From 453aa7867f99080ed36ce78b9f778dc33f4d209e Mon Sep 17 00:00:00 2001
From: Henry Schreiner <henryfs@princeton.edu>
Date: Sat, 13 Jun 2026 01:52:46 -0400
Subject: [PATCH 1/3] =?UTF-8?q?=F0=9F=90=9B=20fix(env):=20reject=20a=20fil?=
 =?UTF-8?q?e=20as=20--env-dir=20and=20drop=20a=20mutable=20default?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

- Raise BuildException when --env-dir points at a regular file instead of
  letting os.makedirs produce a raw FileExistsError
- Change mutable default `constraints: Collection[str] = []` to `()` (B006)
- Add a test asserting the new "not a directory" error path

Assisted-by: ClaudeCode:claude-sonnet-4.6
---
 src/build/env.py  |  5 ++++-
 tests/test_env.py | 13 +++++++++++++
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/src/build/env.py b/src/build/env.py
index 25d08cf1..44857f2f 100644
--- a/src/build/env.py
+++ b/src/build/env.py
@@ -121,6 +121,9 @@ def __enter__(self) -> Self:
             path = tempfile.mkdtemp(prefix='build-env-')
         else:
             path = self._requested_path
+            if os.path.exists(path) and not os.path.isdir(path):
+                msg = f'Build environment location is not a directory: {path}'
+                raise BuildException(msg)
             if os.path.isdir(path):
                 with os.scandir(path) as entries:
                     if next(entries, None) is not None:
@@ -203,7 +206,7 @@ def installed_versions(self, requirements: Collection[str]) -> dict[str, str]:
     def install(
         self,
         requirements: Collection[str],
-        constraints: Collection[str] = [],
+        constraints: Collection[str] = (),
         *,
         _fresh: bool = False,  # Used internally by CLI to support preset PYTHONPATH
     ) -> None:
diff --git a/tests/test_env.py b/tests/test_env.py
index becd53e0..e75243ca 100644
--- a/tests/test_env.py
+++ b/tests/test_env.py
@@ -696,6 +696,19 @@ def test_env_dir_rejects_non_empty_location(tmp_path: pathlib.Path) -> None:
     assert tmp_path.joinpath('sentinel').exists()
 
 
+def test_env_dir_rejects_file_at_location(tmp_path: pathlib.Path) -> None:
+    file_path = tmp_path / 'env-file'
+    file_path.touch()
+
+    with (
+        pytest.raises(build.BuildException, match='Build environment location is not a directory'),
+        build.env.DefaultIsolatedEnv(path=str(file_path)),
+    ):
+        raise AssertionError
+
+    assert file_path.is_file()
+
+
 @pytest.mark.usefixtures('mock_env_create')
 def test_env_dir_accepts_existing_empty_location(tmp_path: pathlib.Path) -> None:
     with build.env.DefaultIsolatedEnv(path=str(tmp_path)) as env:

From d82e39af6c95ee18cc03670988e80381e03f4f8f Mon Sep 17 00:00:00 2001
From: Henry Schreiner <henryfs@princeton.edu>
Date: Sat, 13 Jun 2026 01:53:11 -0400
Subject: [PATCH 2/3] =?UTF-8?q?=F0=9F=93=9D=20docs(changelog):=20add=20fra?=
 =?UTF-8?q?gment=20for=20#1100?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Assisted-by: ClaudeCode:claude-sonnet-4.6
---
 docs/changelog/1100.bugfix.rst | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 docs/changelog/1100.bugfix.rst

diff --git a/docs/changelog/1100.bugfix.rst b/docs/changelog/1100.bugfix.rst
new file mode 100644
index 00000000..e4e98b87
--- /dev/null
+++ b/docs/changelog/1100.bugfix.rst
@@ -0,0 +1 @@
+Raise a friendly :class:`build.BuildException` when ``--env-dir`` points at an existing regular file instead of letting :func:`os.makedirs` surface a raw :exc:`FileExistsError`; also replace the mutable default ``[]`` in :meth:`DefaultIsolatedEnv.install`'s ``constraints`` parameter with an immutable tuple ``()``.

From 528b9fe4572d8eaa19d7196e85370c26a6e53d58 Mon Sep 17 00:00:00 2001
From: Henry Schreiner <henryfs@princeton.edu>
Date: Sat, 13 Jun 2026 01:54:34 -0400
Subject: [PATCH 3/3] =?UTF-8?q?=F0=9F=8E=A8=20style:=20reformat=20changelo?=
 =?UTF-8?q?g=20fragment=20with=20docstrfmt?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Assisted-by: ClaudeCode:claude-opus-4.8
---
 docs/changelog/1100.bugfix.rst | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/docs/changelog/1100.bugfix.rst b/docs/changelog/1100.bugfix.rst
index e4e98b87..ba0e78fa 100644
--- a/docs/changelog/1100.bugfix.rst
+++ b/docs/changelog/1100.bugfix.rst
@@ -1 +1,3 @@
-Raise a friendly :class:`build.BuildException` when ``--env-dir`` points at an existing regular file instead of letting :func:`os.makedirs` surface a raw :exc:`FileExistsError`; also replace the mutable default ``[]`` in :meth:`DefaultIsolatedEnv.install`'s ``constraints`` parameter with an immutable tuple ``()``.
+Raise a friendly :class:`build.BuildException` when ``--env-dir`` points at an existing regular file instead of letting
+:func:`os.makedirs` surface a raw :exc:`FileExistsError`; also replace the mutable default ``[]`` in
+:meth:`DefaultIsolatedEnv.install`'s ``constraints`` parameter with an immutable tuple ``()``.