gh-72327: Suggest using system terminal for pip install in REPL

[ichard26]

Users new to Python packaging often try to use pip from the REPL only to be met with a confusing SyntaxError. If this happens, guide the user to use a system terminal instead to invoke pip. The pip project still receives the occasional issue from users lamenting that "pip is not working!" (pypa/pip#13409)

This is an updated version of #8536.

cc @ncoghlan

• Issue: Give better errors for OS commands, like 'pip', in REPL, script #72327

[brianschubert]

The new REPL has a similar special case for when await is used at the top-level:

cpython/Lib/_pyrepl/console.py

Lines 210 to 217 in 1953713

	except SyntaxError as e:
	if e.args[0] == "'await' outside function":
	python = os.path.basename(sys.executable)
	e.add_note(
	f"Try the asyncio REPL ({python} -m asyncio) to use"
	f" top-level 'await' and run background asyncio tasks."
	)
	self.showsyntaxerror(filename, source=source)

Maybe this could be done in a similar way?

Something like this might work:

:...skipping...
diff --git a/Lib/_pyrepl/console.py b/Lib/_pyrepl/console.py
index 8956fb1242e..833e79ef3c2 100644
--- a/Lib/_pyrepl/console.py
+++ b/Lib/_pyrepl/console.py
@@ -195,7 +195,15 @@ def runsource(self, source, filename="<input>", symbol="single"):
                 ast.PyCF_ONLY_AST,
                 incomplete_input=False,
             )
-        except (SyntaxError, OverflowError, ValueError):
+        except SyntaxError as e:
+            if "pip install" in source:  # maybe use a more general regex?
+                e.add_note(
+                    "Did you mean to run the 'pip' command?."
+                    " If so, <insert tip message here>"
+                )
+            self.showsyntaxerror(filename, source=source)
+            return False
+        except (OverflowError, ValueError):
             self.showsyntaxerror(filename, source=source)
             return False
         if tree.body: