Be more timid about choosing a shebang

pre_commit/commands/install_uninstall.py

@@ -2,15 +2,14 @@ from __future__ import print_function
 from __future__ import unicode_literals
 
 import io
+import itertools
 import logging
 import os.path
 import sys
 
-import pre_commit.constants as C
 from pre_commit import git
 from pre_commit import output
 from pre_commit.clientlib import load_config
-from pre_commit.languages import python
 from pre_commit.repository import all_hooks
 from pre_commit.repository import install_hook_envs
 from pre_commit.util import cmd_output
@@ -51,8 +50,19 @@ def shebang():
     if sys.platform == 'win32':
         py = 'python'
     else:
-        py = python.get_default_version()
-        if py == C.DEFAULT:
+        # Homebrew/homebrew-core#35825: be more timid about appropriate `PATH`
+        path_choices = [p for p in os.defpath.split(os.pathsep) if p]
+        exe_choices = [
+            'python{}'.format('.'.join(
+                str(v) for v in sys.version_info[:i]
+            ))
+            for i in range(3)
+        ]
+        for path, exe in itertools.product(path_choices, exe_choices):
+            if os.path.exists(os.path.join(path, exe)):
+                py = exe
+                break
+        else:
             py = 'python'
     return '#!/usr/bin/env {}'.format(py)
 

tests/commands/install_uninstall_test.py

@@ -18,7 +18,6 @@ from pre_commit.commands.install_uninstall import is_our_script
 from pre_commit.commands.install_uninstall import PRIOR_HASHES
 from pre_commit.commands.install_uninstall import shebang
 from pre_commit.commands.install_uninstall import uninstall
-from pre_commit.languages import python
 from pre_commit.util import cmd_output
 from pre_commit.util import make_executable
 from pre_commit.util import mkdirp
@@ -51,19 +50,21 @@ def test_shebang_windows():
         assert shebang() == '#!/usr/bin/env python'
 
 
-def test_shebang_otherwise():
+def test_shebang_posix_not_on_path():
     with mock.patch.object(sys, 'platform', 'posix'):
-        assert C.DEFAULT not in shebang()
-
-
-def test_shebang_returns_default():
-    with mock.patch.object(sys, 'platform', 'posix'):
-        with mock.patch.object(
-            python, 'get_default_version', return_value=C.DEFAULT,
-        ):
+        with mock.patch.object(os, 'defpath', ''):
             assert shebang() == '#!/usr/bin/env python'
 
 
+def test_shebang_posix_on_path(tmpdir):
+    tmpdir.join('python{}'.format(sys.version_info[0])).ensure()
+
+    with mock.patch.object(sys, 'platform', 'posix'):
+        with mock.patch.object(os, 'defpath', tmpdir.strpath):
+            expected = '#!/usr/bin/env python{}'.format(sys.version_info[0])
+            assert shebang() == expected
+
+
 def test_install_pre_commit(in_git_dir, store):
     assert not install(C.CONFIG_FILE, store)
     assert os.access(in_git_dir.join('.git/hooks/pre-commit').strpath, os.X_OK)