def check_header(header: dict) -> Optional[Blockchain]:
    """Returns any Blockchain that contains header, or None."""
    if type(header) is not dict:
        return None
    with blockchains_lock: chains = list(blockchains.values())
    for b in chains:
        if b.check_header(header):
            return b
    return None
Returns any Blockchain that contains header, or None.
electrum/blockchain.py