for-else

[Shane Geiger]

>  if a == 0:
>     do_task_0()
>  elif a == 1:
>     do_task_1()
>  elif a == 2:
>     do_task_2()
>  else:
>     do_default_task()

The if-elif-else structure that calls functions (like that above) can be
avoided with the code below:


def foo0(): print 'foo0'
def bar0(): print 'bar0'
def foo1(): print 'foo1'
def bar1(): print 'bar1'
def do_default_task(): print 'do_default_task'

do_task = { 0:foo0, 1:foo1, 2:bar0, 3:bar1, }

a = 1

# example of normal usage
if a in do_task.keys(): do_task[a]()
else: do_default_task()

# example of testing all functions in the dict as well as the default
function
for a in do_task.keys() + [8]:  # 8 is a non-existent key in the do_task
dict
    print "a is ",a,"and it gives this output:",
    if a in do_task.keys(): do_task[a]()
    else: do_default_task()





Carl Banks wrote:
> On Mar 4, 10:55 am, "BJörn Lindqvist" <bjou... at gmail.com> wrote:
>   
>> On Tue, Mar 4, 2008 at 4:17 PM, Carl Banks <pavlovevide... at gmail.com> wrote:
>>     
>>>  > for ...:
>>>  >     ...
>>>  > exhausted:
>>>  >     ...
>>>  > broken:
>>>  >     ...
>>>       
>>>  > The meaning is explicit. While "else" seems to mean little there.
>>>  > So I may like something similar for Python 3.x (or the removal of the
>>>  > "else").
>>>       
>>>  I would not be opposed to this on its own merits, but there is a
>>>  rationale behind the name "else".  If you consider a for loop to be a
>>>  rolled-up if...elif...else statement (situations where this is
>>>  reasonable tend to be the same ones were else would be useful), then
>>>  the "else" clause would remain unchanged on the for loop.
>>>       
>>>  For instance, if you have a (trivial) if...elif...else like this:
>>>       
>>>  if a == 0:
>>>     do_task_0()
>>>  elif a == 1:
>>>     do_task_1()
>>>  elif a == 2:
>>>     do_task_2()
>>>  else:
>>>     do_default_task()
>>>       
>>>  You could roll it up into a for...else statement like this:
>>>       
>>>  for i in range(3):
>>>     if a == i:
>>>         do_task[a]()
>>>  else:
>>>     do_default_task()
>>>       
>> You forgot the break statement. The else suite will always be executed
>> in this loop. Kind of proves bearophiles point, for-else is really
>> tricky.
>>     
>
> Ah ha, but that would have been a mistake with or without the else
> clause....
>
>
> Carl Banks
>   

This approach works well for me:



def foo0(): print 'foo0'
def bar0(): print 'bar0'
def foo1(): print 'foo1'
def bar1(): print 'bar1'

def do_default_task(): print 'do_default_task'

do_task = { 0:foo0, 1:foo1, 2:bar0, 3:bar1, }

a = 1 

# example of normal usage
if a in do_task.keys(): do_task[a]()
else: do_default_task()


# example of testing
for i in range(len(do_task.keys)):
     if a in do_task.keys(): do_task[a]()
     else: do_default_task()



-- 
Shane Geiger
IT Director
National Council on Economic Education
sgeiger at ncee.net  |  402-438-8958  |  http://www.ncee.net

Leading the Campaign for Economic and Financial Literacy