List of Functions

[Marko Rauhamaa]

Chris Angelico <rosuav at gmail.com>:

> On Tue, Mar 29, 2016 at 12:40 AM, Marko Rauhamaa <marko at pacujo.net> wrote:
>> Dan Sommers <dan at tombstonezero.net>:
>>
>>> On Mon, 28 Mar 2016 11:58:54 +0300, Marko Rauhamaa wrote:
>>>
>>>> As for Python, I don't feel a great need for anonymous functions.
>>>> However, I keep running into a need for anonymous classes, or,
>>>> rather, classless objects. Not a biggie. I just create a one-off
>>>> inner class and instantiate it, but I do appreciate Java's syntactic
>>>> innovation.
>>>
>>> And I always curse Java for having to create an inner class and a
>>> method when all I need is a simple function. :-)
>>>
>>> I think it's Steven D'Aprano who keeps pointing out that you can
>>> always name your tiny helper functions instead of using lambda:
>>>
>>>     def some_complex_function():
>>>         def f(x) = x + 2
>>>         some_library_that_wants_a_callback(f)
>>>         some_library_that_wants_a_callback(lambda x: x + 2)
>>>
>>> Both calls to some_library_that_wants_a_callback run the same.
>>
>> Yes, but I've come to realize that I quite often need more than a
>> function: I need an object with behavior. The solution is to use a
>> "helper" class.
>
> Can you give an example of code that would benefit from a
> "lambda-class" construct?

Here's a recent example:

========================================================================
            def process(self, source):
                awk = self
                class Source(snake.FieldSource):
                    def emit_fields(self):
                        for fields in snake.get_fields(
                                source, awk.encoding, awk.errors,
                                awk.eol, awk.delim):
                            if awk.selector is None or awk.selector(fields):
                                yield [ fields[i] for i in awk.columns ]
                return Source(self.encoding, self.errors, self.eol, self.delim)
========================================================================

Here's another, older one:

========================================================================
    def __init__(self, server, sock, domain, peer):
        super().__init__(server, sock)
        conn = self
        client_ip = peer[0]

        [...]

        class SPF_HELO(STATE):
            def terminate(self):
                conn.resume()
                conn.spf_query.cancel()
                conn.close()
                if conn.timer is not None:
                    conn.timer.cancel()
                    conn.timer = None
                conn.set_state(ZOMBIE)

            def handle_spf_verdict(self, verdict, reason, *capabilities):
                conn.resume()
                conn.log("verdict {} reason {}".format(verdict, reason))
                # RFC 4408 §2.5.5 calls for leniency for SoftFail
                #if verdict in [ SPFClient.FAIL, SPFClient.SOFT_FAIL ]:
                if verdict in [ SPFClient.FAIL ]:
                    conn.respond(550, "SPF check failed: {}".format(reason))
                    conn.set_state(IDLE)
                    return
                conn.respond(250, server.domain, *capabilities)
                conn.set_state(IDLE)
========================================================================


Marko