[WIP] Try to use user site-packages more often

src/pip/_internal/commands/install.py

@@ -18,7 +18,9 @@
 from pip._internal.resolve import Resolver
 from pip._internal.status_codes import ERROR
 from pip._internal.utils.filesystem import check_path_owner
-from pip._internal.utils.misc import ensure_dir, get_installed_version
+from pip._internal.utils.misc import (
+    ensure_dir, get_installed_version, should_use_user_site,
+)
 from pip._internal.utils.temp_dir import TempDirectory
 from pip._internal.wheel import WheelBuilder
 
@@ -79,10 +81,16 @@ def __init__(self, *args, **kw):
             '--user',
             dest='use_user_site',
             action='store_true',
-            help="Install to the Python user install directory for your "
-                 "platform. Typically ~/.local/, or %APPDATA%\\Python on "
-                 "Windows. (See the Python documentation for site.USER_BASE "
-                 "for full details.)")
+            help="Install into the user site packages."
+        )
+
+        cmd_opts.add_option(
+            '--global',
+            dest='use_user_site',
+            action='store_false',
+            help='Install into the global site packages.',
+        )
+
         cmd_opts.add_option(
             '--root',
             dest='root_path',
@@ -192,7 +200,9 @@ def run(self, options, args):
 
         options.src_dir = os.path.abspath(options.src_dir)
         install_options = options.install_options or []
-        if options.use_user_site:
+
+        # Check if a user installation makes sense.
+        if options.use_user_site is True:
             if options.prefix_path:
                 raise CommandError(
                     "Can not combine '--user' and '--prefix' as they imply "
@@ -206,6 +216,11 @@ def run(self, options, args):
             install_options.append('--user')
             install_options.append('--prefix=')
 
+        # If --user or --global is not passed, use some heuristic to determine
+        # whether the installations should be user or global.
+        if options.use_user_site is None:
+            options.use_user_site = should_use_user_site()
+
         target_temp_dir = TempDirectory(kind="target")
         if options.target_dir:
             options.ignore_installed = True

src/pip/_internal/utils/misc.py

@@ -11,10 +11,12 @@
 import posixpath
 import re
 import shutil
+import site
 import stat
 import subprocess
 import sys
 import tarfile
+import tempfile
 import zipfile
 from collections import deque
 
@@ -28,8 +30,8 @@
 from pip._internal.compat import console_to_str, expanduser, stdlib_pkgs
 from pip._internal.exceptions import InstallationError
 from pip._internal.locations import (
-    running_under_virtualenv, site_packages, user_site, virtualenv_no_global,
-    write_delete_marker_file
+    distutils_scheme, running_under_virtualenv, site_packages, user_site,
+    virtualenv_no_global, write_delete_marker_file,
 )
 
 if PY2:
@@ -877,3 +879,49 @@ def enum(*sequential, **named):
     reverse = dict((value, key) for key, value in enums.items())
     enums['reverse_mapping'] = reverse
     return type('Enum', (), enums)
+
+
+def should_use_user_site():
+    """Determines if pip uses user-site when not explicitly told
+    """
+    # Doing a user installation when a virtualenv is active does not make sense
+    if running_under_virtualenv():
+        return False
+
+    # It won't make sense to install to user site-packages if they are disabled
+    if not site.ENABLE_USER_SITE:
+        return False
+
+    # XXX: Everything beyond this line is for addressing backwards
+    #      compatibility concerns.
+    #      Eventually, the following should be replaced with a return True
+    def _can_create_file_in(folder_path):
+        """Returns whether a file can be created in the given folder
+        """
+        # If the given folder does not exist, try to create it. If there's
+        # going to be a successful installation, the folder probably needs to
+        # be created anyway.
+        try:
+            ensure_dir(folder_path)
+        except EnvironmentError:
+            return False
+
+        # Try to create a temporary file in the folder and delete it.
+        try:
+            with tempfile.TemporaryFile(dir=folder_path):
+                pass
+        except EnvironmentError:
+            return False
+        else:
+            return True
+
+    # This is the "smart" portion. Check if there's any potential global folder
+    # for which pip doesn't have the required permissions. If there's any such
+    # location, pip should perform a user installation instead.
+    for folder in distutils_scheme("").values():
+        if not _can_create_file_in(folder):
+            return True
+
+    # Assume the user wants to do a global installation. This is the most
+    # backwards compatible policy; preserving behaviour for `sudo pip install`
+    return False