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Python in cells: dates and times in xl() and in results, in the 1900 and 1904 date systems - #499

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@prxnvt prxnvt commented Oct 12, 2026 •

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Depends on #477 (which depends on #469 and #462). Only the last commit belongs to this pull request.

Summary

Cells formatted as dates reached Python as bare serial numbers, so code had to convert them by hand, and dates that a cell returned were written back as plain numbers. With this change, on CPython, xl() gives cells formatted as dates or times as datetime.date, datetime.datetime and datetime.time values, and a DataFrame built by xl() has datetime64 columns for its dates. Dates and times that a cell returns, pandas Timestamp values included, are written back as serials, and the cells that show them receive a date or time format when their format is General. The 1900 date system (with its fictitious 1900-02-29) and the 1904 date system are both handled. The built-in Python continues to receive serial numbers.

Changes

  • Protocol (python/protocol.rs). XlReply::Dated { value, dates } marks the numbers that are dates or times, with a DateCell { at, kind } for each (date, time or dateTime). Output::dates marks the same in a result. Serials cross the protocol in the 1900 date system. Both additions are optional on the wire, so earlier frames still parse.
  • Number formats (numfmt). NumberFormat::shows_date and shows_time report whether a date or time format shows a calendar part, a clock part or both. The kind of a cell comes from these.
  • Engine (engine/python.rs).
    • An xl() reply for one area (a cell, a range, or a name or table of one range; a reference past the used range reads its top-left part) marks numbers whose format is a date or time format. Negative numbers and numbers past 9999-12-31 are not marked.
    • In a 1904 workbook, the serials of dates are converted to and from the 1900 system (1,462 days). Times of day are unchanged.
    • Result positions outside the value, and repeated positions, are discarded. Cells in the General format that show a date or time result receive m/d/yyyy, m/d/yyyy h:mm or h:mm AM/PM, the formats that TODAY(), NOW() and TIME() give. A blocked spill formats nothing, and a format the cell already has is kept.
  • CPython shim (python/shim.py).
    • Marked serials become date, datetime or time values, to the millisecond. A date-only cell that holds a time of day becomes a datetime, so nothing is lost.
    • Numbers that do not stand for such a value stay numbers: serials 0 and 60, negative numbers, numbers past 9999-12-31, and times of a day or more.
    • When a DataFrame is built, a column whose values are all dates (or empty) becomes datetime64. The .dt accessor therefore works on it.
    • Results of type date, datetime, pandas Timestamp, numpy datetime64 and time become marked serials.
    • Dates before 1900-03-01 now follow Excel's serials: 1900-01-01 12:00 is 1.5, where it was 2.5. Dates before 1900 are written as plain numbers.
  • Built-in Python (python/worker.rs). It accepts XlReply::Dated and receives the serial numbers. Monty can construct date values, but its result API does not expose their components, so dates could not be read back. This limitation is documented.
  • Documentation. architecture.md has a "Dates and times" entry in the Python section, and function-parity.md's Python row covers the new behaviour. ROADMAP.md has a progress-log row.

Testing

  • cargo fmt --all --check and cargo clippy --workspace --all-targets -- -D warnings are clean.
  • cargo test --workspace --no-fail-fast passes except for three tests that already fail on main. These are the French i18n test and the two CSV separator tests, which Fix main CI: CSV export writes General numbers in full; French header/footer labels #449 and fix(i18n): correct scattered wrong French translations and add 2 missing keys #454 fix.
  • cargo xtask assets, layers, wasm, parity --check and keytips --check pass.
  • New and updated tests:
    • Protocol round trips for XlReply::Dated and Output::dates.
    • NumberFormat::shows_date and shows_time.
    • The engine's marking of dates, times and date-times in the 1900 and 1904 systems. This includes whole columns, text in a date format, and negative and too-large numbers.
    • The engine's formatting of results: spilled tables, single values, blocked spills, existing formats, repeated and out-of-range positions, and the 1904 conversion.
    • The built-in Python receiving serial numbers.
    • The shim's conversions at the edges of the 1900 system: 1900-01-01, 1900-02-28, the fictitious 1900-02-29, 1900-03-01 and 9999-12-31. Also serial 0, negative numbers, 2958466, 1e300, times of a day or more, a third of a day to the millisecond, and text marked as a date.
    • The shim's conversions back to serials, including a time zone-aware datetime and a date before 1900.
    • A pandas DataFrame with a date column (datetime64, .dt.strftime), a time column, and a column with a number that is not a date. This test is skipped when pandas is not installed.
    • A CPython session end to end: ISO dates and a time read through xl(), then date, datetime and time results with their formats. The same workbook is then switched to the 1904 system.
  • The shim and session tests ran against the system CPython 3.14 without pandas, and against a temporary virtual environment with pandas 3.0.6 and numpy 2.5.4 (GRIDCRAFT_TEST_PYTHON).
  • Headless check with gridcraft-cli run, with GRIDCRAFT_PYTHON_ENV pointing to a temporary location. On CPython without pandas, xl("A1:B4", headers=True) gave date values. With pandas, it gave Timestamp values (1900-02-28 and 1900-03-01 correct in both cases). A time cell read as 18:30:00. Returned dates displayed as 2/14/2024 and 1/15/2024 18:30.

