From 042d4844e8fe8b664c7e29c0c66a4fe399c45581 Mon Sep 17 00:00:00 2001
From: Thomas Kluyver <thomas@kluyver.me.uk>
Date: Mon, 9 Sep 2019 15:22:08 +0100
Subject: [PATCH 01/18] Default to --user install in certain conditions

---
 src/pip/_internal/commands/install.py | 20 ++++++++++++--
 src/pip/_internal/utils/filesystem.py | 40 +++++++++++++++++++++++++++
 2 files changed, 58 insertions(+), 2 deletions(-)

diff --git a/src/pip/_internal/commands/install.py b/src/pip/_internal/commands/install.py
index 66071f6e819..2cb9c055d7f 100644
--- a/src/pip/_internal/commands/install.py
+++ b/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
@@ -28,11 +29,11 @@
     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 @@ def run(self, options, args):
             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
 
diff --git a/src/pip/_internal/utils/filesystem.py b/src/pip/_internal/utils/filesystem.py
index f4a389cd92f..db099e4a64b 100644
--- a/src/pip/_internal/utils/filesystem.py
+++ b/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 @@ 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):
+    """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')

From 14e76d0eaf7f4aa59894403fa92eed4d7fea4074 Mon Sep 17 00:00:00 2001
From: Thomas Kluyver <thomas@kluyver.me.uk>
Date: Wed, 9 Oct 2019 08:38:58 +0100
Subject: [PATCH 02/18] Remove unused import

---
 src/pip/_internal/commands/install.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/pip/_internal/commands/install.py b/src/pip/_internal/commands/install.py
index 2cb9c055d7f..b5180f5b514 100644
--- a/src/pip/_internal/commands/install.py
+++ b/src/pip/_internal/commands/install.py
@@ -29,7 +29,7 @@
     InstallationError,
     PreviousBuildDirError,
 )
-from pip._internal.locations import distutils_scheme, site_packages
+from pip._internal.locations import distutils_scheme
 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 172274ac1dcb2839ec8a52bdf07a8d88dd6814d9 Mon Sep 17 00:00:00 2001
From: Thomas Kluyver <thomas@kluyver.me.uk>
Date: Wed, 9 Oct 2019 08:39:07 +0100
Subject: [PATCH 03/18] Add type annotation comments

---
 src/pip/_internal/utils/filesystem.py | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/pip/_internal/utils/filesystem.py b/src/pip/_internal/utils/filesystem.py
index db099e4a64b..6de6ea65d4d 100644
--- a/src/pip/_internal/utils/filesystem.py
+++ b/src/pip/_internal/utils/filesystem.py
@@ -119,6 +119,7 @@ def replace(src, dest):
 # 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.
@@ -130,6 +131,7 @@ def test_writable_dir(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_'

From f7132d9c6d898c10e0c9146a572818f46be4581e Mon Sep 17 00:00:00 2001
From: Thomas Kluyver <thomas@kluyver.me.uk>
Date: Wed, 9 Oct 2019 08:43:42 +0100
Subject: [PATCH 04/18] Fix OSError catching for Python 2

---
 src/pip/_internal/utils/filesystem.py | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/src/pip/_internal/utils/filesystem.py b/src/pip/_internal/utils/filesystem.py
index 6de6ea65d4d..88d214a32ab 100644
--- a/src/pip/_internal/utils/filesystem.py
+++ b/src/pip/_internal/utils/filesystem.py
@@ -1,3 +1,4 @@
+import errno
 import os
 import os.path
 import random
@@ -142,13 +143,15 @@ def _test_writable_dir_win(path):
         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
+        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.unlink(file)
             return True

From a6b13b637b1cd7208f478734817212e409ff8a17 Mon Sep 17 00:00:00 2001
From: Thomas Kluyver <thomas@kluyver.me.uk>
Date: Wed, 9 Oct 2019 08:43:58 +0100
Subject: [PATCH 05/18] Fix maximum line length

