Stop the exporter naming a serial it is not sending

python/exporter/cli.py

@@ -480,9 +480,15 @@ async def sign_in(arguments: argparse.Namespace):
 
     # Named as it will actually appear. The model and OS come from FindMy.py, which presents as a
     # MacBook Pro, so somebody who was only told the serial goes looking for a device they own.
+    #
+    # **Read off the account rather than written here.** Serials are drawn per install, so a
+    # literal in this sentence names a device the user does not have and will not find - which is
+    # worse than saying nothing, because they go looking and conclude the entry is somebody
+    # else's. The same mistake as the screen this replaced in the Android app.
     print("\nSigning in registers this exporter as a device on your Apple account. It appears in",
           file=sys.stderr)
-    print("your device list as a MacBook Pro on macOS 13.4.1, serial 0PENTAGXPORT - that is this",
+    print("your device list as a MacBook Pro on macOS 13.4.1, serial "
+          f"{account.serial} - that is this",
           file=sys.stderr)
     print("program, not a Mac you own. Remove it any time at account.apple.com > Devices.\n",
           file=sys.stderr)

python/exporter/wizard.py

@@ -466,10 +466,14 @@ async def _read_icloud(self, asker: Asker):
             # Said before the password is asked for, and naming the entry as it will appear: the
             # model and OS come from FindMy.py, which presents as a MacBook Pro, so somebody told
             # only the serial goes looking for a Mac they own.
+            #
+            # **Read off the account rather than written here.** Serials are drawn per install, so
+            # a literal names a device this user does not have, and they go looking for it and
+            # conclude the entry belongs to somebody else.
             asker.ask(lambda: messagebox.showinfo(
                 "Signing in registers a device",
                 "Signing in adds an entry to your Apple account's device list: a MacBook Pro on"
-                " macOS 13.4.1, serial 0PENTAGXPORT.\n\n"
+                f" macOS 13.4.1, serial {account.serial}.\n\n"
                 "That is this program, not a Mac you own. You can remove it at any time at"
                 " account.apple.com under Devices, from any browser.",
             ))

python/test/test_cli.py

@@ -285,8 +285,19 @@ async def _nothing_ticked(*_args, **_kwargs):
 class FakeAccount:
     """Enough of an `AsyncAppleAccount` for the sign-in flow: it can be closed, and it says so."""
 
+    SERIAL = "0PENTAGXK7QX"
+    """
+    A drawn serial, deliberately not `0PENTAGXPORT`.
+
+    The notice printed before the password prompt used to write the constant into the sentence by
+    hand, and this fake had no `serial` at all - so nothing could tell the difference between the
+    sentence naming the account and the sentence naming a literal. It named a literal for as long
+    as serials were constant, and kept naming it afterwards.
+    """
+
     def __init__(self) -> None:
         self.closed = False
+        self.serial = FakeAccount.SERIAL
 
     async def close(self) -> None:
         self.closed = True
@@ -348,6 +359,23 @@ class TestRememberingTheDevice:
     not recognise. See `exporter.device`.
     """
 
+    def test_the_notice_names_the_serial_this_run_presents(self, apple, capsys):
+        """
+        The sentence shown before the password prompt tells the user what to look for.
+
+        **It used to be a literal**, written when every install shared one serial and left there
+        when they stopped. A user on a drawn serial was told to look for `0PENTAGXPORT`, would not
+        find it in their device list, and would reasonably conclude the entry they did find
+        belonged to somebody else - which is the belief that gets it removed, taking the session
+        with it. The same defect was fixed on the Android side's registered-device screen.
+        """
+        asyncio.run(cli.sign_in(signing_in()))
+
+        notice = capsys.readouterr().err
+
+        assert FakeAccount.SERIAL in notice
+        assert "0PENTAGXPORT" not in notice
+
     def test_an_ordinary_sign_in_is_remembered(self, apple):
         # The path almost every run takes, and the one that was storing nothing: the call sat in
         # the terms handler, so only an account with unaccepted terms kept its identity.