FIX: preserve PYTHONPATH under --python=same discovery

asv/benchmark.py

@@ -49,6 +49,25 @@ def _help(args):
 }
 
 
+def _apply_path_env():
+    """Apply ``ASV_PYTHONPATH`` to ``sys.path`` for this process.
+
+    Managed environments already drop bare ``PYTHONPATH`` in
+    ``Environment.run_executable`` (isolation). ``ExistingEnvironment`` /
+    ``--python=same`` inherits the host environment, including
+    ``PYTHONPATH``, which in-tree builds rely on for a build directory.
+    Do not strip host ``PYTHONPATH`` here; that broke discovery under
+    ``--python=same`` (gh-1537) and also undid ``ASV_PYTHONPATH`` after
+    ``run_executable`` rewrote it to ``PYTHONPATH``.
+    """
+    asv_pythonpath = os.environ.get('ASV_PYTHONPATH')
+    if not asv_pythonpath:
+        return
+    for path in reversed(asv_pythonpath.split(os.pathsep)):
+        if path and path not in sys.path:
+            sys.path.insert(0, path)
+
+
 def main():
     # Remove asv package directory from `sys.path`. This script file resides
     # there although it's not part of the package, so Python prepends it to
@@ -68,24 +87,7 @@ def main():
     mode = sys.argv[1]
     args = sys.argv[2:]
 
-    env = os.environ.copy()
-    # --- Modify sys.path for the current interpreter ---
-    if 'ASV_PYTHONPATH' in env:
-        new_paths = env['ASV_PYTHONPATH'].split(os.pathsep)
-        for path in reversed(new_paths):  # Add to the front to prioritize
-            if path not in sys.path:
-                sys.path.insert(0, path)
-        # Remove ASV_PYTHONPATH from env, as it's no longer needed after sys.path update
-        env.pop('ASV_PYTHONPATH')
-    else:
-        # Clean up sys.path if PYTHONPATH was set but ASV_PYTHONPATH is not
-        if 'PYTHONPATH' in env:
-            old_paths = env['PYTHONPATH'].split(os.pathsep)
-            for path in old_paths:
-                if path in sys.path:
-                    sys.path.remove(path)
-
-            env.pop('PYTHONPATH')
+    _apply_path_env()
 
     if mode in commands:
         commands[mode](args)

changelog.d/1537.bugfix.rst

@@ -0,0 +1,3 @@
+Preserve host ``PYTHONPATH`` under ``--python=same`` / discovery
+(do not strip it when ``ASV_PYTHONPATH`` is unset). Managed environments
+still drop bare ``PYTHONPATH`` for isolation.

docs/source/env_vars.rst

@@ -23,7 +23,9 @@ behavior are also set:
 - ``PATH``: environment-specific binary directories prepended
 - ``PIP_USER``: ``false``
 - ``PYTHONNOUSERSITE``: ``True`` (for conda environments only)
-- ``PYTHONPATH``: unset (if really needed, can be overridden by setting ``ASV_PYTHONPATH``)
+- ``PYTHONPATH``: unset for asv-managed environments (if really needed,
+  set ``ASV_PYTHONPATH`` instead). With ``--python=same`` / an existing
+  environment, the host ``PYTHONPATH`` is retained.
 
 .. note::
 

test/test_benchmark_path_env.py

@@ -0,0 +1,63 @@
+# Licensed under a 3-clause BSD style license - see LICENSE.rst
+"""Path env handling in asv.benchmark (PYTHONPATH / ASV_PYTHONPATH)."""
+
+import importlib
+import sys
+
+import pytest
+
+from asv.benchmark import _apply_path_env
+
+
+@pytest.fixture
+def path_marker(tmp_path, monkeypatch):
+    """A unique module importable only via an extra sys.path entry."""
+    marker = f"asv1537_marker_{tmp_path.name.replace('-', '_')}"
+    root = tmp_path / "pyroot"
+    root.mkdir()
+    (root / f"{marker}.py").write_text("value = 42\n", encoding="utf-8")
+    yield str(root), marker
+    sys.modules.pop(marker, None)
+    while str(root) in sys.path:
+        sys.path.remove(str(root))
+
+
+def test_pythonpath_kept_when_asv_pythonpath_unset(path_marker, monkeypatch):
+    # gh-1537: --python=same + host PYTHONPATH (e.g. scipy build dir)
+    root, marker = path_marker
+    monkeypatch.setenv("PYTHONPATH", root)
+    monkeypatch.delenv("ASV_PYTHONPATH", raising=False)
+    # Interpreter startup puts PYTHONPATH on sys.path; simulate that.
+    if root not in sys.path:
+        sys.path.insert(0, root)
+
+    _apply_path_env()
+
+    mod = importlib.import_module(marker)
+    assert mod.value == 42
+
+
+def test_asv_pythonpath_applied(path_marker, monkeypatch):
+    root, marker = path_marker
+    monkeypatch.delenv("PYTHONPATH", raising=False)
+    monkeypatch.setenv("ASV_PYTHONPATH", root)
+    assert root not in sys.path
+
+    _apply_path_env()
+
+    assert root in sys.path
+    mod = importlib.import_module(marker)
+    assert mod.value == 42
+
+
+def test_asv_pythonpath_prepended_before_existing(path_marker, monkeypatch, tmp_path):
+    root, marker = path_marker
+    other = tmp_path / "other"
+    other.mkdir()
+    monkeypatch.setenv("ASV_PYTHONPATH", root)
+    monkeypatch.delenv("PYTHONPATH", raising=False)
+    sys.path.insert(0, str(other))
+
+    _apply_path_env()
+
+    assert sys.path.index(root) < sys.path.index(str(other))