", "--verbose", "pip").stdout
+    match = re.search(r"Metadata-Version:\s(\d+\.\d+)", verbose).group(1)
+    if tuple(map(int, match.split('.'))) >= (2, 4) and "License-Expression" in verbose:
+        license_str = "License-Expression"
+    else:
+        license_str = "License"
+
     result = script.pip("show", "pip")
     lines = result.stdout.splitlines()
     expected = {
@@ -226,7 +236,7 @@ def test_all_fields(script: PipTestEnvironment) -> None:
         "Home-page",
         "Author",
         "Author-email",
-        "License",
+        f"{license_str}",
         "Location",
         "Editable project location",
         "Requires",
@@ -410,3 +420,27 @@ def test_show_populate_homepage_from_project_urls(
     result = script.pip("show", "simple", cwd=pkg_path)
     lines = result.stdout.splitlines()
     assert "Home-page: https://example.com" in lines
+
+
+def test_show_license_expression(script: PipTestEnvironment, data: TestData) -> None:
+    """
+    Show License-Expression if present in metadata >= 2.4.
+    """
+    wheel_file = data.packages.joinpath("license.dist-0.1-py2.py3-none-any.whl")
+    script.pip("install", "--no-index", wheel_file)
+    result = script.pip("show", "license.dist")
+    lines = result.stdout.splitlines()
+    assert "License-Expression: MIT AND MIT-0" in lines
+    assert "License: The legacy license declaration" not in lines
+
+
+def test_show_license_for_metadata_24(script: PipTestEnvironment, data: TestData) -> None:
+    """
+    Show License if License-Expression is not there for metadata >= 2.4.
+    """
+    wheel_file = data.packages.joinpath("license.dist-0.2-py2.py3-none-any.whl")
+    script.pip("install", "--no-index", wheel_file)
+    result = script.pip("show", "license.dist")
+    lines = result.stdout.splitlines()
+    assert "License-Expression: " not in lines
+    assert "License: The legacy license declaration" in lines