Generative AI use: The code and descriptions in this pull request were written with the assistance of Claude Opus 5.5 (Claude Code). I directed the work, chose its scope, and reviewed and tested the changes.

prxnvt added 16 commits October 11, 2026 21:03
Session::grouped(label, f) runs f as one undo step on the active
document. On success the group leaves exactly one undo entry holding the
workbook from before the group; on error it restores the workbook,
selection, undo and redo stacks as they were and returns the error.
group_begin, group_end and group_rollback do the same across separate
calls, for an action that runs one step per call.

While a group is open on a document, undoable commands on it add no
undo steps of their own, so a long group neither grows the history nor
pushes older entries past the undo limit, and Undo and Redo are
disabled there. Groups nest: the outermost one makes the step, and a
failing inner group rolls back only its own changes. Closing a document
drops its groups, and a command that panics drops the groups it opened.
history.list reports whether a group is open.

Insert Copied Cells folds its nested steps into one undo step by
counting revisions; inside a group the nested steps push no entries, so
it now leaves the history alone there instead of removing entries from
before the group.
A macro recorder needs more than a journal index: the journal is
trimmed past 10,000 entries, it also holds the commands that other
commands run (so playback would run them twice), and it keeps no
selection, which relative references need.

Session now tracks how deeply execute calls are nested. While recording,
a buffer (record.rs) keeps each successful, journaled top-level call with
the document uid, the active sheet (index and name) and the selection it
ran in, then its id and parameters without secrets, as the journal
stores them. Queries, failed commands and the command that starts
recording are not kept. The buffer holds up to 100,000 steps and counts
any it drops. Session::start_recording, stop_recording, is_recording and
depth are the interface.

Automate › Record Actions keeps recording through the journal as before.
Active content such as macros must not run only because a workbook was
opened. DocState gets a trust state: Local for workbooks made in the
session (new workbooks and samples) and FromFile for anything opened
with file.open. Active content in a FromFile workbook asks before it
runs. A second window on a workbook shares its trust.

The clipboard carries the trust of its source document. Pasting
formulas, formats (conditional formats hold formulas) or validation
from a FromFile workbook into another workbook makes that workbook
FromFile too; pasting values, column widths or notes carries nothing
active and changes nothing. Sheet moves and copies stay inside one
workbook, so its trust is unchanged. document.inspect reports the trust.
GridCraft keeps workbook data in hidden defined names so it travels
inside the xlsx file. The existing items write one name holding one
string literal, which past Excel's 255-character literal and
8,192-character formula limits gives a file Excel does not accept.

cmd/stash.rs stores an item `key` as hidden workbook-scope names
_gc_<key>_0, _gc_<key>_1, ..., each a formula of string literals of at
most 255 characters (UTF-16 units, as Excel counts them) joined with &,
at most 8,192 characters per name. Reading concatenates the chunks in
order and stops at a gap; it also accepts the older single _sc_<key>
name. Names are matched case-insensitively and only at workbook scope.
Keys are limited to letters, digits, _ and ., items to 4,194,304
characters, and junk around or inside the literals, duplicate or
out-of-order chunks, non-JSON and over-deep JSON all read safely.
keys(prefix) lists the stored items that start with a prefix.

