gh-72327: Add help message for pip in REPL

Lib/site.py

@@ -390,6 +390,22 @@ def setcopyright():
         files, dirs)
 
 
+def _register_detect_pip_usage_in_repl():
+    old_excepthook = sys.excepthook
+
+    def detect_pip_usage_in_repl(typ, value, traceback):
+        old_excepthook(typ, value, traceback)
+        if typ is SyntaxError and (
+            "pip install" in value.text or "pip3 install" in value.text
+        ):
+            print("\nThe Python package manager (pip) can only be used from"
+                  " outside of Python.\nPlease try the `pip` command in a"
+                  " separate terminal or command prompt.",
+                  file=sys.stderr)
+
+    sys.excepthook = detect_pip_usage_in_repl
+
+
 def sethelper():
     builtins.help = _sitebuiltins._Helper()
 
@@ -558,6 +574,10 @@ def main():
     setquit()
     setcopyright()
     sethelper()
+    # Detect REPL by prompt, -i flag or PYTHONINSPECT env var being set.
+    if getattr(sys, 'ps1',
+               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 +455,32 @@ 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()
+        self.save_stderr = sys.stderr
+
+    def tearDown(self):
+        sys.stderr = self.save_stderr
+        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):
+                stream = io.StringIO()
+                sys.stderr = stream
+
+                try:
+                    exec(pip_cmd, {}, {})
+                except SyntaxError as exc:
+                    sys.excepthook(SyntaxError, exc, exc.__traceback__)
+
+                self.assertIn("the `pip` command", stream.getvalue())
+
+
 class StartupImportTests(unittest.TestCase):
 
     def test_startup_imports(self):

Misc/ACKS

@@ -1826,3 +1826,4 @@ Jelle Zijlstra
 Gennadiy Zlobin
 Doug Zongker
 Peter Åstrand
+Tom Viner

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.