first_known

first_known(observations, *, key_columns, known_at_column=KNOWN_AT_COLUMN)

Keep the first-known observation per key, dropping every later revision.

The first report of a figure is the one a market could trade on. A later restatement is a different fact and must not displace it.

Parameters

Name	Type	Description	Default
observations	pd.DataFrame	Point-in-time rows, possibly several per key.	required
key_columns	Sequence[str]	Columns identifying one figure.	required
known_at_column	str	Column holding when each row became knowable.	KNOWN_AT_COLUMN

Returns

Name	Type	Description
	pd.DataFrame	One row per key: the one with the earliest known_at.

Raises

Name	Type	Description
	PanelError	A column is absent or known_at is not tz-aware UTC.