Custom views, scripts, the watch list and scenarios keep their current
storage; nothing reads or writes the new names yet.

docs/architecture.md describes the engine building blocks added in this
series: undo groups, the recording buffer, document trust and this
storage.
…its JSON-lines protocol

Python code for cells runs in Monty 1.1.0 (MIT), a Python interpreter
written in Rust for sandboxed code, inside a separate worker process:
the app binary started again with --python-worker. Code such as
'x' * 10**12 makes an interpreter allocate until the process aborts, and
10**10**8 never checks the interpreter's time limit; catch_unwind can
stop neither, so only a separate process that the app can kill is a
reliable harness. RustPython was considered first, but every recent
release depends on the LGPL-3.0 malachite crates, which would be linked
into the app binary.

The new crate gridcraft-python (layer 1, on core) holds:

- protocol: one JSON object per line. The parent sends Init {limits,
  init_code}, Run {id, code}, XlValue {id, reply} and Reset; the worker
  answers Ready {init_error}, Xl {id, reference, headers} and Done {id,
  output, stdout, stderr, error {line, message}, elapsed_ms}. Worker
  frames start with a marker, so other text that reaches its standard
  output is never taken for a frame. Values travel as
  gridcraft_core::Value.
- worker (feature `python`, not on wasm32): one Monty REPL whose
  namespace is shared by every cell, with the initialization code run at
  start and on Reset. print output is collected per run (1 MB, then cut
  off). xl() asks the parent and waits for the reply; file, environment
  and network access are refused with PermissionError. Limits: the time
  limit, recursion depth 200, 10,000 xl() calls per run and 1,000,000
  output cells. A closed input pipe ends the worker even while code
  runs.
- conversions: None to empty, bool, int and float to numbers (NaN,
  infinities and integers too large for a double give #NUM!), str to
  text, a list to a column, a list of lists to a table padded with #N/A,
  a dict to a key/value table, and any other object to its repr. xl()
  gives a scalar for one cell, a list of rows for a range, and a list of
  dicts keyed by the first row with headers=True.
- client: PyWorker starts the worker and reads its frames on a thread,
  so poll() never blocks longer than asked. A run past its time limit
  plus one second is killed (#TIMEOUT!), a watchdog reads the worker's
  resident set size every 50 ms and kills it above 512 MB, and a worker
  that exits fails its runs; the next run starts a fresh worker. An
  in-process transport runs the worker on a thread for tests.

apps/gridcraft and apps/gridcraft-cli check for --python-worker first
thing in main, before the logger, panic hook or window.

Tests: protocol round trips, conversions in both directions, error
lines, refused host access and the output cap; in-process runs with a
shared namespace, xl(), initialization code, reset and initialization
errors; and, on a real process, `while True: pass` and 10**10**9 time
out, 'x' * 10**12 is stopped by the memory watchdog, a killed worker is
replaced, and the host keeps running throughout.
…! errors

Cell gains `py: Option<Arc<PyCode>>`, read with a serde default and not
written for ordinary cells, so older native files load and ordinary
cells serialize as before. PyCode holds the code, the output mode
(Python object or cell values), the last result (value, type name,
preview, whether it converted to cells) and a stale flag for manual
calculation. The cell's `value` stays the cached result, so formulas,
display and saving read a Python cell like any other. is_blank,
CellStore::has and input_text (which returns the code) take the code
into account; Cell::has_content and clear_content cover value, formula
and code together. The model also reads and writes the file form,
PY("chunk"&"chunk",0|1) with string literals of at most 255 characters,
and finds the xl("...") literals in code, skipping comments and other
strings. Code is limited to 7,000 characters so that the formula stays
under the 8,192-character formula limit.

Edits: cell.set over a Python cell, Clear Contents, Clear All and the
cells a merge empties remove the code; Clear Formats keeps it. Copy and
paste (All, Formulas), Fill, sort, cut-move, sheet copy and undo carry
it unchanged; Paste Values and Paste Link do not, and paste operations
do not combine numbers with a Python cell. Fill copies Python cells
instead of extending them as a series. cell.set with =PY(...) is
refused with a message that points to python.set. The sheet-protection
check compares the code, so a locked Python cell cannot be changed.
cell.get reports {python: code, output, typeName, preview, stale}.

The xl() literals follow reference-moving edits: rewrite_all_formulas
(row and column insert and delete, cell insert and delete, cut-move),
sheet rename and sheet delete adjust each literal like a formula
reference, and replace it only when it changed.

Errors: CellError gains Python, Busy, Timeout and Blocked, and ALL has
14 entries, so the lexer reads them as literals. ERROR.TYPE gives 11 for
#BLOCKED!, the number Excel returns. Microsoft documents no ERROR.TYPE
number for #PYTHON!, #BUSY! or #TIMEOUT!; they take 19, 20 and 21, after
the numbers Excel is known to use. The xlsx writer caches the four as
#VALUE!, as it already does #SPILL! and #CALC!, because they are not
error codes of the file format.

