Default to --user install in certain conditions

news/1668.feature

@@ -0,0 +1,2 @@
+Default to doing a user install (as if ``--user`` was passed) when the main
+site-packages directory is not writeable and user site-packages are enabled.

src/pip/_internal/commands/install.py

@@ -13,6 +13,7 @@
 import operator
 import os
 import shutil
+import site
 from optparse import SUPPRESS_HELP
 
 from pip._vendor import pkg_resources
@@ -32,7 +33,7 @@
 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,
@@ -291,17 +292,16 @@ def run(self, options, args):
 
         options.src_dir = os.path.abspath(options.src_dir)
         install_options = options.install_options or []
+
+        options.use_user_site = decide_user_install(
+            options.use_user_site,
+            prefix_path=options.prefix_path,
+            target_dir=options.target_dir,
+            root_path=options.root_path,
+            isolated_mode=options.isolated_mode,
+        )
+
         if options.use_user_site:
-            if options.prefix_path:
-                raise CommandError(
-                    "Can not combine '--user' and '--prefix' as they imply "
-                    "different installation locations"
-                )
-            if virtualenv_no_global():
-                raise InstallationError(
-                    "Can not perform a '--user' install. User site-packages "
-                    "are not visible in this virtualenv."
-                )
             install_options.append('--user')
             install_options.append('--prefix=')
 
@@ -594,6 +594,50 @@ 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 decide_user_install(
+        use_user_site,
+        prefix_path,
+        target_dir,
+        root_path,
+        isolated_mode,
+):
+    """Determine whether to do a user install based on the input options.
+
+    If use_user_site is True/False, that is checked for compatibility with
+    other options. If None, the default behaviour depends on other options
+    and the environment.
+    """
+    if use_user_site:
+        if prefix_path:
+            raise CommandError(
+                "Can not combine '--user' and '--prefix' as they imply "
+                "different installation locations"
+            )
+        if virtualenv_no_global():
+            raise InstallationError(
+                "Can not perform a '--user' install. User site-packages "
+                "are not visible in this virtualenv."
+            )
+    if use_user_site in (True, False):
+        return use_user_site
+
+    if prefix_path or target_dir:
+        return False  # user install incompatible with --prefix/--target
+
+    # Default behaviour: prefer non-user installation if that looks possible.
+    # If we don't have permission for that and user site-packages are visible,
+    # choose a user install.
+    return site.ENABLE_USER_SITE and not site_packages_writable(
+        root=root_path, isolated=isolated_mode
+    )
+
+
 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 +1,7 @@
+import errno
 import os
 import os.path
+import random
 import shutil
 import stat
 from contextlib import contextmanager
@@ -113,3 +115,55 @@ def replace(src, dest):
 
 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):
+    # type: (str) -> bool
+    """Check if a directory is writable.
+
+    Uses os.access() on POSIX, tries creating files on Windows.
+    """
+    # If the directory doesn't exist, find the closest parent that does.
+    while not os.path.isdir(path):
+        parent = os.path.dirname(path)
+        if parent == path:
+            break  # Should never get here, but infinite loops are bad
+        path = parent
+
+    if os.name == 'posix':
+        return os.access(path, os.W_OK)
+
+    return _test_writable_dir_win(path)
+
+
+def _test_writable_dir_win(path):
+    # type: (str) -> bool
+    # 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:
+            fd = os.open(file, os.O_RDWR | os.O_CREAT | os.O_EXCL)
+        except OSError as e:
+            if e.errno == errno.EEXIST:
+                continue
+            if e.errno == errno.EPERM:
+                # 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 dir is not writable.
+                return False
+            raise
+        else:
+            os.close(fd)
+            os.unlink(file)
+            return True
+
+    # This should never be reached
+    raise EnvironmentError(
+        'Unexpected condition testing for writable directory'
+    )

tests/functional/test_install.py

@@ -248,7 +248,6 @@ def test_basic_editable_install(script):
         in result.stderr
     )
     assert not result.files_created
-    assert not result.files_updated
 
 
 @pytest.mark.svn

tests/functional/test_install_upgrade.py

@@ -245,7 +245,9 @@ def test_upgrade_to_same_version_from_url(script):
         'https://files.pythonhosted.org/packages/source/I/INITools/INITools-'
         '0.3.tar.gz',
     )
-    assert not result2.files_updated, 'INITools 0.3 reinstalled same version'
+    assert script.site_packages / 'initools' not in result2.files_updated, (
+        'INITools 0.3 reinstalled same version'
+    )
     result3 = script.pip('uninstall', 'initools', '-y')
     assert_all_changes(result, result3, [script.venv / 'build', 'cache'])