firestore: field path with a quoted name ending in a backslash is misparsed

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Since Oct 19, 2026.

Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
78/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
python
Domain
backend, databases

Research direction

Start with google/cloud/firestore_v1/field_path.py and inspect the QUOTED token pattern used by FieldPath.from_api_repr(); reproduce the failure with the provided FieldPath example. Check the field path parser tests and add coverage for a quoted segment ending in a backslash followed by another segment. Done when the API representation parses back to the original parts and the update targets the nested field described in the issue.

Written by the indexing model from the issue text.

Description

Package: google-cloud-firestore (google/cloud/firestore_v1/field_path.py), current main.

What happens

FieldPath.to_api_repr() escapes a trailing backslash correctly, but the parser cannot read the result back when another quoted segment follows it. FieldPath.from_api_repr() raises ValueError, and FieldPath.from_string() catches that error and falls back to a plain split on ., so it silently returns parts that still contain the backticks and escape characters.

Because update() and set(..., merge=[...]) go through FieldPath.from_string() / parse_field_path(), the write goes to the wrong field.

Minimal repro
from google.cloud.firestore_v1.field_path import FieldPath
from google.cloud.firestore_v1 import _helpers

s = FieldPath("dir\\", "file name").to_api_repr()
print(s)                                  # `dir\\`.`file name`
print(FieldPath.from_string(s).parts)     # ('`dir\\\\`', '`file name`')
FieldPath.from_api_repr(s)                # ValueError: Invalid path

pbs = _helpers.pbs_for_update("projects/p/databases/d/documents/c/doc", {s: 1}, None)
print(list(pbs[0].update.fields))         # ['`dir\\\\`']
What should happen

The string produced by to_api_repr() should parse back to the same parts, ('dir\\', 'file name'), and the update should target dir\ > file name.

Root cause

The QUOTED token pattern is r"`(?:\\`|[^`])*?`". In `dir\\`.`file name` the lazy match takes the second backslash together with the closing backtick as an escaped backtick (\`), so the token runs on to the next backtick and swallows `.`. What remains (file name`) does not tokenize, so parsing fails. The pattern needs to treat a backslash as an escape for the next character, not only for a backtick, and should not let a bare backslash match on its own.

Dominant language
Python
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