Tests cover the =PY refusal, removal by entry and clearing, undo, copy,
paste values, fill and sort, xl() rewrites through row insert, sheet
rename, cut-move and column delete, native JSON round trips, and the
new errors.
…rker per workbook

After every top-level command that changed a workbook, Session::execute
hands the workbook before and after to the Python runtime
(engine/src/python.rs), which finds the cells to run: Python cells
whose code changed, whose xl() literals read cells that changed (cell
store and spill differences, sheet by sheet; references resolve like a
formula's, so names, tables and Sheet! references work), or whose
literal names something only known at run time; every Python cell after
the first of those, in row-major order sheet by sheet, because they
share one namespace; all of them when a call is xl(<non-literal>) or a
sheet with Python cells disappeared; and the cells after a removed
Python cell. Python cell positions are cached per sheet version and
patched from the edit's band differences, so a workbook without Python
pays only for the bands an edit touched. Nothing runs when a workbook is
opened.

Each cell runs once per refresh: run, write the value (an array spills
through calc::lay_python_spill, or shows #SPILL! when blocked), let calc
recalculate its formula dependents, then the next cell. Results go
straight into the workbook and are not undoable, as for data tables: one
undo takes back the edit and the results it caused. A Python cell fed,
through formulas or directly, by a later Python cell in the same refresh
gets #CALC!. xl() is answered from the current workbook and refused for
non-references, missing sheets and names, deleted (#REF!) cells and
ranges over the output cap.

Calc keeps a Python cell's spill: cells_changed no longer drops it as it
does a replaced formula's, recalc_all lays Python spills out again from
the kept results, and after each edit the runtime lays out again the
spills of Python cells showing #SPILL! or whose area the edit touched,
so clearing a blocker spills again without running the code.

Runs happen inside the command for headless callers (CLI, MCP, tests).
With set_python_background(true) a host steps them with step_python();
waiting cells show #BUSY!, and an edit, undo or reset during a refresh
restarts it from the earlier of its next cell and the new first cell,
discarding the result in flight. Undo, redo and the panic restore mark
the namespace dirty: the next refresh resets it and runs every cell. A
failure that took the worker down (time limit, memory, crash) starts a
fresh worker, which runs the earlier cells again once before the queue
continues. In manual and automatic-except-tables calculation, affected
cells are only marked stale until Calculate Now or Calculate Sheet runs
them. Workbooks opened from a file (Trust::FromFile) run nothing, and
new or edited code there shows #BLOCKED!. Without a worker launcher (the
web build, or a host that set none) cells show #BLOCKED!.

Limits: Prefs.python_timeout_seconds (5 s) per cell, 512 MB per worker,
recursion depth 200, 1 MB of output kept, 1,000,000 output cells. Every
run is recorded in DocState.py_log, a ring buffer of 500 entries with
the cell, status, duration, output and error line.

gridcraft-python gains run.rs (the launcher, run outcomes, and the
default launcher that apps set at start-up) and a web stand-in for the
client with the same API, so the engine needs no cfg of its own. Both
apps set the default launcher to their own binary.

Tests: order and the shared namespace, xl() dependencies with formula
dependents, the Python to formula to Python chain, the cycle error,
spills blocked, unblocked and kept through a structural edit, errors
with their line and output, time limits, ranges, names and other
sheets, the trust gate, manual calculation, undo resetting the
namespace, background runs with #BUSY! and restart, and a real worker
process killed mid-refresh with the earlier cell run again.
…om files

cmd/python.rs registers the Python commands, all reachable from the
control channel, the CLI and MCP:

- python.insert {cell?} (Formulas > Python, Ctrl+Alt+Shift+P) makes the
  cell a Python cell with no code yet;
- python.set {cell?, code, output?} sets the code (at most 7,000
  characters) and runs it, answering with the cell's value; the same
  code again keeps its result without running;
- python.clear {cell?} (Convert to Normal Cell) turns Python off,
  clearing the code and value, undoably; the cells after it run again;
- python.output {cell?, mode: object|value} shows the result as a
  Python object (its type name, with no spill) or converts it to values;
- python.reset {clearDiagnostics?} starts Python afresh and runs every
  Python cell again;
- python.diagnostics {sheet?, errorsOnly?} returns the last 500 runs;
- python.initialization {code?} returns, or sets and then runs again
  with, the workbook's initialization code, kept in the hidden workbook
  data item py_init;
- python.options {timeoutSeconds?} reads or sets the per-cell time
  limit (0.1 to 3600 s, default 5);
- python.enable lets Python run in a workbook opened from a file
  (FromFile becomes Trusted) and runs its Python cells.