---
 src/pip/_internal/commands/install.py | 4 +++-
 src/pip/_internal/utils/filesystem.py | 4 +++-
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/src/pip/_internal/commands/install.py b/src/pip/_internal/commands/install.py
index b5180f5b514..dfeb9c04ff6 100644
--- a/src/pip/_internal/commands/install.py
+++ b/src/pip/_internal/commands/install.py
@@ -607,7 +607,9 @@ def get_lib_location_guesses(*args, **kwargs):
 
 
 def site_packages_writable(**kwargs):
-    return all(test_writable_dir(d) for d in get_lib_location_guesses(**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):
diff --git a/src/pip/_internal/utils/filesystem.py b/src/pip/_internal/utils/filesystem.py
index 88d214a32ab..90e344a1465 100644
--- a/src/pip/_internal/utils/filesystem.py
+++ b/src/pip/_internal/utils/filesystem.py
@@ -157,4 +157,6 @@ def _test_writable_dir_win(path):
             return True
 
     # This should never be reached
-    raise EnvironmentError('Unexpected condition testing for writable directory')
+    raise EnvironmentError(
+        'Unexpected condition testing for writable directory'
+    )

From f2b1882b6a0438d4fead602ff3c3e08e9e8ea7ae Mon Sep 17 00:00:00 2001
From: Thomas Kluyver <thomas@kluyver.me.uk>
Date: Wed, 9 Oct 2019 08:46:33 +0100
Subject: [PATCH 06/18] Add news file

---
 news/1668.feature | 2 ++
 1 file changed, 2 insertions(+)
 create mode 100644 news/1668.feature

diff --git a/news/1668.feature b/news/1668.feature
new file mode 100644
index 00000000000..d200841ea7f
--- /dev/null
+++ b/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.

From a9405b22fcae2fc3f5dc7d2ef79e3bbcdea32363 Mon Sep 17 00:00:00 2001
From: Thomas Kluyver <thomas@kluyver.me.uk>
Date: Wed, 9 Oct 2019 08:48:33 +0100
Subject: [PATCH 07/18] Ensure necessary install_options are set for default
 user install

---
 src/pip/_internal/commands/install.py | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/pip/_internal/commands/install.py b/src/pip/_internal/commands/install.py
index dfeb9c04ff6..f7bbceaec37 100644
--- a/src/pip/_internal/commands/install.py
+++ b/src/pip/_internal/commands/install.py
@@ -316,6 +316,8 @@ def run(self, options, args):
                 options.use_user_site = False
             elif site.ENABLE_USER_SITE:
                 options.use_user_site = True
+                install_options.append('--user')
+                install_options.append('--prefix=')
 
         target_temp_dir = None  # type: Optional[TempDirectory]
         target_temp_dir_path = None  # type: Optional[str]

From 086ab4c52aa1112f57acfabe166a4095681978df Mon Sep 17 00:00:00 2001
From: Thomas Kluyver <thomas@kluyver.me.uk>
Date: Wed, 9 Oct 2019 09:46:12 +0100
Subject: [PATCH 08/18] Fix exclusive-mode open for Python 2

---
 src/pip/_internal/utils/filesystem.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/pip/_internal/utils/filesystem.py b/src/pip/_internal/utils/filesystem.py
index 90e344a1465..f076f738d52 100644
--- a/src/pip/_internal/utils/filesystem.py
+++ b/src/pip/_internal/utils/filesystem.py
@@ -141,8 +141,7 @@ def _test_writable_dir_win(path):
         name = basename + ''.join(random.choice(alphabet) for _ in range(6))
         file = os.path.join(path, name)
         try:
-            with open(file, mode='xb'):
-                pass
+            fd = os.open(file, os.O_RDWR | os.O_CREAT | os.O_EXCL)
         except OSError as e:
             if e.errno == errno.EEXIST:
                 continue
@@ -153,6 +152,7 @@ def _test_writable_dir_win(path):
                 return False
             raise
         else:
+            os.close(fd)
             os.unlink(file)
             return True
 

