Dispatch table of methods with various return value types

[Loris Bennett]

dn <PythonList at DancesWithMice.info> writes:

> On 19/11/2020 02:13, Loris Bennett wrote:
>> dn <PythonList at DancesWithMice.info> writes:
>>
>> Firsty, thanks for taking the time to write such a detailed reply.
>
> Bitte!
>
>
>>>>>> I have a method for manipulating the membership of groups such as:
>>>>>>
>>>>>>        def execute(self, operation, users, group):
>>>>>>            """
>>>>>>            Perform the given operation on the users with respect to the
>>>>>>            group
>>>>>>            """
>>>>>>
>>>>>>            action = {
>>>>>>                'get': self.get,
>>>>>>                'add': self.add,
>>>>>>                'delete': self.delete,
>>>>>>            }
>>>>>>
>>>>>>            return action.get(operation)(users, group)
>>>>>>
>>>>>> The 'get' action would return, say, a dict of users attribute, whereas
>>>>>> the 'add/delete' actions would return, say, nothing, and all actions
>>>>>> could raise an exception if something goes wrong.
>>>>>>
>>>>>> The method which calls 'execute' has to print something to the terminal,
>>>>>> such as the attributes in the case of 'get' and 'OK' in the cases of
>>>>>> 'add/delete' (assuming no exception occurred).
>>>>>>
>>>>>> Is there a canonical way of dealing with a method which returns different
>>>>>> types of data, or should I just make all actions return the same data
>>>>>> structure so that I can generate a generic response?
>>>>>
>>>>>
>>>>> Is the problem caused by coding the first step before thinking of the overall
>>>>> task? Try diagramming or pseudo-coding the complete solution (with multiple
>>>>> approaches), ie the operations AND the printing and exception-handling.
>>>>
>>>> You could have a point, although I do have a reasonable idea of what the
>>>> task is and coming from a Perl background, Python always feels a bit
>>>> like pseudocode anyway (which is one of the things I like about Python).
>>>
>>> +1 the ease of Python, but can this be seductive?
>>>
>>> Per the comment about Perl/Python experience, the operative part is the
>>> "thinking", not the tool - as revealed in responses below...
>>>
>>> Sometimes we design one 'solution' to a problem, and forget (or 'brainwash'
>>> ourselves into thinking) that there might be 'another way'.
>>>
>>> It may/not apply in this case, but adjusting from a diagram-first methodology,
>>> to the habit of 'jumping straight into code' exhibited by many colleagues,
>>> before readjusting back to (hopefully) a better balance; I felt that
>>> coding-first often caused me to 'paint myself into a corner' with some
>>> 'solutions, by being too-close to the code and not 'stepping back' to take a
>>> wider view of the design - but enough about me...
>>>
>>>
>>>>> Might it be more appropriate to complete not only the get but also its
>>>>> reporting, as a unit. Similarly the add and whatever happens after that; and the
>>>>> delete, likewise.
>>>>
>>>> Currently I am already obtaining the result and doing the reporting in
>>>> one method, but that makes it difficult to write tests, since it
>>>> violates the idea that one method should, in general, just do one thing.
>>>> That separation would seem appropriate here, since testing whether a
>>>> data set is correctly retrieved from a database seems to be
>>>> significantly different to  testing whether the
>>>> reporting of an action is correctly laid out and free of typos.
>>>
>>> SRP = design thinking! +1
>>
>> I knew the idea, but I didn't now the TLA for it ;-)
>
> Yes, there are plenty of those!
>
> You may be interested in reading about "Clean Code", instigated (IIRC) by "Uncle
> Bob" (Robert Martin). NB Python may/not be used for book-examples. Just the
> other day I came across "Clean Code in Python", Mariano Anaya, PacktPub, 2018. I
> have yet to read it, but the contents page seemed to 'tick all the boxes'. The
> book is two years old, and IIRC he presented at EuroPython a few years before
> that (YouTube videos on-line - in case you prefer that medium, or want to gain a
> flavor before spending money...). All of these TLAs, and others comprising the
> "SOLID Principles" appear in the ToC, along with plenty of others, eg YAGNI and
> EAFP; plus some specific to Python, eg MRO.

