Append monthly files under a declared schema · Python (pandas)

Inputs (three files, all text)
  jan(id, amount)            feb(id, amount, channel)            bad(id, currency)

Rules
  Schema: required columns id and amount; optional column channel. No other columns are allowed.
  A file missing a required column, or containing a column outside the schema, is rejected as a whole.
  Keep every row of accepted files. A missing optional column gives a missing channel.
  source_file is the name of the file the row came from (jan, feb, …).
  amount is always a whole number in accepted files.

Output (row order does not matter)
  clean(id text, amount int64, channel text, source_file text)