py-screen

A terminal workspace of my own, in Python: not a clone of screen or tmux, built for fun, made more powerful and improved by daily use. The design is in spec/; the current version is spec/spec.v3.md.

Status: M1 in progress (Linux; macOS and Windows on the platform layer, tested on CI). One server, many windows, attach/detach, hot reload, flood-safe typing.

How it works

client (pys attach) ──► brain (Python, restartable) ──► keeper (tiny, stable) ──► ptys

• keeper owns every pty and child process and is the only reader of their output (a byte pump with a ring buffer per window). It restarts the brain when it exits.
• brain holds a pyte screen per window, renders the current one plus a status line, handles keys and answers RPCs. pys reload restarts it; shells never notice, because the ptys live in the keeper. The new brain replays the ring buffers, then nudges each pty (SIGWINCH) so full-screen apps repaint.
• client puts the terminal in raw mode, forwards keys and size, prints frames, and reconnects by itself across a brain reload.

Floods: the screen model is lossless

Every byte a window writes is parsed, in order, whether the window is shown or hidden, attached or not. That matters most for apps that draw by diff, like Claude Code: they write only the cells they think changed and never rewrite a cell they think is right, so a single lost byte leaves wrong cells for good (and a lost mode switch, such as the alternate screen or bracketed paste, breaks input too). Until 2026-11-01 py-screen parsed only the tail of a flood and dropped a hidden window's output past 128 KB; that is what left stray characters in Claude's status line.

Parsing is the slow part (pyte in Python; py-screen's fast paths for plain text, line feeds and CSI sequences take it from 0.85 to 1.7 MB/s of scrolling text), so:

• Backpressure, not dropping. The keeper gives each window a credit of 256 KB un-acked output and stops reading that pty past it. The brain acks bytes only once it has parsed them, so a flooding app simply waits on its write, as under GNU screen. One window can't fill the shared stream, so another window's echo never waits behind it.
• Small steps. The brain parses in steps of 2 ms (2 KB per window per turn, shown windows first, the others round robin), then goes back to keys and frames.
• Keys first. For 100 ms after a key, only shown windows are parsed: nothing else stands between a key and its echo (about 2 ms median while a hidden window floods).

A flood runs at the parser's speed. With the vterm engine (libvterm, in C) that is faster than GNU screen: seq 1 5000000 takes 1.6 s against screen's 13.4 s, a 50 MB coloured log 0.9 s against 8.1 s (bench/README.md has the history).

Screen engines. Each window's screen is a term.Term; PYS_TERM picks the engine:

PYS_TERM=vterm makes a missing extension an error instead of a quiet fallback.

A sequence that pyte can't apply (an extra parameter, a ? where none is defined, an unknown erase value) is ignored, as terminals do; the bytes after it are still parsed.

Releases

Rolling releases: every push to main that passes is published as <YYYYMMDD>.<N>, the version and the tag alike, on PyPI as py-screen-mux and as a GitHub release, once, never replaced. Pushes that change only docs, specs or bench results publish nothing. A release must pass, in release.yml:

• the suite on Linux and macOS, and the RPC smoke test on Windows, on both screen engines (vterm and pyte);
• the vterm engine built with ASan + UBSan and fuzzed against libvterm's own screen (a long run every night, in sanitize.yml);
• a parse-speed check: vterm's lead over pyte must keep at least half of the newest recorded run's (release/bench_gate.py, against bench/results/parse-*.json);
• each wheel, built by cibuildwheel from the sdist, installed and smoke-tested on the vterm engine on its own system: Linux x86_64 and aarch64 (manylinux), macOS arm64 and x86_64, Windows x86_64 and ARM64.

Install with uv, which fetches Python 3.14 if needed and picks the wheel for your system:

uv tool install py-screen-mux      # the command is pys

The package is py-screen-mux on PyPI because py-screen is too close to an existing pyscreen there; the import is py_screen and the command pys.

Elsewhere, the sdist builds with a C compiler, or runs the pure-Python pyte engine without one.

Use

uv tool install -e .       # puts `pys` in ~/.local/bin, running this checkout (edits apply on reload)
pys                        # attach (starts the server on first use)
pys ls [--json]     # windows
pys new [--select] [-- cmd …]
pys send 0 'make test' --enter
pys capture 0       # the window's screen as text
pys reload          # new brain, same shells
pys kill            # end everything

Keys: the prefix is C-a, or C-] when you run pys inside GNU screen ($STY set), so the two don't fight. Override with PREFIX in the config or PYS_PREFIX.

