Fix #3523

docs/changelog.rst

@@ -29,6 +29,9 @@ To update |kitty|, :doc:`follow the instructions <binary>`.
 - GNOME: Fix maximize state not being remembered when focus changes and window
   decorations are hidden (:iss:`3507`)
 
+- Fix reading :option:`kitty --session` from ``STDIN`` not working when the
+  :option:`kitty --detach` option is used (:iss:`3523`)
+
 
 0.20.1 [2021-04-19]
 ----------------------

kitty/main.py

@@ -306,6 +306,9 @@ def _main() -> None:
         create_opts(cli_opts, debug_config=True)
         return
     if cli_opts.detach:
+        if cli_opts.session == '-':
+            from .session import PreReadSession
+            cli_opts.session = PreReadSession(sys.stdin.read())
         detach()
     if cli_opts.replay_commands:
         from kitty.client import main as client_main

kitty/session.py

@@ -137,6 +137,14 @@ def parse_session(raw: str, opts: Options, default_title: Optional[str] = None)
     yield finalize_session(ans)
 
 
+class PreReadSession(str):
+
+    def __new__(cls, val: str) -> 'PreReadSession':
+        ans: PreReadSession = str.__new__(cls, val)
+        ans.pre_read = True  # type: ignore
+        return ans
+
+
 def create_sessions(
     opts: Options,
     args: Optional[CLIOptions] = None,
@@ -146,6 +154,9 @@ def create_sessions(
     default_session: Optional[str] = None
 ) -> Generator[Session, None, None]:
     if args and args.session:
+        if isinstance(args.session, PreReadSession):
+            session_data = '' + str(args.session)
+        else:
             if args.session == '-':
                 f = sys.stdin
             else: