fix: ignore non-Python sources in HTML reports

CHANGELOG.rst

@@ -2,6 +2,9 @@
 Changelog
 =========
 
+* Avoid HTML coverage reports trying to parse non-Python source files such as
+  Jinja templates. See `#748 <https://github.com/pytest-dev/pytest-cov/issues/748>`_.
+
 7.1.0 (2026-03-21)
 ------------------
 

src/pytest_cov/engine.py

@@ -186,7 +186,19 @@ def summary(self, stream):
         # Produce html report if wanted.
         if 'html' in self.cov_report:
             output = self.cov_report['html']
-            self.cov.html_report(ignore_errors=True, directory=output)
+            html_options = {}
+            if (
+                self.cov.config.source is None
+                and not self.cov.config.source_pkgs
+                and not self.cov.config.run_include
+                and self.cov.config.report_include is None
+            ):
+                # Coverage data can include executable code compiled from
+                # non-Python sources, such as Jinja templates.  Restrict the
+                # default HTML report to Python files so the report generator
+                # does not try to parse those sources as Python.
+                html_options['include'] = ['*.py']
+            self.cov.html_report(ignore_errors=True, directory=output, **html_options)
             stream.write(f'Coverage HTML written to dir {self.cov.config.html_dir if output is None else output}\n')
 
         # Produce xml report if wanted.

tests/test_pytest_cov.py

@@ -257,6 +257,27 @@ def test_html(testdir):
     assert result.ret == 0
 
 
+def test_html_ignores_non_python_sources(testdir):
+    template = testdir.makefile('.jinja2', template='{% invalid jinja %}')
+    script = testdir.makepyfile(
+        f"""
+def test_template():
+    exec(compile('value = 1\\n', {str(template)!r}, 'exec'))
+"""
+    )
+
+    result = testdir.runpytest('-v', '--cov', '--cov-report=html', script)
+
+    result.stdout.fnmatch_lines(
+        [
+            '*_ coverage: platform *, python * _*',
+            'Coverage HTML written to dir htmlcov',
+            '*1 passed*',
+        ]
+    )
+    assert result.ret == 0
+
+
 def test_html_output_dir(testdir):
     script = testdir.makepyfile(SCRIPT)