Dispatch table of methods with various return value types
Hi,
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?
Cheers,
Loris
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Re: Dispatch table of methods with various return value types
On 17/11/2020 22:01, Loris Bennett wrote:
Hi,
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.
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.
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)?
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Re: Dispatch table of methods with various return value types
dn writes:
> On 17/11/2020 22:01, Loris Bennett wrote:
>> Hi,
>>
>> 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).
> 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.
> 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.
Cheers,
Loris
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Re: Unable to set up Python correctly
According to my understanding, when the "Select Interpreter" option is used, only the current session is affected. It does not change the path of the default interpreter that VS Code tries to use. My suggestion would be to try to change the path specified in "Python: Default Interpreter Path" setting. -- https://mail.python.org/mailman/listinfo/python-list
Re: Dispatch table of methods with various return value types
On 17/11/2020 23:35, Loris Bennett wrote:
dn writes:
On 17/11/2020 22:01, Loris Bennett wrote:
Hi,
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
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...
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 c
