tervatrix-rtl

Correct right-to-left text for Hebrew and Arabic, in one dependency-free module. Pure standard library, Python 3.9+, MIT.

It fixes the four bidi bugs that ship in almost every RTL application and are invisible in review, because they only appear once real data lands in the layout:

what the user sees	why	the fix
+972-4-000-0000 renders as 0000-000-4-972+	the dashes are neutral characters and inherit the paragraph's RTL direction	prepare_outbound() wraps it in a left-to-right isolate
opening hours 09:00-18:00 render as 18:00-09:00, closing time first	two number runs with a neutral between them get reordered as a unit	the pattern treats a time range as one run
a whole Hebrew line flips to left-aligned because it starts with 2026 or WhatsApp	the first-strong-character rule guesses the paragraph direction	an invisible right-to-left mark is prefixed to the line
a prepared string no longer equals the string it came from	the marks are invisible but real characters	strip_bidi(), the exact inverse

Extracted from a WhatsApp agent answering Hebrew, Arabic and English customers in production.

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Install

pip install tervatrix-rtl

No dependencies. Nothing to configure.

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The 30-second version

from tervatrix_rtl import prepare_outbound

message = "שעות פתיחה: 09:00-18:00\nלשאלות: +972-4-000-0000"

send(prepare_outbound(message, "he"))

That one call wraps the time range and the phone number in left-to-right isolates and marks the line that starts with a digit. An English string is returned byte for byte untouched, so English never carries invisible control characters into your logs, your tests or your database.

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API

prepare_outbound(text, language=None)

The single call to make on any string before you render or send it. Runs isolate_ltr_runs then force_rtl_paragraphs. Returns LTR languages unchanged.

Omit language and it is detected from the script the text is written in.

isolate_ltr_runs(text)

Wraps URLs, www. hosts, e-mail addresses, phone numbers, prices and time ranges in U+2066 … U+2069.

Idempotent, and composable: it processes only what lies outside any isolate that is already there. So you can wrap one value by hand with isolate(), drop it into a sentence, and still pass the whole sentence through prepare_outbound() — the hand-wrapped value survives untouched and everything around it is still protected.

force_rtl_paragraphs(text)

Prefixes U+200F to every line that does not already begin with a strong RTL character. Applied line by line, which is why an isolate is never allowed to cross a newline — a pair split over two lines would never balance.

isolate(value)

Wraps one value unconditionally. Use it when you already know what the value is and the pattern is deliberately too strict to catch it — a bare order id like A7, a two-digit table number.

f"הזמנה {isolate(order.id)} מוכנה"

format_time_range(start, end, separator="-")

format_time_range("09:00", "18:00") gives an isolated 09:00-18:00 that cannot be reordered.

detect_language(text) → 'he' | 'ar' | 'en'

Counts characters per script rather than stopping at the first hit, so one Latin brand name inside a Hebrew sentence does not switch the whole string to English. URLs, e-mail addresses and phone numbers are removed before counting - they are not evidence of a language, and a single link otherwise carries enough Latin letters to flip a short Hebrew message to English. Falls back to 'en' for digits-only, emoji-only and empty input.

is_rtl(language) and dir_for(language)

is_rtl accepts a bare subtag or a full tag: he, he-IL, ar_EG, fa-IR, ur. dir_for returns 'rtl' or 'ltr', ready to drop into an HTML dir attribute.

strip_bidi(text)

Removes every bidi control character, including the legacy embedding marks (U+202A–U+202E) found in old translation files.

Call this before comparing strings. The marks are invisible, so an unstripped string fails an equality check for no visible reason — in a test assertion, a database WHERE, a de-duplication key. strip_bidi(prepare_outbound(x)) is exactly x.

LRI, PDI, RLM, RTL_LANGUAGES

The control characters (U+2066, U+2069, U+200F) and the tuple of RTL primary subtags, exported so you can assert on them in your own tests.

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Where it belongs in a web app

Do the isolating once, on the way out, not scattered through templates:

# FastAPI / Flask / Django - one place, at the edge
from tervatrix_rtl import dir_for, prepare_outbound

@app.get("/order/{order_id}")
def order_page(order_id: str, lang: str = "he"):
    order = repo.get(order_id)
    return render(
        "order.html",
        dir=dir_for(lang),
        lang=lang,
        summary=prepare_outbound(order.summary, lang),
    )

And strip on the way in, before anything is compared or stored:

from tervatrix_rtl import strip_bidi

clean = strip_bidi(request.form["name"]).strip()

That second line matters more than it looks. Text pasted from a chat app, a PDF or a spreadsheet routinely carries U+200F and U+202A–U+202E. Stored unstripped, the row never matches on lookup and the bug is invisible in every log and every database client.

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Tests

No framework to install; unittest ships with Python.

python -m unittest discover -s tests -t .

They cover language detection, isolate placement, idempotency, isolate/newline balance, the round trip through strip_bidi, and a parity suite asserting byte-identical output with the JavaScript implementation.

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Licence

MIT — see LICENSE. Use it commercially, fork it, ship it. No warranty.

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Where this came from

This package is the RTL layer of a WhatsApp agent that answers Hebrew, Arabic and English customers on the official Meta Cloud API. Two more pieces of it are public and free:

• @tervatrix/rtl-ui — the identical logic in JavaScript, plus logical-property CSS utilities and an optional React wrapper. Same function names, same behaviour, so a string prepared on either side of a project is byte-identical.
• wa-agent-starter — a complete, working WhatsApp agent in FastAPI: Meta webhook, HMAC signature verification, language detection, keyword FAQ, this RTL handling, and 50 passing tests. MIT, self-hosted, no account with us.

If you need more than a keyword bot, wa-agent-kit is the paid version. It adds a Qdrant knowledge base so the agent answers questions nobody wrote a keyword for, multi-tenant support for many business numbers on one deployment, human handover, an admin panel, an AI-provider abstraction, and 12 months of updates.

→ Get wa-agent-kit