Default to --user install in certain conditions

src/pip/_internal/commands/install.py

@@ -13,6 +13,7 @@ import logging
 import operator
 import os
 import shutil
+import site
 from optparse import SUPPRESS_HELP
 
 from pip._vendor import pkg_resources
@@ -28,11 +29,11 @@ from pip._internal.exceptions import (
     InstallationError,
     PreviousBuildDirError,
 )
-from pip._internal.locations import distutils_scheme
+from pip._internal.locations import distutils_scheme, site_packages
 from pip._internal.operations.check import check_install_conflicts
 from pip._internal.req import RequirementSet, install_given_reqs
 from pip._internal.req.req_tracker import RequirementTracker
-from pip._internal.utils.filesystem import check_path_owner
+from pip._internal.utils.filesystem import check_path_owner, test_writable_dir
 from pip._internal.utils.misc import (
     ensure_dir,
     get_installed_version,
@@ -305,6 +306,17 @@ class InstallCommand(RequirementCommand):
             install_options.append('--user')
             install_options.append('--prefix=')
 
+        elif options.use_user_site is None:
+            if options.prefix_path or options.target_dir:
+                options.use_user_site = False
+            elif site_packages_writable(
+                    root=options.root_path,
+                    isolated=options.isolated_mode
+            ):
+                options.use_user_site = False
+            elif site.ENABLE_USER_SITE:
+                options.use_user_site = True
+
         target_temp_dir = None  # type: Optional[TempDirectory]
         target_temp_dir_path = None  # type: Optional[str]
         if options.target_dir:
@@ -594,6 +606,10 @@ def get_lib_location_guesses(*args, **kwargs):
     return [scheme['purelib'], scheme['platlib']]
 
 
+def site_packages_writable(**kwargs):
+    return all(test_writable_dir(d) for d in get_lib_location_guesses(**kwargs))
+
+
 def create_env_error_message(error, show_traceback, using_user_site):
     """Format an error message for an EnvironmentError
 

src/pip/_internal/utils/filesystem.py

@@ -1,5 +1,6 @@
 import os
 import os.path
+import random
 import shutil
 import stat
 from contextlib import contextmanager
@@ -113,3 +114,42 @@ if PY2:
 
 else:
     replace = _replace_retry(os.replace)
+
+
+# test_writable_dir and _test_writable_dir_win are copied from Flit,
+# with the author's agreement to also place them under pip's license.
+def test_writable_dir(path):
+    """Check if a directory is writable.
+
+    Uses os.access() on POSIX, tries creating files on Windows.
+    """
+    if os.name == 'posix':
+        return os.access(path, os.W_OK)
+
+    return _test_writable_dir_win(path)
+
+
+def _test_writable_dir_win(path):
+    # os.access doesn't work on Windows: http://bugs.python.org/issue2528
+    # and we can't use tempfile: http://bugs.python.org/issue22107
+    basename = 'accesstest_deleteme_fishfingers_custard_'
+    alphabet = 'abcdefghijklmnopqrstuvwxyz0123456789'
+    for i in range(10):
+        name = basename + ''.join(random.choice(alphabet) for _ in range(6))
+        file = os.path.join(path, name)
+        try:
+            with open(file, mode='xb'):
+                pass
+        except FileExistsError:
+            continue
+        except PermissionError:
+            # This could be because there's a directory with the same name.
+            # But it's highly unlikely there's a directory called that,
+            # so we'll assume it's because the parent directory is not writable.
+            return False
+        else:
+            os.unlink(file)
+            return True
+
+    # This should never be reached
+    raise EnvironmentError('Unexpected condition testing for writable directory')