From 357c1322221290ee640454fe28fce7f7dfeae95d Mon Sep 17 00:00:00 2001
From: Thomas Kluyver <thomas@kluyver.me.uk>
Date: Wed, 9 Oct 2019 10:13:29 +0100
Subject: [PATCH 09/18] Handle nonexistant directory when checking for write
 access

---
 src/pip/_internal/utils/filesystem.py | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/src/pip/_internal/utils/filesystem.py b/src/pip/_internal/utils/filesystem.py
index f076f738d52..bedd662958a 100644
--- a/src/pip/_internal/utils/filesystem.py
+++ b/src/pip/_internal/utils/filesystem.py
@@ -125,6 +125,13 @@ def test_writable_dir(path):
 
     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)
 

From 4a4f1ca1cfbfa7d8a6ac4ae55f8aae8501eb2fbc Mon Sep 17 00:00:00 2001
From: Thomas Kluyver <thomas@kluyver.me.uk>
Date: Wed, 9 Oct 2019 13:54:46 +0100
Subject: [PATCH 10/18] Relax failing tests due to changing site-packages mtime

---
 tests/functional/test_install.py         | 1 -
 tests/functional/test_install_upgrade.py | 4 +++-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/tests/functional/test_install.py b/tests/functional/test_install.py
index 2e980d5402d..d6a2ce0ee9c 100644
--- a/tests/functional/test_install.py
+++ b/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
diff --git a/tests/functional/test_install_upgrade.py b/tests/functional/test_install_upgrade.py
index 36b518b2546..6d2eeb5dc4e 100644
--- a/tests/functional/test_install_upgrade.py
+++ b/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'])
 

From 5f1468274987348b569aa586eeca4363494d0357 Mon Sep 17 00:00:00 2001
From: Thomas Kluyver <thomas@kluyver.me.uk>
Date: Wed, 9 Oct 2019 17:34:16 +0100
Subject: [PATCH 11/18] Factor out code for handling the --user option & its
 default

---
 src/pip/_internal/commands/install.py | 70 ++++++++++++++++++---------
 1 file changed, 47 insertions(+), 23 deletions(-)

diff --git a/src/pip/_internal/commands/install.py b/src/pip/_internal/commands/install.py
index f7bbceaec37..84feb023bc5 100644
--- a/src/pip/_internal/commands/install.py
+++ b/src/pip/_internal/commands/install.py
@@ -292,33 +292,19 @@ 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=')
 
-        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
-                install_options.append('--user')
-                install_options.append('--prefix=')
-
         target_temp_dir = None  # type: Optional[TempDirectory]
         target_temp_dir_path = None  # type: Optional[str]
         if options.target_dir:
@@ -614,6 +600,44 @@ def site_packages_writable(**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
 

From e94e7877ab09ad11c7eb53ccad50097faa0ebede Mon Sep 17 00:00:00 2001
From: Pradyun Gedam <pradyunsg@gmail.com>
Date: Sat, 19 Oct 2019 11:23:27 +0100
Subject: [PATCH 12/18] Polish & clarify decide_user_install() function

---
 src/pip/_internal/commands/install.py | 45 +++++++++++++++++----------
 1 file changed, 28 insertions(+), 17 deletions(-)

diff --git a/src/pip/_internal/commands/install.py b/src/pip/_internal/commands/install.py
index 84feb023bc5..f6a5ac53e6a 100644
--- a/src/pip/_internal/commands/install.py
+++ b/src/pip/_internal/commands/install.py
@@ -601,19 +601,25 @@ def site_packages_writable(**kwargs):
 
