fix: keep template variable in HTML attribute name on one line (#1185) - #2206

Fixes #1185

Root cause

The attribute tokenizer (Config.attribute_pattern in src/djlint/settings.py) matched an attribute name with the subpattern (?:\w|-|\.|\:|@|/(?!>))+, which only accepts plain name characters. When a template variable is embedded in an attribute name — e.g. data-{{ st_controller }}-target — the run stops at the {{, so data- is matched as one attribute name, the {{ st_controller }} is then picked up by the standalone-template-tag branch, and -target="documentFile" is matched as a third, separate attribute.

During attribute expansion (src/djlint/formatter/attributes.py), each token is emitted on its own line, so the single attribute is corrupted into three lines.

Reproduction

Input:

<input type="file" name="file" class="d-none" data-{{ st_controller }}-target= "documentFile" multiple />

Actual (wrong) output on 1.36.4 and current master:

<input type="file"
       name="file"
       class="d-none"
       data-
       {{ st_controller }}
       -target="documentFile"
       multiple />

Corrected output with this change:

<input type="file"
       name="file"
       class="d-none"
       data-{{ st_controller }}-target="documentFile"
       multiple />

Fix

Extend the attribute-name subpattern so a template tag ({{ ... }} or {% ... %}) may appear inside a name. The name must still start with a real name character, and the embedded tag uses an atomic group (?>...) to avoid pathological backtracking:

(?:\w|-|\.|\:|@|/(?!>))                       # a name character
(?:
    (?:\w|-|\.|\:|@|/(?!>))                    # more name characters
  | (?>{{ ... }}|{% ... %})                    # or an embedded template tag
)*
| required | checked

The change is localized to the name portion of the pattern only. Because a name must begin with a name character, a standalone template tag ({{ x }}, {% if %}...{% endif %}) still falls through to the existing standalone branch, so that behaviour is unchanged. Values, quoting, and boolean attributes are untouched.

Tests

Added a regression case template_var_in_attribute_name to tests/test_html/test_attributes.py. It fails on master (producing the three-line output shown above) and passes with this fix.

The full test suite passes locally (678 passed; the one unrelated failure, test_python_call, fails identically on unmodified master in this environment because it invokes a bare python binary that is not on PATH here). ruff check and ruff format --check are clean on the changed files. I also smoke-tested standalone template tags, {% if %}-wrapped booleans, template vars in values, and multiple template vars within one name to confirm no regressions.

Disclosure: prepared with AI assistance; reviewed and verified locally.