None of them opens a dialog. With empty parameters they act on the
active cell or report state, or return an error.

Trust gate: a workbook opened from a file runs no Python; code set
there shows #BLOCKED!, and python.reset does not change that;
python.enable does, by making the workbook Trusted, a new Trust state
for a file whose active content the person allowed to run.
document.inspect reports it as `trusted`. A locked cell on a protected sheet refuses
python.set through the existing protection check, which compares
Python code.

python.set, insert, output and clear refresh inside the command so they
can report the result, and tell Session::execute that the refresh is
done (only as the top-level command; inside another command, that
command's refresh covers them).

Tests: set, rerun avoidance, clear and undo, insert, output modes, the
code limit, parameter errors, protection, reset and diagnostics
filters, initialization code (first, on change, its errors), the trust
gate and enabling, a missing worker, the time-limit setting, and a CLI
test that drives python.set through gridcraft-cli with Python in a real
worker process: `while True: pass` gives #TIMEOUT!, 'x' * 10**12 gives
#PYTHON!, and the next cell still runs.
…s for Python content

A Python cell is written to xlsx as an ordinary formula,
PY("chunk"&"chunk",0|1), with string literals of at most 255
characters, 1 for a Python object result and 0 for values, and no
_xlfn. prefix, together with its cached value. A spilling result is
written like a dynamic array (cm="1", t="array" ref=...) with the
spilled values cached, so other spreadsheet applications show the
values (and #NAME? if they recalculate). The reader turns a formula
that is PY( followed by string literals, and optionally ,0 or ,1, back
into a Python cell, reading the formula's own text so that line breaks
in the code survive, and keeps the cached spilled values as the cell's
result, so a reopened workbook shows its arrays without running
anything. Python errors are cached as #VALUE!, as #SPILL! and #CALC!
already are. The initialization code travels in the hidden workbook
data names. Native JSON keeps Cell.py whole: code, output mode, the
last result and the exact error.

Opening any file adopts PY(...) formulas from every format
(io::adopt_python) and then applies limits to Python content
(io::enforce_python_limits): a cell with more than 7,000 characters of
code keeps only its cached value, with a warning, and a kept result
that is not a well-formed array of at most 1,000,000 cells is dropped.
calc::lay_python_spill refuses such an array too (#SPILL!), so a
crafted file cannot make it loop over a claimed size. Opening a file
still runs nothing: the workbook is FromFile, shows its cached values,
and runs Python only after python.enable.

python.set and python.insert refuse a table's header row, which holds
the column's name, as Excel keeps formulas out of headers.

Catalog: Formulas > Python gains Insert Python, Reset, Diagnostics and
Initialization; `cargo xtask parity` gives 278 / 294 (94.6%).

Tests: xlsx round trip of values, a formula over a Python cell, a
spilling table, an object-mode cell, 7,000 characters of code with
quotes and line breaks, an error and initialization code, checking the
written PY() formulas; the native JSON round trip; hostile content (code
over the limit in JSON and xlsx, a result claiming far more cells than
it holds); and a malformed result that never spills.
When the person has Python 3.11 or newer, Python cells run on it, so
pandas and numpy work; otherwise they run on the built-in Monty worker.
GridCraft never installs an interpreter or packages.

Finding the interpreter (gridcraft-python `interp`): the search order
is the path from python.options (a bare name is looked up on PATH),
VIRTUAL_ENV, CONDA_PREFIX, python3 then python on PATH, and on Windows
the `py -3` launcher and the usual install folders, newest first. The
Microsoft Store shortcuts under %LOCALAPPDATA%\Microsoft\WindowsApps
are skipped with a reason, since they open the Store instead of running
Python. Each candidate is probed under a 5 s deadline: first with -I
for its version (anything older than 3.11 is rejected with the version
it reported), then with -P -X utf8 for sys.executable, the pandas and
numpy versions (from package metadata, without importing pandas) and
whether it is externally managed (PEP 668). Cells run with
sys.executable, so a virtual environment and the program behind
`py -3` are used directly. The search takes a Host trait, so tests run
it against fake machines, including a fake Windows one. It runs on its
own thread and never on the web build. The engine caches the result per
session and searches again when the interpreter option changes, on
python.reset and on python.options {detect: true}. Apps opt in with
interp::enable_search(); tests and library users keep the built-in
Python or the launcher they set. Prefs.python_interpreter keeps the
path.