 
 def decide_user_install(
-        use_user_site,
-        prefix_path,
-        target_dir,
-        root_path,
-        isolated_mode,
+    use_user_site,  # type: Optional[bool]
+    prefix_path,  # type: Optional[str]
+    target_dir,  # type: Optional[str]
+    root_path,  # type: Optional[str]
+    isolated_mode,  # type: bool
 ):
+    # type: (...) -> bool
     """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 is False, no additional checks are done.
+    If use_user_site is True, it is checked for compatibility with other
+    options.
+    If use_user_site is None, the default behaviour depends on the environment,
+    which is provided by the other arguments.
     """
-    if use_user_site:
+    if use_user_site is False:
+        return False
+
+    if use_user_site is True:
         if prefix_path:
             raise CommandError(
                 "Can not combine '--user' and '--prefix' as they imply "
@@ -624,17 +630,22 @@ def decide_user_install(
                 "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
+        return True
+
+    # If we are here, user installs have not been explicitly requested/avoided
+    assert use_user_site is None
 
+    # user install incompatible with --prefix/--target
     if prefix_path or target_dir:
-        return False  # user install incompatible with --prefix/--target
+        return False
+
+    # If user installs are not enabled, choose a non-user install
+    if not site.ENABLE_USER_SITE:
+        return False
 
-    # 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
+    # If we don't have permissions for a non-user install, choose a user install
+    return not site_packages_writable(
+        root=root_path, isolated=isolated_mode,
     )
 
 

From ecac7a17b51dbfb59a554633a165e4889baac776 Mon Sep 17 00:00:00 2001
From: Thomas Kluyver <thomas@kluyver.me.uk>
Date: Sat, 19 Oct 2019 11:28:51 +0100
Subject: [PATCH 13/18] Line length

---
 src/pip/_internal/commands/install.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/pip/_internal/commands/install.py b/src/pip/_internal/commands/install.py
index f6a5ac53e6a..cb7b972ee17 100644
--- a/src/pip/_internal/commands/install.py
+++ b/src/pip/_internal/commands/install.py
@@ -643,7 +643,7 @@ def decide_user_install(
     if not site.ENABLE_USER_SITE:
         return False
 
-    # If we don't have permissions for a non-user install, choose a user install
+    # If we don't have permission for a non-user install, choose a user install
     return not site_packages_writable(
         root=root_path, isolated=isolated_mode,
     )

From 79f05916300a0fe41c70b227eadc8fcda4eda170 Mon Sep 17 00:00:00 2001
From: Thomas Kluyver <thomas@kluyver.me.uk>
Date: Sun, 20 Oct 2019 21:23:49 +0100
Subject: [PATCH 14/18] Don't check write access on the same path twice

---
 src/pip/_internal/commands/install.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/pip/_internal/commands/install.py b/src/pip/_internal/commands/install.py
index cb7b972ee17..851a9537974 100644
--- a/src/pip/_internal/commands/install.py
+++ b/src/pip/_internal/commands/install.py
@@ -596,7 +596,7 @@ def get_lib_location_guesses(*args, **kwargs):
 
 def site_packages_writable(**kwargs):
     return all(
-        test_writable_dir(d) for d in get_lib_location_guesses(**kwargs)
+        test_writable_dir(d) for d in set(get_lib_location_guesses(**kwargs))
     )
 
 

From 16174f4ad07664d71be72276997f6b0e3a0fd6d4 Mon Sep 17 00:00:00 2001
From: Thomas Kluyver <thomas@kluyver.me.uk>
Date: Sun, 20 Oct 2019 21:28:56 +0100
Subject: [PATCH 15/18] Add logging for decide_user_install

---
 src/pip/_internal/commands/install.py | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/src/pip/_internal/commands/install.py b/src/pip/_internal/commands/install.py
index 851a9537974..ead9f0fe9c7 100644
--- a/src/pip/_internal/commands/install.py
+++ b/src/pip/_internal/commands/install.py
@@ -617,6 +617,7 @@ def decide_user_install(
     which is provided by the other arguments.
     """
     if use_user_site is False:
+        logger.debug("Non-user install by explicit request")
         return False
 
     if use_user_site is True:
@@ -630,6 +631,7 @@ def decide_user_install(
                 "Can not perform a '--user' install. User site-packages "
                 "are not visible in this virtualenv."
             )
+        logger.debug("User install by explicit request")
         return True
 