I had a look at the Europython 2016 video and found it very instructive.
I'm a not very familiar with using exceptions, but when I have tried to
use them, its seems to have generated a lot of code clutter.  The
approach shown in the video seems to be an elegant solution for certain
category of exception handling code repetition.

>>> TDD = early testing! +1
>>>
>>> Agreed: The tasks are definitely separate. The first is data-related. The second
>>> is about presentation.
>>>
>>> In keeping with the SRP philosophy, keep the split of execution-flow into the
>>> three (or more) functional-tasks by data-process, but turn each of those tasks
>>> into two steps/routines. (once the reporting routine following "add" has been
>>> coded, and it comes time to implement "delete", it may become possible to repeat
>>> the pattern, and thus 're-use' the second-half...)
>>>
>>> Putting it more formally: as the second-half is effectively 'chosen' at the same
>>> time as the first, is the reporting-routine "dependent" upon the data-processor?
>>>
>>> 	function get( self, ... )
>>> 		self.get_data()
>>> 		self.present_data()
>>>
>>> 	function add( self, ... )
>>> 		self.add_data()
>>> 		self.report_success_fail()
>>>
>>> 	...
>>>
>>> Thus, the functional task can be tested independently of any reporting follow-up
>>> (for example in "get"); whilst maintaining/multiplying SRP...
>>
>> The above approach appeals to me a lot.  Slight downsides are that
>> such 'metafunctions' by necessity non-SRP functions and that, as there
>> would be no point writing tests for such functions, some tools which try
>> to determine test coverage might moan.
>
> First comes (Python) 'duty': the word "meta", perhaps more in the context of
> "meta-classes" has particular meaning in Python, that may not align with
> expectations generated by understanding the term "meta" in other contexts!

OK, thanks for pointing that out.

> Second, we return to earlier comments about up-front design. Consider "Stepwise
> Decomposition" (https://en.wikipedia.org/wiki/Top-down_and_bottom-up_design) and
> how solving a 'large problem' is likened to pealing an onion, ie one 'layer' at
> a time. Thus there is a sub-problem, eg report on xyz; this divides into smaller
> problems: (i) from where do I find the data on xyz, and (ii) how do I present
> this.
>
> If you code top-down, then it may be that there are three subroutines (functions
> in Python) which implement the three of these. Further, that only the two
> "smaller" routines appear to be doing any 'work'. However, remember that the
> function names both document the solution and reproduce the specification. Thus
> three well-chosen names will add value and ease understanding for you/us, six
> months later...
>
> If you code bottom-up and have TDD-built the two "smaller" functions, then
> adding the 'higher' function as an 'umbrella' will tie them together - for the
> reasons/results mentioned above.

My programs are fairly small, so there isn't much between the top and the
bottom - probably a maximum of five calls.   In fact, if calls did get
much deeper, I would think about farming some of the code out to a
separate module, if possible.

> There are different types of testing. Unit testing is straightforward with
> pytest or similar. This takes care of tests such as 'did "get" realise the
> correct data?' and 'after "delete" does this data exist?'. These are likely
> tests of functions at the lowest and/or lower levels of the implementation -
> hence the name.
>
> When it comes to reporting, life becomes more complicated. Whereas pytest will
> capture and allow testing of sysout, when we move to Qt, gtk, or some other UI
> took-kit, we need to find a compatible testing tool. If presentation is HTML,
> then web-page testing is accomplished with the likes of Selenium.

Luckily I just have to deal with console output.

> If we are talking UX (User Experience) testing, then the information-presented
> is almost-irrelevant. If you have a user on the dev.team (see also Agile teams),
> then (s)he will perform such 'testing' manually (and give rapid feedback). Thus,
> no tool required, as such.

As I am the main user, I usually give myself fairly rapid feedback ;-)

> NB If you are concerned about the actual information being presented, surely
> that will have already been tested as accurate by the unit test mentioned
> earlier?

This is true.  Although obviously, if the information is correct, but
somehow mangled by the presentation, that a problem.  I have more than
once shot myself in the foot by, say, getting a correct value of zero
indicating success, but mistakenly converting this into the string
indicating failure.

