add build-lower-constraints command

Add a command to merge lower constraints lists together, as a
precursor to removing the global lower-constraints.txt list in this
repository.

Change-Id: Ia238607b9feb292842971d0fc161ae6294150e1f
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

README.rst

@@ -51,6 +51,26 @@ are normally constrained::
 
   edit-constraints oslo.db "-e file://opt/stack/oslo.db#egg=oslo.db"
 
+build-lower-constraints
+-----------------------
+
+Combine multiple lower-constraints.txt files to produce a list of the
+highest version of each package mentioned in the files. This can be
+used to produce the "highest minimum" for a global lower constraints
+list (a.k.a., the "TJ Maxx").
+
+To use the script, run::
+
+    $ tox -e venv -- build-lower-constraints input1.txt input2.txt
+
+Where the input files are lower-constraints.txt or requirements.txt
+files from one or more projects.
+
+If the inputs are requirements files, a lower constraints list for the
+requirements is produced. If the inputs are lower-constraints.txt, the
+output includes the highest version of each package referenced in the
+files.
+
 Proposing changes
 =================
 

openstack_requirements/cmds/build_lower_constraints.py

@@ -0,0 +1,72 @@
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+# implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Merge multiple lower-constraints.txt files to find the highest values.
+
+"""
+
+import argparse
+import collections
+
+from openstack_requirements import requirement
+
+import packaging.specifiers
+import packaging.version
+
+
+def read_requirements_file(filename):
+    with open(filename, 'rt') as f:
+        body = f.read()
+    return requirement.parse(body)
+
+
+def get_requirements_version(req):
+    """Find the version for a requirement.
+
+    Use the version attached to >=, ==, or ===, depending on the type
+    of input requirement.
+
+    """
+    for specifier in packaging.specifiers.SpecifierSet(req.specifiers):
+        if '>=' in specifier.operator or '==' in specifier.operator:
+            return packaging.version.parse(specifier.version)
+    raise ValueError('could not find version for {}'.format(req))
+
+
+def merge_constraints_sets(constraints_sets):
+    "Generator of Requirements with the maximum version for each constraint."
+    all_constraints = collections.defaultdict(list)
+    for constraints_set in constraints_sets:
+        for constraint_name, constraint in constraints_set.items():
+            all_constraints[constraint_name].extend(constraint)
+    for constraint_name, constraints in sorted(all_constraints.items()):
+        val = max((c[0] for c in constraints), key=get_requirements_version)
+        yield val.to_line()
+
+
+def main():
+    parser = argparse.ArgumentParser()
+    parser.add_argument(
+        'lower_constraints',
+        nargs='+',
+        help='lower-constraints.txt files',
+    )
+    args = parser.parse_args()
+
+    constraints_sets = [
+        read_requirements_file(filename)
+        for filename in args.lower_constraints
+    ]
+
+    merged = list(merge_constraints_sets(constraints_sets))
+    print(''.join(merged))

openstack_requirements/tests/test_build_lower_constraints.py

@@ -0,0 +1,57 @@
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+import testtools
+
+from openstack_requirements.cmds import build_lower_constraints
+from openstack_requirements import requirement
+
+
+class BuildLowerConstraintsTest(testtools.TestCase):
+
+    def test_one_input_file(self):
+        inputs = [
+            requirement.parse('package==1.2.3'),
+        ]
+        expected = [
+            'package==1.2.3\n',
+        ]
+        self.assertEqual(
+            expected,
+            list(build_lower_constraints.merge_constraints_sets(inputs))
+        )
+
+    def test_two_input_file_same(self):
+        inputs = [
+            requirement.parse('package==1.2.3'),
+            requirement.parse('package==1.2.3'),
+        ]
+        expected = [
+            'package==1.2.3\n',
+        ]
+        self.assertEqual(
+            expected,
+            list(build_lower_constraints.merge_constraints_sets(inputs))
+        )
+
+    def test_two_input_file_differ(self):
+        inputs = [
+            requirement.parse('package==1.2.3'),
+            requirement.parse('package==4.5.6'),
+        ]
+        expected = [
+            'package==4.5.6\n',
+        ]
+        self.assertEqual(
+            expected,
+            list(build_lower_constraints.merge_constraints_sets(inputs))
+        )

setup.cfg

@@ -41,3 +41,4 @@ console_scripts =
     normalize-requirements = openstack_requirements.cmds.normalize_requirements:main
     check-python2-support = openstack_requirements.cmds.check_py2:main
     check-constraints = openstack_requirements.cmds.check_exists:main
+    build-lower-constraints = openstack_requirements.cmds.build_lower_constraints:main