     # If we are here, user installs have not been explicitly requested/avoided
@@ -637,16 +639,22 @@ def decide_user_install(
 
     # user install incompatible with --prefix/--target
     if prefix_path or target_dir:
+        logger.debug("Non-user install due to --prefix or --target option")
         return False
 
     # If user installs are not enabled, choose a non-user install
     if not site.ENABLE_USER_SITE:
+        logger.debug("Non-user install because user site-packages disabled")
         return False
 
     # If we don't have permission for a non-user install, choose a user install
-    return not site_packages_writable(
-        root=root_path, isolated=isolated_mode,
-    )
+    if site_packages_writable(root=root_path, isolated=isolated_mode):
+        logger.debug("Non-user install because site-packages writeable")
+        return False
+
+    logger.info("Defaulting to user installation because normal site-packages "
+                "is not writeable")
+    return True
 
 
 def create_env_error_message(error, show_traceback, using_user_site):

From fbc0588c0135574f6ad61928bd9ca70564bf4ef4 Mon Sep 17 00:00:00 2001
From: Thomas Kluyver <thomas@kluyver.me.uk>
Date: Sun, 20 Oct 2019 21:48:18 +0100
Subject: [PATCH 16/18] Add unit tests of decide_user_install()

---
 src/pip/_internal/commands/install.py |  8 +++---
 tests/unit/test_command_install.py    | 36 ++++++++++++++++++++++++++-
 2 files changed, 39 insertions(+), 5 deletions(-)

diff --git a/src/pip/_internal/commands/install.py b/src/pip/_internal/commands/install.py
index ead9f0fe9c7..3f19ade8927 100644
--- a/src/pip/_internal/commands/install.py
+++ b/src/pip/_internal/commands/install.py
@@ -602,10 +602,10 @@ def site_packages_writable(**kwargs):
 
 def decide_user_install(
     use_user_site,  # type: Optional[bool]
-    prefix_path,  # type: Optional[str]
-    target_dir,  # type: Optional[str]
-    root_path,  # type: Optional[str]
-    isolated_mode,  # type: bool
+    prefix_path=None,  # type: Optional[str]
+    target_dir=None,  # type: Optional[str]
+    root_path=None,  # type: Optional[str]
+    isolated_mode=False,  # type: bool
 ):
     # type: (...) -> bool
     """Determine whether to do a user install based on the input options.
diff --git a/tests/unit/test_command_install.py b/tests/unit/test_command_install.py
index 1a3fee5c9ae..34635cd6df9 100644
--- a/tests/unit/test_command_install.py
+++ b/tests/unit/test_command_install.py
@@ -1,6 +1,6 @@
 from mock import Mock, call, patch
 
-from pip._internal.commands.install import build_wheels
+from pip._internal.commands.install import build_wheels, decide_user_install
 
 
 class TestWheelCache:
@@ -61,3 +61,37 @@ def test_build_wheels__wheel_not_installed(self, is_wheel_installed):
         ]
 
         assert build_failures == ['a']
+
+
+class TestDecideUserInstall:
+    @patch('site.ENABLE_USER_SITE', True)
+    @patch('pip._internal.commands.install.site_packages_writable')
+    def test_prefix_and_target(self, sp_writable):
+        sp_writable.return_value = False
+
+        assert decide_user_install(
+            use_user_site=None, prefix_path='foo'
+        ) is False
+
+        assert decide_user_install(
+            use_user_site=None, target_dir='bar'
+        ) is False
+
+    @patch('pip._internal.commands.install.site_packages_writable')
+    def test_user_site_enabled(self, sp_writable):
+        sp_writable.return_value = False
+
+        with patch('site.ENABLE_USER_SITE', True):
+            assert decide_user_install(use_user_site=None) is True
+
+        with patch('site.ENABLE_USER_SITE', False):
+            assert decide_user_install(use_user_site=None) is False
+
+    @patch('site.ENABLE_USER_SITE', True)
+    @patch('pip._internal.commands.install.site_packages_writable')
+    def test_site_packages_access(self, sp_writable):
+        sp_writable.return_value = True
+        assert decide_user_install(use_user_site=None) is False
+
+        sp_writable.return_value = False
+        assert decide_user_install(use_user_site=None) is True