> Regarding the comment about "moan[ing]" tools. Who's in-charge here? When it is
> helping you it is a "tool". What is something that is getting in your way,
> causing you frustration, or otherwise interfering with your happiness and
> productivity?
>
> Pointy-headed managers [a reference to the Dilbert cartoons] have often tried to
> create/impose 'rules' on developers. One of my favorites is: "there will be at
> least one comment for every ten lines of code". Do you need to strain-the-brain
> to know what happens?
>
>     # this code has a comment
>     ...
>
>     # add one to x
>     x += 1
>
> I'm afraid the idea of 100% code-coverage is a nirvana that is probably not
> worth seeking. See also @Ned's comments (about his own coverage.py tool)
> https://nedbatchelder.com/blog/200710/flaws_in_coverage_measurement.html
>
> The car's speedo might tell you that it can motor-along at some incredible
> speed, but using the information sensibly might attract less attention from the
> Highway Patrol!

You're right, of course, but as I am my own pointy-headed boss, as far as
setting development rules goes, I don't have to worry about coverage too
much.  However, as I code fairly sporadically, it is a useful metric to
check how well I am doing with my self-set TDD goal.

>>>>> Otherwise the code must first decide which action-handler, and later,
>>>>> which result-handler - but aren't they effectively the same decision?
>>>>> Thus, is the reporting integral to the get (even if they are in
>>>>> separate routines)?
>>>>
>>>> I think you are right here.  Perhaps I should just ditch the dispatch
>>>> table.  Maybe that only really makes sense if the methods being
>>>> dispatched are indeed more similar.  Since I don't anticipate having
>>>> more than half a dozen actions, if that, so an if-elif-else chain
>>>> wouldn't be too clunky.
>>>
>>> An if...elif...else 'ladder' is logically-easy to read, but with many choices it
>>> may become logistically-complicated - even too long to display at-once on a
>>> single screen.
>>>
>>> Whereas, the table is a more complex solution (see 'Zen of Python') that only
>>> becomes 'simple' with practice.
>>>
>>> So, now we must balance the 'level(s)' of the team likely to maintain the
>>> program(me) against the evaluation of simple~complex. Someone with a ComSc
>>> background will have no trouble coping with the table - and once Python's
>>> concepts of dictionaries and functions as 'first-class objects' are understood,
>>> will take to it like the proverbial "duck to water". Whereas, someone else may
>>> end-up scratching his/her head trying to cope with 'all the above'.
>>
>> The team?  L'équipe, c'est moi :-) Having said that I do try to program
>> not only with my fictitious replacement in mind, should I be hit by the
>> proverbial bus, but also my future self, and so tend to err on the side
>> of 'simple'.
>
> +1 "simple"
> +1 "ego-less programming"
>
> German, English, and now French?

That's by country of residence, birth, and almost forgotten schooling.

