Why python doesn't use syntax like function(, , x) for default parameters?

Dmitry Anikin anikin#remove_this# at vstu.ru
Fri Mar 10 21:13:29 CET 2006


Some example (from real life).
def ChooseItems(StartDate, EndDate, Filter):
#function returns a set of some items in chronological order
#from required interval possibly using filter

ChooseItems() #get everything
ChooseItems('01.01.2000', ,SomeFilter) #get everything after a date using filter
ChooseItems(, '01.01.2000') #get everything before a date
ChooseItems(, , SomeFilter) #get everything using filter

Now compare this to something which (I hope) is rather pythonian

Seq[:] #get everything
Seq[2::3] #get everything after an index using filter (filter every third value)
Seq[:3] #get everythin before an index
Seq[::4] #get everything using a filter

Do you see any significant difference?

I understand that many do not need such a syntax, I don't understand
why someone would be AGAINST it. I don't propose to CHANGE anything
in python (right now this syntax is error anyway). What I propose is just
ADD another way of calling a function with keyword parameters but using
POSITIONS instead of NAMES. And sometimes position is easier to
remember than name. Anyway, who wants names let them use names.
Who wants positions let them use positions. But to have a CHOICE is
always good. As far as the choice itself doesn't damage anything,
and I don't think that my does.

I think that if we compare
ChooseItems('01.01.2000', ,SomeFilter)
and
ChooseItems(StartDate='01.01.2000', Filter=SomeFilter)
the first one is more readable, 'cos you see
what is meant right away. In second one you have to
actually READ the keyword names to understand.
It's not the common case, of course, but still, why
not have a choice to use it?

Some other examples which might benefit
SetDate(year, month, day)
SetDate(, month+1) # set next month, leaving year and day
SetDate(, , 31) # set to end of month, not changing year
#(wrong date adjusted automatically, of course)

FormatFloat(Float, Length, Precision, FormatFlags)
You might want precision, leaving length default, or just use FormatFlags

In fact, I became so used to convenience of such syntax that
it was a disappointment not to find it in python.

Please, don't try to scare me with 25-parameter functions.
This is not for them. But to remember positions of two to
five parameters is actually easier (if their order has some
logic) then what are their names: startDate ? beginDate?
firstDate? openDate? Date1?

The same approach can be used with tuples:
(, , z) = func() # returning three element tuple()
You think
z = func()[2]
is actually more clear? - By the way, I want THIRD value,
not SECOND. And tuples don't have keyword names, do they?
And what about
(a, , b)  = func()
...well, maybe I got carried away a little...

Finally, if syntax
func (None, None, 10)
seems natural to you, I propose to make it even more
natural: I don't want some "None" passed as argument,
I don't want anything at all passed, so I just use empty space
func ( , , 10)
And the called func don't have to bother with checking
None for EACH argument but will happily use defaults instead.



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