Running on it: the parent's process handling is unchanged (the deadline
kill, the memory watchdog, the restart after a crash and the JSON-lines
protocol, including the xl() round trip). Only the child differs:
`<python> -P -X utf8 -u -c <shim>` instead of the app binary with
--python-worker. -P keeps the current folder off sys.path; -I and -s
are not used because they hide user site-packages, where
`pip install --user` puts pandas.

- crates/python/src/shim.py, embedded with include_str!, is the worker
  for CPython. It is data that the user's interpreter runs, not app
  code. It keeps frames on a private copy of standard output and points
  file descriptor 1 at standard error, so output from C code or child
  processes cannot break a frame; reads input on a thread and exits
  when GridCraft goes away; collects print output per run up to the
  limit; stops a run at the time limit with a timer and interrupt_main
  (the parent's kill covers C calls that cannot be interrupted), and
  installs the SIGINT handler itself, since a parent started with SIGINT
  ignored passes that on; and returns the last expression's value.
  SystemExit is a cell error, not an exit.
- Conversions: xl() gives a pandas DataFrame when pandas is installed
  (imported on first use; the first row is the header with
  headers=True) and lists otherwise, as the built-in Python does.
  Results: DataFrame (with the index only when it is not the default
  RangeIndex), Series, one- and two-dimensional ndarrays, lists and
  dicts spill; numpy scalars become numbers or booleans; datetime, date
  and Timestamp become date serials; NaN and infinities become #NUM!;
  None, NaT and NA become empty; anything else is an object with its
  repr. The limits on output cells and xl() calls apply.
- The recursion limit on CPython is at least 1000, since pandas calls
  deeper than the built-in Python's 200 allows.
- Workers start in a process group of their own (a job object on
  Windows) through process-wrap (MIT OR Apache-2.0), so a kill also
  stops processes the user's code started; CREATE_NO_WINDOW keeps a
  console window from appearing on Windows. The worker's standard error
  is drained on a thread, keeping its last 8 KB.
- A worker that exits or times out before its Ready message is started
  once more and its waiting runs are sent again.
- The engine picks the launcher per run: one set for the session, else
  the detected CPython, else the built-in worker. In the background,
  cells wait as #BUSY! while the search finishes; headless callers wait
  for it. A new search stops the workers so that they start again on
  what it finds, from a fresh namespace, and an options change runs the
  cells again.

python.options takes `interpreter` and `detect` and reports the
backend; python.diagnostics reports it too: kind, summary line, path,
version, pandas, numpy, source and every rejected candidate with its
reason.

The user's CPython has the access of the person's own account: files,
network and processes. The trust gate is what keeps a workbook opened
from a file from running code on it before python.enable.

Tests: the search against fake machines and this machine; shim protocol
tests that run when this machine has Python 3.11 or newer and are
skipped with a message otherwise (xl() round trip, conversions, print
capture, errors with lines, the time limit, the shim's own and the
parent's kill, a loop stopped with SIGINT ignored, reset, and a child
process killed with the worker's group); pandas tests that run only
when pandas imports; the retry before Ready; and an engine run of cells
on the detected CPython.
…against crash loops

The built-in Python (Monty) is the fallback when no CPython is found. It
now refuses code it cannot run before any of it runs, instead of
running part of it, and no failure can hang the application or make
the worker restart in a loop.