Without the prefix: Alt-0…Alt-9 select, F2 opens the list, Alt-Tab / Alt-Shift-Tab go to the next / previous window. Desktops usually keep Alt-Tab for themselves (GNOME: org.gnome.desktop.wm.keybindings switch-applications), so it reaches py-screen only where the desktop lets it through, e.g. over ssh from another machine. Inside GNU screen, screen's own bindkey lines may also take the Alt keys first.

Mouse

On by default, like tmux's mouse on:

Every report is routed to the pane under the pointer and rewritten in that pane's coordinates, in the encoding the app asked for (SGR 1006, or the old X10 form). A press grabs its pane: the drag and the release go there even if the pointer leaves it. MOUSE = False in the config gives plain drags back to the terminal; the mouse is then pys's only while the focused app asks for it (still translated per pane).

Scrollback

Each window keeps the rows that scroll off the top of its screen (SCROLLBACK, 5000 by default; the alternate screen of full-screen apps keeps none, as in xterm). The wheel or a touch drag (a phone's terminal sends it as wheel reports), or Shift-PgUp / Shift-PgDn, scroll the pane back and forth through them; the caption shows [rows back/rows kept]. The view is only a view: the app keeps running and drawing (Claude Code's input box included), and output that arrives meanwhile doesn't move the rows you are reading. Any key that goes to the app takes the pane back to live. ESC [ 3 J (what clear sends) empties the history. pys capture --history prints it.

A row costs about 36 bytes per cell up to its last visible one: a window full of 100-column lines holds ~18 MB at 5000 rows. Keeping rows has a price on floods too: about 0.2 µs a line (see bench/README.md).

Config

~/.config/pys/config.py (plain Python, read on every brain start, so C-a R applies an edit): status widgets and extra key bindings. See src/py_screen/config.py for the helpers.

WIDGETS = [cmd(["git", "-C", "{cwd}", "branch", "--show-current"], every=10), load(), clock()]
BIND = {"T": ("top", ["htop"])}   # C-a T opens a window running htop
MOUSE = True                      # False: plain drags select text in the terminal
SCROLLBACK = 5000                 # rows of history per window; 0 = none

The layout (splits, focus, renamed titles) is saved on every change, so a reload keeps it.

Sockets live in $XDG_RUNTIME_DIR/pys/ (PYS_DIR overrides; keep it short: an AF_UNIX path is limited to 108 bytes). Server log: server.log in that dir.

Test and measure

uv run pytest -s                                      # M0 done-criteria (spec §6)
uv run python bench/m0_speed.py --work <scratch dir>  # speed vs GNU screen (spec §8.1)

Results of each run are kept in bench/results/, one file per run, and bench/README.md tells how the speed evolved, one row per run.

Platforms

Everything that differs between systems lives in src/py_screen/platform/ (spec §5.2); the keeper, brain and client import nothing else that is OS-specific.

CI: Linux runs the full suite on every push; macOS (full suite) and Windows (tests/test_rpc_smoke.py, plus spike/win_check.py) run when the platform layer changes, or by hand (platform workflow).

Known gaps (M1)

• A mouse report split across two reads is passed on as keys (rare; the outer terminal writes each report at once).
• After a brain restart the screens are rebuilt from the keeper's last 2 MB per window; if that ring wrapped, older state is gone and full-screen apps get a SIGWINCH to repaint. Keeping the screens across a planned reload is open.
• Floods are bounded by pyte's speed (see "Floods").
• Scrollback is a view only: no copy mode or search yet (select with Shift+drag in your terminal), and a resize doesn't rewrap old rows.
• Windows: pys attach needs a real console (Windows Terminal or conhost, not mintty); a new window doesn't inherit the focused window's directory (it isn't read from the child yet), and the load widget is empty. The pty harness tests are POSIX-only, so Windows CI runs the RPC smoke test.
• Windows named pipes keep the default DACL (other local users may open them); instead every connection must first send a per-session key from the user-private runtime dir. A per-user DACL would close the pipe itself; still to do.