Errors

Errors are product surface: typed, importable one level deep, and written to name the offending source, the rows, both timestamps where temporal, and a concrete fix. This catalog is captured from the installed package’s inheritance tree.

import mostlyright as mr

try:
    frame = mr.align(spine, source)
except mr.LeakageError as e:
    ...   # the guard tripped; e names the source, rows, and fix
except mr.ContractError:
    ...   # a frame does not conform to its contract
except mr.NoDataError:
    ...   # nothing came back

mr.LeakageError, mr.ContractError, and mr.NoDataError are importable one deep (the requests.HTTPError convention). Everything below inherits MostlyRightError unless noted.

The core families

Family	Members	Raised when
Leakage	LeakageError, OpenMeteoSeamlessLeakageError	A source row postdates the decision cutoff
Contract	ContractError, LabelAlignmentError, SchemaValidationError, IssuedAtMissingError, UnitsContractError	A frame, label, or schema does not conform
No data	NoDataError → LiveStreamError → NoLiveDataError → NoCWOPDataError	Nothing came back, at increasing specificity
Availability	DataAvailabilityError, SourceUnavailableError, PayloadTooLargeError	A source, extra, or payload cannot be served
Provenance	SourceMismatchError	Train/serve source identity diverges
Temporal	TemporalDriftError	Clock or timeline inconsistency

Domain families

Domain	Members
NWP	NwpError → NwpModelNotAvailableError, NwpModelRetiredError, GribIntegrityError, HistoricalDepthError, NoLiveForNwpError, StormNotFoundError
Satellite	SatelliteError → GoesS3Error, GoesDataCorruptError (→ UnitsContractError), ProductNotRegisteredError, StationOutOfGridError
Econ	IndicatorNotYetReleasedError, EarningsError → EarningsFactCorruptError
Markets	PolymarketStrikeError, PolymarketEventError, PolymarketSettlementError, TooEarlyToSettleError, DeferredMarketError

One deliberate exception in markets: KalshiTickerError subclasses ValueError, not MostlyRightError, because a malformed ticker is a caller-input error in the standard Python sense. Catch it separately when parsing user-supplied tickers.

What an error message owes you

The SDK’s error-copy contract: every user-facing error carries the offending source id, the entity or rows involved, both timestamps where the failure is temporal, and a concrete fix. Example, verbatim shape:

Leakage detected in source 'weather.forecasts.gfs': 3 row(s) carry a
knowledge_time after the spine decision_time cutoff 2025-01-02T00:00:00+00:00.
Fix: filter the source to knowledge_time <= your decision_time before calling
align(), or call mr.provenance(frame) to inspect the offending rows.

Unknown entities fail with a nearest-match hint (unknown entity 'KNYX'; did you mean 'KNYC'?), never fuzzy auto-correction.

See also

• Temporal safety: the guard behind LeakageError
• Markets: venue error semantics in context
• API surface: domains and signatures