events/callbacks - best practices

[Peter Otten]

HankC wrote:

> Greetings:
> 
> I'm been programming Delphi for a while and am trying to get into the
> Python mindset.  In Delphi, you can have a component class that has
> properties, methods, and events.  Properties and methods translate
> pretty easily to Python but I'm not sure about the best way to handle
> events.  For example, from fxn retrbinary in ftplib there is:
> 
>         while 1:
>             data = conn.recv(blocksize)
>             if not data:
>                 break
>             callback(data)
>         conn.close()
>  
> In delphi this would be something like:
> 
>         while 1:
>             data = conn.recv(blocksize)
>             if not data:
>                 break
>             if assigned(OnProgress) then
>                 callback(data)
>         conn.close()
>  
> In other words, the class can handle a number of events but passing a
> callback function is not mandatory.  If you want to handle an event,
> it is assigned and dealt with.
> 
> I'm especially interested in non-visual components (like the ftplib)
> and the examples I can find all deal with the events of visual
> components that interjects more complexity than I'd like at this stage
> of my knowledge.
> 
> My goal is to rewrite a number of non visual components in Python.
> What I'd like to find is a 'non visual component design in Python'
> guide somewhere.  So...  I'm more than willing to study if I can find
> the resources - if you have any please lay them on me!  Part of my
> problem, I'm sure, is poor terminology use.
> 
> Of course, any general comments are welcomed - I'm not really
> expecting a tutorial here, though :-)
> 
> Thanks!

As of C++ Builder 3, Delphi events are just function pointers with IDE
support. IMHO it is useless to replicate this mechanism in Python. Just
make a base class with empty methods instead. Override instead of using
function pointers, if the behaviour need not change at runtime.

# delphi style, as far as I remember.
# now say this isn't bloated with a straight face :-)
class TDelphi:
    def __init__(self):
        self.OnStart = None
    def start(self):
        if self.OnStart:
            self.OnStart(self)


d = TDelphi() # IDE

# callback would typically be a method of a form
class Form:
    def MyOnStart(self, sender): # prototype generated by IDE
        print "start 0"

f = Form() # IDE
d.OnStart = f.MyOnStart # IDE
d.start()

# oo style
class OO:
    def start(self):
        pass

# use case 1
o = OO()
def start():
    print "start 1"
o.start =  start
o.start()

# use case 2
class Override(OO):
    def start(self):
        print "start 2"
o = Override()
o.start()

Peter