Errors

Mostly Right fails loudly when data is unavailable, temporally unsafe, or incompatible with a contract. Catch the four stable root exceptions for application control flow, then reach for a specialized subtype when you need a narrower recovery path.

Stable root exceptions

import mostlyright as mr

try:
    table = mr.weather.training_table("KNYC", from_date, to_date)
except mr.LeakageError:
    # A row was not knowable by the decision cutoff.
    ...
except mr.ContractError:
    # An input or output frame broke a documented contract.
    ...
except mr.NoDataError:
    # No usable rows were available.
    ...
except mr.MostlyrightError:
    # Any other structured SDK error.
    ...

MostlyrightError, LeakageError, ContractError, and NoDataError are available directly from mostlyright. Structured errors expose error_code, source, request_id, and a JSON-safe to_dict() payload when those fields apply.

TypeScript mirrors the same teaching families:

import {
  ContractError,
  LeakageError,
  MostlyrightError,
  NoDataError,
} from "mostlyright";

TypeScript errors expose errorCode, source, requestId, and toDict().

Common specialized errors

Import specialized Python exceptions from mostlyright.core.exceptions unless a domain documents a closer import path.

Family	Common members	Raised when
Availability	DataAvailabilityError, SourceUnavailableError, PayloadTooLargeError	A source, date window, cache entry, or payload cannot be served
Live data	LiveStreamError, NoLiveDataError, NoCWOPDataError	A live source has no fresh usable report
Temporal safety	LeakageError, OpenMeteoSeamlessLeakageError, TemporalDriftError	Data crosses a decision cutoff or two timelines disagree
Contracts	ContractError, SchemaValidationError, LabelAlignmentError, IssuedAtMissingError, UnitsContractError	A frame, label, timestamp, or unit breaks its contract
Provenance	SourceMismatchError	Research and live source identities do not match
Forecasts	NWPError, NWPModelNotAvailableError, NWPModelRetiredError, GRIBIntegrityError, HistoricalDepthError, NoLiveForNWPError, StormNotFoundError	A forecast model or requested depth is unavailable or invalid
Satellite	SatelliteError, GOESS3Error, GOESDataCorruptError, ProductNotRegisteredError, StationOutOfGridError	A satellite product, object, or station projection fails
Economy and finance	IndicatorNotYetReleasedError, EarningsError, EarningsFactCorruptError, CaptureNotAvailableError, KalshiCountRuleViolationError	A release or earnings artifact is not yet available or fails validation

Missing live data

weather.live.latest() raises NoLiveDataError instead of returning an old report as if it were current.

import asyncio
from mostlyright import weather
from mostlyright.core.exceptions import NoLiveDataError

try:
    report = asyncio.run(weather.live.latest("KNYC"))
except NoLiveDataError as error:
    print(error.source)
    print(error.to_dict())

This is a normal operational branch. Retry according to your policy, switch to another active station, or stop the live decision. Do not silently substitute archived or stale data.

Data availability reasons

DataAvailabilityError adds a stable reason value and an operator-facing hint. The reason enum is shared across Python and TypeScript:

• model_unavailable
• out_of_window
• cache_miss
• source_404
• source_5xx
• rate_limited

Branch on reason, not message text.

HTTP errors

TherminalError was renamed to HttpError in 2.0. The HTTP family also includes NotFoundError, RateLimitError, ValidationError, AuthenticationError, ForbiddenError, and ServerError.

Market-specific errors

Polymarket settlement can raise PolymarketEventError, PolymarketSettlementError, PolymarketStrikeError, or TooEarlyToSettleError.

KalshiTickerError is intentionally a ValueError, not a MostlyrightError, because a malformed ticker is caller input:

from mostlyright.markets.kalshi import KalshiTickerError, parse_ticker

try:
    parsed = parse_ticker(user_supplied_ticker)
except KalshiTickerError:
    ...

See also

• Temporal safety: decision cutoffs and leakage checks
• API reference: stable and experimental entry points
• SDK migration: 2.0 renames and removals