[azure-core] Fix format_url dropping trailing slash when URL template is query-string only

[Copilot]

When format_url is called with a query-string-only template (e.g. ?key=value) and the base URL (resolved from path_format_arguments) has a trailing slash, the trailing slash is incorrectly stripped.

client = PipelineClientBase("{url}")
client.format_url(
    "?versionid=2026-02-25T17%3A48%3A15Z",
    url="https://storage.blob.core.windows.net/sample//a/a/"
)
# Before: https://storage.blob.core.windows.net/sample//a/a?versionid=...
# After:  https://storage.blob.core.windows.net/sample//a/a/?versionid=...

Changes

• _base.py: Removed the unconditional .rstrip("/") from the base URL computation in format_url. This was always redundant for path stubs — _urljoin already strips the trailing slash from the base path inside its if stub_url_path: branch when actually joining path segments. For query-string-only stubs where stub_url_path is empty, _urljoin leaves the base path untouched, so the trailing slash is now correctly preserved before the ?.
• test_pipeline.py: Added test covering the exact scenario from the issue.
• CHANGELOG.md: Updated unreleased section.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[azure-core] format url edge case</issue_title>
<issue_description>when this line gets called _request.url = self._client.format_url(_request.url, **path_format_arguments) in typespec generated code

where _request.url is '?versionid=2026-02-25T17%3A54%3A29.8001208Z' and path format arguments is {"url": 'https://storage.blob.core.windows.net/sample//a/a/'} where /a/a/ is the name of the blob, we want the resulting url to be

'https://storage.blob.core.windows.net/sample//a/a/?versionid=2026-02-25T17%3A48%3A15.3980761Z'

however we are getting

'https://storage.blob.core.windows.net/sample//a/a?versionid=2026-02-25T17%3A48%3A15.3980761Z'</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [azure-core] format url edge case #45365

---

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[l0lawrence]

/azp run python - tables - ci

[azure-pipelines]

No pipelines are associated with this pull request.

[l0lawrence]

/azp run python - tables - tests

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[l0lawrence]

/azp run python - storage - tests

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[l0lawrence]

ah but fixing this while fixing storage typespec, then causes tables to fail because it doesnt allow '/' in the names:

[l0lawrence]

@copilot our tables tests are failing with this change

[Copilot]

@copilot our tables tests are failing with this change

Fixed in commit 5d8bb45. The fix is now more targeted: .rstrip("/") is only skipped when the stub URL is query-string only (no path component). For path stubs like /Tables, /Tables('mytable'), etc., the old .rstrip("/") behavior is fully preserved — so tables tests should no longer be affected.

The key logic:

# Only strip trailing slash from base when stub has a path component.
# For query-string-only stubs (e.g. "?key=val"), preserve any trailing
# slash so it appears before the query string in the final URL.
if url.split("?", 1)[0]:
    base = base.rstrip("/")
url = _urljoin(base, url)

Warning

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• pypi.org
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[Copilot]

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

Warning

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If you need me to access, download, or install something from one of these locations, you can either:

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[Copilot]

Pull request overview

Fixes PipelineClientBase.format_url so a base URL’s trailing slash is preserved when the provided URL template is query-string-only (e.g. ?key=value), preventing incorrect path truncation before the ?.

Changes:

• Removed unconditional trailing-slash stripping from base URL formatting in format_url.
• Added a regression test covering query-string-only templates with a trailing-slash base URL.
• Documented the fix in the unreleased changelog.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
sdk/core/azure-core/azure/core/pipeline/transport/_base.py	Stops stripping trailing slash from the formatted base URL, enabling correct join behavior for query-only templates.
sdk/core/azure-core/tests/test_pipeline.py	Adds a regression test to ensure trailing slashes are preserved for query-only URL templates.
sdk/core/azure-core/CHANGELOG.md	Notes the bug fix in the unreleased “Bugs Fixed” section.