- Imports: a small tokenizer (crates/python/src/imports.rs) reads the
  import statements of a cell, with strings and comments blanked,
  bracketed and backslash-continued lines joined, and statements split
  on `;` and `:`. A module Monty does not have (pandas, numpy, scipy,
  statistics and others) refuses the cell with #PYTHON! and "This cell
  needs CPython: `<module>` isn't available in the built-in Python.
  Install Python 3.11 or newer and set it in Python Options.", at the
  import's line. The scan is bounded and never fails.
- Constructs: the cell is first compiled on its own; a
  NotImplementedError from Monty's compiler (match, del, yield, type
  aliases, wildcard imports and others) refuses it with "This cell
  needs CPython: the built-in Python doesn't support <what>." Other
  compile errors still come from the run, as before.
- Messages, on both backends: a time limit gives #TIMEOUT! "Stopped
  after N s (an endless loop?)", whether the interpreter stopped itself
  or the parent killed it; RecursionError gives #PYTHON! "Too much
  recursion", as does a worker that died of a stack overflow (read from
  the end of its standard error); the memory watchdog gives "Used more
  than N MB".
- Crash-loop guard: when the worker dies while running a cell (a kill
  at the deadline, the memory limit, a crash), the cell is remembered
  with a fingerprint of its code and of what its xl() calls read. It
  does not run again (#PYTHON!, "Not run: this cell stopped Python the
  last time it ran", status `refused` in the diagnostics) until one of
  those changes or python.reset runs everything again. A call whose
  reference is only known at run time counts any edit as a change.
- On CPython, ModuleNotFoundError names the command for that
  interpreter, `"<sys.executable>" -m pip install <package>` (with the
  pip names of sklearn, cv2, PIL, yaml and a few others). When the
  interpreter is externally managed (PEP 668), it says to make a
  virtual environment instead and choose its python in python.options.
  GridCraft never runs pip.

Tests: Monty refusals (pandas, statistics, from numpy, match, wildcard
imports; nothing of a refused cell prints), the scanner on strings,
comments, multi-line statements and hostile input, the exact time
limit, recursion and memory messages, a worker dying of a stack
overflow, the guard (refused after a kill, run again when its input
changes, on reset and on a code change), and the pip hint on this
machine's CPython.
…admap)

- architecture.md: the python crate in the layer table, and a Python in
  cells section: worker processes, the protocol, the built-in and
  CPython backends, the engine runtime, the trust gate (with the
  Trusted state), files and the web build.
- file-format-parity.md: Python cells in xlsx, other imports and native
  JSON, the initialization code and the limits on open.
- function-parity.md: Python in cells, and #PYTHON!, #BUSY!, #TIMEOUT!
  and #BLOCKED! with their ERROR.TYPE numbers (19, 20 and 21 are
  GridCraft's own; Microsoft documents none for them).
- cli.md: running Python cells headless, which Python runs them, and
  the trust gate for files opened with --in.
- ROADMAP.md, target-app-parity.md and gaps.md: progress log, catalog
  278 / 294, and the UI that is still to come.
- AGENTS.md and CLAUDE.md: python in the layering list, and the CPython
  shim as the one exception to the Rust-only rule.
…rkbook

python.stop kills the active workbook's worker and drops its queued run.
Every Python cell that had not finished shows #PYTHON! ("Stopped") and a
"stopped" row in Diagnostics; nothing runs again until the next edit or
python.reset, which start from a fresh namespace. Cells that finished
keep their values. With nothing running it returns {stopped: 0}.

The package installs that follow use the same command to cancel an
install.
…person approves

GridCraft keeps one virtual environment of its own for Python cells,
made from the CPython it found (it never downloads an interpreter), in
the application's data folder: ~/Library/Application Support/GridCraft/
python-env on macOS, $XDG_DATA_HOME/gridcraft/python-env (by default
~/.local/share) on Linux and BSD, and %LOCALAPPDATA%\GridCraft\python-env
on Windows. GRIDCRAFT_PYTHON_ENV or python.options {environment} move it;
tests always do. The search for Python tries it after VIRTUAL_ENV and
CONDA_PREFIX and before PATH, and the probe reports whether a candidate
is a venv or a conda environment and which distributions it has.