From c4d92bbb4ea704069de8381e3629348db32c3818 Mon Sep 17 00:00:00 2001
From: Thomas Kluyver <thomas@kluyver.me.uk>
Date: Mon, 21 Oct 2019 08:52:44 +0100
Subject: [PATCH 17/18] Use pytest parametrize for decide_user_install tests

---
 tests/unit/test_command_install.py | 35 +++++++++++++++---------------
 1 file changed, 17 insertions(+), 18 deletions(-)

diff --git a/tests/unit/test_command_install.py b/tests/unit/test_command_install.py
index 34635cd6df9..13d644a8587 100644
--- a/tests/unit/test_command_install.py
+++ b/tests/unit/test_command_install.py
@@ -1,3 +1,4 @@
+import pytest
 from mock import Mock, call, patch
 
 from pip._internal.commands.install import build_wheels, decide_user_install
@@ -77,21 +78,19 @@ def test_prefix_and_target(self, sp_writable):
             use_user_site=None, target_dir='bar'
         ) is False
 
-    @patch('pip._internal.commands.install.site_packages_writable')
-    def test_user_site_enabled(self, sp_writable):
-        sp_writable.return_value = False
-
-        with patch('site.ENABLE_USER_SITE', True):
-            assert decide_user_install(use_user_site=None) is True
-
-        with patch('site.ENABLE_USER_SITE', False):
-            assert decide_user_install(use_user_site=None) is False
-
-    @patch('site.ENABLE_USER_SITE', True)
-    @patch('pip._internal.commands.install.site_packages_writable')
-    def test_site_packages_access(self, sp_writable):
-        sp_writable.return_value = True
-        assert decide_user_install(use_user_site=None) is False
-
-        sp_writable.return_value = False
-        assert decide_user_install(use_user_site=None) is True
+    @pytest.mark.parametrize(
+        "enable_user_site,site_packages_writable,result", [
+            (True, True, False),
+            (True, False, True),
+            (False, True, False),
+            (False, False, False),
+        ])
+    def test_most_cases(
+        self, enable_user_site, site_packages_writable, result, monkeypatch,
+    ):
+        monkeypatch.setattr('site.ENABLE_USER_SITE', enable_user_site)
+        monkeypatch.setattr(
+            'pip._internal.commands.install.site_packages_writable',
+            lambda **kw: site_packages_writable
+        )
+        assert decide_user_install(use_user_site=None) is result

From 42b1ef3537343fa1787480b45daa353241b4705a Mon Sep 17 00:00:00 2001
From: Thomas Kluyver <thomas@kluyver.me.uk>
Date: Mon, 21 Oct 2019 13:52:50 +0100
Subject: [PATCH 18/18] Clarify comment

---
 src/pip/_internal/commands/install.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/pip/_internal/commands/install.py b/src/pip/_internal/commands/install.py
index 3f19ade8927..12d58f6e056 100644
--- a/src/pip/_internal/commands/install.py
+++ b/src/pip/_internal/commands/install.py
@@ -647,7 +647,8 @@ def decide_user_install(
         logger.debug("Non-user install because user site-packages disabled")
         return False
 
-    # If we don't have permission for a non-user install, choose a user install
+    # If we have permission for a non-user install, do that,
+    # otherwise do a user install.
     if site_packages_writable(root=root_path, isolated=isolated_mode):
         logger.debug("Non-user install because site-packages writeable")
         return False