>>> Given that Python does not (yet) have a switch/case construct, does the table
>>> idea assume a greater importance? Could it be (reasonably) expected that
>>> pythonista will understand such more readily?
>>>
>>>
>>> IMHO the table is easier to maintain - particularly 'six months later', but
>>> likely 'appears' as a 'natural effect' of re-factoring*, once I've implemented
>>> the beginnings of an if-ladder and 'found' some of those common follow-up
>>> functions.
>>> * although, like you, I may well 'see' it at the design-stage, particularly if
>>> there are a number (more) cases to implement!
>>>
>>> Is functional "similar"[ity] (as above) the most-appropriate metric? What about
>>> the number of decision-points in the code? (ie please re-consider "effectively
>>> the same decision")
>>>
>>> 	# which data-function to execute?
>>> 	if action==get
>>> 		do get_data
>>> 	elif action == add
>>> 		do add_data
>>> 	elif ...
>>>
>>> 	...
>>>
>>> 	# now 'the work' has been done, what is the follow-through?
>>> 	if action=get
>>> 		do present_data
>>> 	elif action == add
>>> 		report success/fail
>>> 	...
>>
>> In my current case this is there is a one-to-one relationship between
>> the 'work' and the 'follow-through', so this approach doesn't seem that
>> appealing to me.  However I have other cases in which the data to be
>> displayed comes from multiple sources where the structure above might
>> be a good fit.
>
> I hope to have addressed this above.
>
> To help (I hope), consider if, in the proverbial six-months time, you are asked
> to add logging to each of these various actions. Now, there are three tasks:
> 'work', 'follow-through', and 'logging'; to be done for each of the n-action
> choices.
>
> Would an 'umbrella function' which acts as both the single destination for an
> action-choice, and as a 'distributor' for the various specific tasks that must
> be executed, start to appear more viable?
>
>
>> Having said that, I do prefer the idea of having a single jumping off
>> point, be it a dispatch table or a single if-then-else ladder, which
>> reflects the actions which the user can take and where the unpleasant
>> details of, say, how the data are gathered are deferred to a lower level
>> of the code.
>
> +1
>
>
>>> Back to the comment about maintainability - is there a risk that an extension
>>> requested in six months' time will tempt the coding of a new "do" function AND
>>> induce failure to notice that there must be a corresponding additional function
>>> in the second 'ladder'?
>>>
>>> This becomes worse if we re-factor to re-use/share some of the follow-throughs,
>>> eg
>>>
>>> 	...
>>> 	elif action in [ add, delete, update]
>>> 		report success/fail
>>> 	...
>>>
>>> because, at first glance, the second 'ladder' appears to be quite dissimilar -
>>> is a different length, doesn't have the condition-clause symmetry of the first,
>>> etc! So, our fictional maintainer can ignore the second, correct???
>>>
>>> Consider SRP again, and add DRY: should the "despatch" decision be made once, or
>>> twice, or... ?
>>
>> With my non-fictional-maintainer-cum-six-month-older-self hat on I think
>> you have made a good case for the dispatch table, which is my latent
>> preference anyway, especially in connection with the 'do/display'
>> metafunctions and the fact that in my current case DRY implies that the
>> dispatch decision should only be made once.
>
> +1 Definitely!
>
>
> See also @Wulfraed's point about OOP (Object-Oriented Programming)! If we were
> talking about people, then I'd expect to find a class Person, or similar, in the
> code. That means that "get" and "delete" might refer to database
> transactions. Hence, they should be part of the Person class, rather than
> functions 'in the wild'. Thus, where we have used the term "function" (or even
> "subroutine"), we should have said "method". A class is a (very good) way to
> 'collect' related functionality and keep it 'together'!
>
>
> Another aspect, following-on from UI comments (above). If you are using a
> framework, the presentation code will largely fit within those
> objects. Therefore, logically-separate from manipulating the
> source-object. Another consideration (maybe) for how to structure and relate the
> routines...
>
> As a general rule, I keep print() out of functions which 'calculate' - 
> even, out of the Person class. This facilitates re-use, where the next use may
> want to present the results differently, or merely to use the calculation as
> 'input' and not present 'it' at all!
>
>
>> Thanks again for the input!
>
> It is good that you are reviewing your code and considering alternate
> approaches! Many others 'here' will have benefited from considering your
> points/questions...
>
>
> You may like to pass some information about the Free University:
> - is Python the primary teaching language

I don't know.  It probably depends quite a bit on the subject.

> - is Python widely used within various schools/departments

Anyone doing ML, which these days seems to be everyone, tends to use
Python because of torch.  People doing genomics seem to like it too.

> - is instruction in English, or...

Mainly German, for undergrads.  In the Sciences, which is were most of
my contacts are, some of the Master's students and quite a few of people
doing doctorates seem to mainly speak English  

> - what does "Free" mean

The implication is that it is ideologically free.  The university was
founded in 1948 in the context of Berlin being divided into different
sectors after WWII.  The old Berlin University (of Einstein and Planck
fame) was in the Soviet sector, so a new university was set up in the
western part of the city in the American sector.

> - is it also $free

All state universities in Germany are $free.  You might have to pay for
some exotic Master's courses, particularly if they are in English. 

> - is it open to (non-German) foreigners

Yes.

> Tschüss!

A presto! ;-)