Improve how we manage which PDM to use

Move our PDM version pin to a separate file. This both makes it easier
to manage updates to the same and also allows the pinned version to be
usable outside of the venv_sync script. Add a simple script to update
version pins in this file. Teach venv_upgrade to use this to update the
PDM version pin in addition to updating the PDM lock file. Also, modify
venv_sync to tell PDM to not complain about newer versions.

setup/python/requirements.txt

@@ -0,0 +1,12 @@
+# This file describes does NOT describe the requirements for Drake itself, but
+# for setting up the Python virtual environment that Drake will manage. It is
+# used by Drake's @python, and also when preparing a virtual environment to
+# support a Drake installation.
+#
+# Specific version pins in this file are updated automatically by
+# tools/workspace/python/venv_upgrade.
+
+# TODO(jeremy.nimmer) Ideally, we also would pin all of the dependencies of
+# PDM here, but it's not obvious to me how to do that in a way which is easy to
+# upgrade/re-pin over time.
+pdm==2.22.0

tools/workspace/python/update_requirements.py

@@ -0,0 +1,43 @@
+#!/usr/bin/env python3
+
+import json
+import sys
+import urllib.request
+
+
+def get_latest_version(name):
+    r = urllib.request.urlopen(f'https://pypi.org/pypi/{name}/json')
+    d = json.loads(r.read())
+    return d['info']['version']
+
+
+def update_requirements(requirements_path):
+    updated = False
+
+    lines = []
+    with open(requirements_path, 'r') as f:
+        for line in f:
+            line = line.strip()
+
+            if not line.startswith('#') and '==' in line:
+                name, current = line.split('==')
+                latest = get_latest_version(name)
+                if latest != current:
+                    line = f'{name}=={latest}'
+                    updated = True
+
+            lines.append(line)
+
+    if updated:
+        with open(requirements_path, 'w') as f:
+            f.write('\n'.join(lines))
+        print(requirements_path)
+
+
+def main(args):
+    for p in args:
+        update_requirements(p)
+
+
+if __name__ == '__main__':
+    main(sys.argv[1:])

tools/workspace/python/venv_sync

@@ -53,32 +53,34 @@ fi
 # Place the venv(s) in a sibling directory to the output base. That should be a
 # suitable disk location for build artifacts, but without polluting the actual
 # output base that Bazel owns.
+readonly drake_root="$(cd "$(dirname $0)/../../.." && pwd)"
 readonly bazel_output_base="$(cd "${repository}/../.." && pwd)"
 readonly drake_python="${bazel_output_base}.drake_python"
 mkdir -p "${drake_python}"
 
 # Install PDM into a virtual environment. We segregate PDM from the environment
 # it is managing, so that changes to the managed environment cannot break PDM.
+readonly setup="${drake_root}/setup/python"
 readonly venv_pdm="${drake_python}/venv.pdm"
 mkvenv "${venv_pdm}"
-# TODO(jeremy.nimmer) Ideally, we also would pin all of the dependencies of
-# PDM here, but it's not obvious to me how to do that in a way which is easy to
-# upgrade/re-pin over time.
-"${venv_pdm}/bin/pip" install -U pdm==2.22.0
+"${venv_pdm}/bin/pip" install -U -r "${setup}/requirements.txt"
 
 # Don't nag about new versions of PDM; we'll update the above pin when we want
 # to use something newer, and otherwise want to use the pinned version, so
 # being informed about newer versions is just noise.
 export PDM_CHECK_UPDATE=0
 
 # Prepare the PDM "project directory".
-readonly drake_root="$(cd "$(dirname $0)/../../.." && pwd)"
-readonly setup="${drake_root}/setup/python"
 readonly project="${drake_python}/project"
 mkdir -p "${project}"
 ln -nsf "${setup}/pyproject.toml" "${project}/pyproject.toml"
 ln -nsf "${setup}/pdm.lock" "${project}/pdm.lock"
 
+# Don't nag about new versions of PDM; venv_upgrade will check for those.
+# Aside from that, we want to use the pinned version, so being informed about
+# newer versions is just noise.
+export PDM_CHECK_UPDATE=0
+
 # Prepare the venv that will hold Drake's requirements.
 readonly venv_drake="${drake_python}/venv.drake"
 mkvenv "${venv_drake}"

tools/workspace/python/venv_upgrade

@@ -34,11 +34,21 @@ check_working_tree() {
     fi
 }
 
+# Ensure GNU sed is available.
+if [ "$(uname)" == "Darwin" ]; then
+    readonly sed=gsed
+else
+    readonly sed=sed
+fi
+${sed} --version
+
 # Chdir to the Drake root.
 cd "$(dirname $0)/../../.."
 readonly drake_python="$(bazel info output_base).drake_python"
 readonly project="${drake_python}/project"
 readonly venv_pdm="${drake_python}/venv.pdm"
+readonly python="${venv_pdm}/bin/python"
+readonly workspace="$(pwd)/tools/workspace/python"
 
 # If committing, check that the working tree is clean (enough). Note that the
 # lock file is ignored.
@@ -47,11 +57,17 @@ readonly venv_pdm="${drake_python}/venv.pdm"
 # Ensure venv exists.
 bazel fetch @python --force
 
+# Determine if PDM itself is up to date; if not, update its pin.
+readonly venv_requirements="./setup/python/requirements.txt"
+files_to_commit+=( "$(
+    "${python}" "${workspace}/update_requirements.py" "${venv_requirements}"
+)" )
+
 # Remove and recreate the lock file.
 readonly lockfile="./setup/python/pdm.lock"
 rm -f "${lockfile}"
 "${venv_pdm}/bin/pdm" lock -p "${project}" -L "${lockfile}"
-files_to_commit+=($(git diff --name-only HEAD -- "${lockfile}"))
+files_to_commit+=( "$(git diff --name-only HEAD -- "${lockfile}")" )
 
 # If committing, do the commit.
 if [ -n "${commit}" ]; then