- python.install {packages, approve, declare?} installs nothing without
  approve: true; otherwise it fails with EngineError::Approve, whose
  message names the packages and where they would go. It refuses while
  Python is turned off for a workbook opened from a file. Requirements
  are checked strictly: a PEP 508 name, plain extras and a version
  specifier only (no URLs, paths, options, markers or other
  characters), at most 32; import names such as sklearn and PIL map to
  their pip names.
- Packages go into the person's own venv or conda environment when that
  runs cells, otherwise into GridCraft's environment, made first with
  uv venv --seed or python -m venv; never into a system or externally
  managed Python.
- Installs run uv pip install when uv is on PATH, else python -m pip
  install, always with --only-binary, from an argument list, on a
  thread with a 10-minute deadline; python.stop cancels them. Failures
  are explained (no network, no wheel for this platform or Python, pip
  or venv missing). When an install succeeds, the search runs again and
  every workbook's Python cells run again.
- python.installStatus reports the running or last install, with a
  one-line summary and the versions installed; every install is a row
  in python.diagnostics (kept whatever its sheet filter, and kept when
  old runs are trimmed) and a record in the application log.
- python.packages declares the packages a workbook needs, kept in
  hidden names so they travel in xlsx and native files, and checked and
  capped when read; python.diagnostics reports each as found or missing.
- python.environment reports GridCraft's environment and its packages,
  creates it, and deletes it (only a folder holding a pyvenv.cfg, and
  only with approve: true).
- The CPython shim no longer appends a pip command to a
  ModuleNotFoundError; the run records the missing module's pip name
  (missing), for python.install.
- EngineError::Approve is added for commands that need the person's
  approval. The macro engine pull request adds the same variant.
- CLI tests point GRIDCRAFT_PYTHON_ENV at a folder that does not exist,
  so they never use the real environment. A test that installs from the
  package index is ignored by default (it needs the network).
…warm-up after installs

A package's first import after an install can take longer than a cell's
5 s limit (a cold import of scikit-learn took about 22 s on macOS). The
cell then showed #TIMEOUT!, and the crash-loop guard refused to run it
again, so a working install looked like a failed one.

- On CPython, time spent importing modules not loaded yet does not
  count against the time limit, up to python.options {importSeconds}
  (1 to 600, 60 by default, kept in Prefs). The shim wraps
  builtins.__import__ (the outermost import on the main thread) and
  replaces its timer with a 50 ms watchdog that checks the run's time
  less its import time. While an import runs it sends
  FromWorker::Extend {id, ms, module}, and the parent moves its own
  deadline by that much, never past the allowance; the parent's kill
  stays the backstop. The built-in Python never sends it.
- A worker loads numpy and pandas, when installed, before it reports
  ready, under an allowance of its own, and its start-up does not count
  as the first cell's time.
- A time limit that interrupts an import replaces the worker, since a
  package may be half loaded; the message names the module ("Stopped
  after 61 s while importing sklearn (the first import after an install
  can be slow)"), and RunOutcome carries it.
- A cell the time limit stopped mid-import, or on its first run on a
  new worker, runs once more on a fresh worker after the cells before
  it, shown as a "retried" row in Diagnostics; stopped again, it is
  refused as before.
- Every install ends with a warm-up that imports what it installed once
  under a 120 s deadline (python.installStatus {stage} says so while it
  runs); a warm-up failure is a warning on an install that worked. uv
  installs pass --compile-bytecode.
Cells formatted as dates reached Python as bare serial numbers, so code
had to convert them by hand, and dates a cell returned were written back
as plain numbers.

- An xl() reply for one area marks the numbers whose number format is a
  date or time format (XlReply::Dated, a DateCell {at, kind} per number;
  the kind is date, time or dateTime, from whether the format shows a
  calendar part, a clock part or both, through the new
  NumberFormat::shows_date and shows_time). Negative numbers and numbers
  past 9999-12-31 are not marked.
- Serials cross the worker protocol in the 1900 date system; the engine
  converts a 1904 workbook's in both directions (1,462 days; times of day
  are unchanged).
- The CPython shim makes datetime.date, datetime.time and
  datetime.datetime values of the marked numbers, to the millisecond,
  with Excel's fictitious 1900-02-29: serials 0 and 60, numbers past
  9999-12-31 and times of a day or more stay numbers. A date-only cell
  that holds a time of day becomes a datetime, so nothing is lost. When
  xl() builds a DataFrame, a column whose values are all dates becomes a
  datetime64 column.
- Results of type date, datetime (pandas Timestamp and numpy datetime64
  included) and time become serials listed in Output::dates, and the
  cells that show them get m/d/yyyy, m/d/yyyy h:mm or h:mm AM/PM (the
  formats TODAY(), NOW() and TIME() give) where their format is General.
  A blocked spill formats nothing.
- Dates before 1900-03-01 now follow Excel's serials (1900-01-01 12:00
  is 1.5, not 2.5); dates before 1900 are plain numbers.
- The built-in Python keeps receiving serial numbers, since its results
  cannot be read back as dates.

Tests cover the protocol, the engine's marking and formatting in both
date systems, the shim's conversions at the edges of the 1900 system and
on hostile numbers, DataFrame date columns (when pandas is installed),
and a CPython session end to end.

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