Python type checking and "dns.rdata.Rdata"

[Martin T]

Hi.

Let's say, that I have a following code snippet which finds the zone
serial number:

zone = dns.zone.Zone("example.com")
dns.query.inbound_xfr("192.168.0.44", zone)
for (name, ttl, rdata) in zone.iterate_rdatas("SOA"):
serial_nr = rdata.serial

When I type check this code with mypy, then it detects the "rdata" as
"dns.rdata.Rdata" and complains that '"Rdata" has no attribute
"serial"'.

How to workaround this problem?


thanks,
Martin

[Bob Halley]

dnspython's design was around "duck typing", so many things return generic types.  You've got two basic options:

1) Tell mypy what the actual type is:

    from dns.rdtypes.ANY.SOA import SOA

    from typing import cast

   
    for (name, ttl, rdata) in zone.iterate_rdatas("SOA"):

        rdata = cast(SOA, rdata)
        serial_nr = rdata.serial   # ok
        bogus = rdata.bogus        # mypy rejects; also fails at runtime

2) Use an rdata variable of type Any, either implicitly from an Any-typed API or explicitly:

    # implicit
    for rdata in zone.find_rdataset("@", "SOA"):
        serial_nr = rdata.serial   # ok as the iterator's values are Any
        bogus = rdata.bogus        # mypy does not complain; fails at runtime

    # explicit
    rdata: Any

    for (name, ttl, rdata) in zone.iterate_rdatas("SOA"):

        serial_nr = rdata.serial   # ok
        bogus = rdata.bogus        # mypy does not complain; fails at runtime

[Martin T]

Hi Bob,

thanks! Casting the SOA type worked fine.


Martin

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