gh-72327: Add help message for pip in REPL

Lib/site.py

@@ -390,6 +390,23 @@ def setcopyright():
         files, dirs)
 
 
+def _register_detect_pip_usage_in_repl():
+    old_excepthook = sys.excepthook
+
+    def detect_pip_usage_in_repl(typ, value, traceback):
+        if typ is SyntaxError and (
+            "pip install" in value.text or "pip3 install" in value.text
+        ):
+            value.msg = ("The Python package manager (pip) can only be used"
+                         " from outside of Python.\n"
+                         "Please try the `pip` command in a"
+                         " separate terminal or command prompt.")
+
+        old_excepthook(typ, value, traceback)
+
+    sys.excepthook = detect_pip_usage_in_repl
+
+
 def sethelper():
     builtins.help = _sitebuiltins._Helper()
 
@@ -558,6 +575,11 @@ def main():
     setquit()
     setcopyright()
     sethelper()
+    # Detect REPL by prompt, -i flag or PYTHONINSPECT env var being set.
+    if (hasattr(sys, 'ps1') or sys.flags.interactive or
+        os.environ.get('PYTHONINSPECT')
+    ):
+        _register_detect_pip_usage_in_repl()
     if not sys.flags.isolated:
         enablerlcompleter()
     execsitecustomize()

Lib/test/test_site.py

@@ -8,6 +8,7 @@
 import test.support
 from test.support import captured_stderr, TESTFN, EnvironmentVarGuard
 import builtins
+import io
 import os
 import sys
 import re
@@ -454,6 +455,28 @@ def test_license_exists_at_url(self):
         self.assertEqual(code, 200, msg="Can't find " + url)
 
 
+class DetectPipUsageInReplTests(unittest.TestCase):
+    def setUp(self):
+        self.old_excepthook = sys.excepthook
+        site._register_detect_pip_usage_in_repl()
+
+    def tearDown(self):
+        sys.excepthook = self.old_excepthook
+
+    def test_detect_pip_usage_in_repl(self):
+        for pip_cmd in [
+            'pip install a', 'pip3 install b', 'python -m pip install c'
+        ]:
+            with self.subTest(pip_cmd=pip_cmd):
+                try:
+                    exec(pip_cmd, {}, {})
+                except SyntaxError as exc:
+                    with captured_stderr() as err_out:
+                        sys.excepthook(SyntaxError, exc, exc.__traceback__)
+
+                self.assertIn("the `pip` command", err_out.getvalue())
+
+
 class StartupImportTests(unittest.TestCase):
 
     def test_startup_imports(self):

Misc/ACKS

@@ -1691,6 +1691,7 @@ Olivier Vielpeau
 Kannan Vijayan
 Kurt Vile
 Norman Vine
+Tom Viner
 Pauli Virtanen
 Frank Visser
 Johannes Vogel

Misc/NEWS.d/next/Library/2018-07-28-17-04-34.bpo-28140.w-sgGe.rst

@@ -0,0 +1 @@
+Give better errors for ``pip install`` command typed into the REPL.