Respect PATHEXT when looking for Win executable

... as idiomatic in Windows programming.

pipenv/core.py

@@ -126,10 +126,10 @@ def which(command, location=None, allow_global=False):
     if not allow_global:
         if os.name == 'nt':
             p = find_windows_executable(
-                os.path.join(location, 'Scripts'), command
+                os.path.join(location, 'Scripts'), command,
             )
         else:
-            p = os.sep.join([location] + ['bin/{0}'.format(command)])
+            p = os.path.join(location, 'bin', command)
     else:
         if command == 'python':
             p = sys.executable
@@ -1508,7 +1508,7 @@ def pip_install(
 
 
 def pip_download(package_name):
-    cache_dir = Path(PIPENV_CACHE_DIR)    
+    cache_dir = Path(PIPENV_CACHE_DIR)
     pip_config = {
         'PIP_CACHE_DIR': fs_str(cache_dir.as_posix()),
         'PIP_WHEEL_DIR': fs_str(cache_dir.joinpath('wheels').as_posix()),

pipenv/utils.py

@@ -835,20 +835,18 @@ def get_windows_path(*args):
 def find_windows_executable(bin_path, exe_name):
     """Given an executable name, search the given location for an executable"""
     requested_path = get_windows_path(bin_path, exe_name)
-    if os.path.exists(requested_path):
+    if os.path.isfile(requested_path):
         return requested_path
 
-    # Ensure we aren't adding two layers of file extensions
-    exe_name = os.path.splitext(exe_name)[0]
-    files = [
-        '{0}.{1}'.format(exe_name, ext) for ext in ['', 'py', 'exe', 'bat']
-    ]
-    exec_paths = [get_windows_path(bin_path, f) for f in files]
-    exec_files = [
-        filename for filename in exec_paths if os.path.isfile(filename)
-    ]
-    if exec_files:
-        return exec_files[0]
+    try:
+        pathext = os.environ['PATHEXT']
+    except KeyError:
+        pass
+    else:
+        for ext in pathext.split(os.pathsep):
+            path = get_windows_path(bin_path, exe_name + ext.strip().lower())
+            if os.path.isfile(path):
+                return path
 
     return find_executable(exe_name)
 

tests/unit/test_utils_windows_executable.py

@@ -0,0 +1,30 @@
+import os
+
+import mock
+import pytest
+
+from pipenv import utils
+
+
+# This module is run only on Windows.
+pytestmark = pytest.mark.skipif(
+    os.name != 'nt',
+    reason="only relevant on windows",
+)
+
+
+@mock.patch('os.path.isfile')
+@mock.patch('pipenv.utils.find_executable')
+def test_find_windows_executable(mocked_find_executable, mocked_isfile):
+    mocked_isfile.return_value = False
+    mocked_find_executable.return_value = None
+    found = utils.find_windows_executable('fake/path', 'python')
+    assert found is None
+
+    assert mocked_isfile.call_count > 1
+
+    calls = [mock.call('fake/path/python')] + [
+        mock.call('fake/path/python{0}'.format(ext.lower()))
+        for ext in os.environ['PATHEXT'].split(';')
+    ]
+    assert mocked_isfile.mock_calls == calls