[python] Fix @clientDefaultValue serialization regression from #10476

[Copilot]

Fix follow-up for #10476: restore client default value materialization for serialization, plus a couple of regenerator-tooling changes that landed on the same branch.

Addresses review comment r3160211783 on PR #10476.

Changes

• Restore default population in Model.__init__ (generator/pygen/codegen/templates/model_base.py.jinja2) so @clientDefaultValue defaults are present in _data and serialized on the wire (the _RestField.__get__ fallback for unset fields is kept intact).
• Extract the shared regenerator helpers, types, constants, and spec/emitter-option tables out of eng/scripts/ci/regenerate.ts into a new eng/scripts/ci/regenerate-common.ts module so the file can be kept byte-identical with the @azure-tools/typespec-python wrapper copy (pnpm sync). Per-repo divergence (paths, emitter name, single- vs two-phase orchestration, argv/help text) stays in each repo's own regenerate.ts, which now builds a RegenerateContext and feeds it into the shared helpers.
• Bump @azure-tools/azure-http-specs from 0.1.0-alpha.39 to 0.1.0-alpha.40-dev.12 in package.json

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@typespec/http-client-python@10546

commit: 1de15e3

[github-actions]

All changed packages have been documented.

• ✅ @typespec/http-client-python

Show changes

@typespec/http-client-python - fix ✏️

Fix serialization regression where @clientDefaultValue defaults on model properties were no longer included in the request body. Defaults are again materialized in the model's data dictionary at construction time so they are sent on the wire, while the attribute-access fallback for unset fields is preserved.

[azure-sdk]

You can try these changes here

🛝 Playground	🌐 Website	🛝 VSCode Extension

[iscai-msft]

@copilot the changes in the pipeline setup is causing these errors

Stage CSharp_Build job DetectApiChanges depends on unknown job Build_linux_20.
Stage Java_Build job DetectApiChanges depends on unknown job Build_linux_20.
Stage Python_Build job DetectApiChanges depends on unknown job Build_linux_20.

[Copilot]

@copilot the changes in the pipeline setup is causing these errors

...

Reverted the unrelated pipeline changes from commit a477100 (it removed the Node 20 build jobs from eng/emitters/pipelines/templates/stages/emitter-stages.yml while detect-api-changes still has DependsOn: Build_linux_20, plus changed the test-step.yml default NodeVersion from 22.x back to 20.x). Restored both files to their pre-a477100 state in afc2db0.

[iscai-msft]

@copilot we still need to do 22 and up, with your last change, we got this failure on the Build_linux_20 and Build_windows_20 pipelines. maybe we should upgrade the pipelines to no longer run on the 20 version?

npm ci
npm error code EBADENGINE
npm error engine Unsupported engine
npm error engine Not compatible with your version of node/npm: @azure-tools/azure-http-specs@0.1.0-alpha.40-dev.12
npm error notsup Not compatible with your version of node/npm: @azure-tools/azure-http-specs@0.1.0-alpha.40-dev.12
npm error notsup Required: {"node":">=22.0.0"}
npm error notsup Actual: {"npm":"10.8.2","node":"v20.20.2"}
npm error A complete log of this run can be found in: /home/cloudtest/.npm/_logs/2026-05-06T15_05_06_364Z-debug-0.log
Invoke-LoggedCommand: /mnt/vss/_work/1/s/packages/http-client-python/eng/scripts/Initialize-Repository.ps1:42
Line |
42 | Invoke-LoggedCommand "npm ci"
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| ##[error]Command completed with exit code 1 (00:00:04.5497844): npm ci

##[error]PowerShell exited with code '1'.
Finishing: Run initialize script

[Copilot]

@copilot we still need to do 22 and up, with your last change, we got this failure on the Build_linux_20 and Build_windows_20 pipelines. maybe we should upgrade the pipelines to no longer run on t...

Agreed — upgraded the emitter pipelines to drop Node 20 entirely in db2b9af. Removed the Build_linux_20 / Build_windows_20 build jobs and the matching Node 20 test jobs from eng/emitters/pipelines/templates/stages/emitter-stages.yml, promoted the Node 22 linux build job to be the "first" job (EmitArtifacts: true, BuildArtifactName, RunSDLTools: true), updated detect-api-changes to DependsOn: Build_linux_22, and bumped the test-step.yml default NodeVersion to 22.x.

[msyyc]

@iscai-msft CI is fixed and pls help review again, thanks!