[Python-ideas] parameter omit

[Tal Einat]

On 5/11/07, Aaron Brady <castironpi at comcast.net> wrote:
>
> Note DefaultArg needs never get referenced literally in declarations, only
> in calls.
>
> def ghi( j=None, k=None, l=None ):
>         if j is None: j= Something
>         if k is None: k= Something2
>         if l is None: l= Something3
>
> No.  Here, I do it all at once.
> def ghi( j=Something, k=Something2, l=Something3 ):


Maybe I'm missing something here... But why would someone not use the latter
syntax unless the value must be calculated when the function is called, as
opposed to when the function is defined? In such cases, None fits the bill.
(Default arguments are evaluated at function definition.)

Calls like ghi( MyThing ) still work, as does:
> ghi( MyThing, l=MyThing3 ).
>
> Everybody's on board so far and nothing's changed.  Omissions, positional,
> and keyword arguments still work as we've grown to know and love.
>
> What we can do, is always know the parameter.  The change is to know the
> default value in every situation.  It's always DefaultArg.
>
> ghi( MyThing, DefaultArg, Mything3 )
> as well as
> ghi( DefaultArg, MyThing2 )


IMHO this is far less readable than using keyword arguments and omitting
those arguments for which you'd like to use the default value. I personally
like the "keyword-only arguments" PEP (3102) because it could help make
function calls more readable.


Perhaps we could learn from experience?

I've recently had to automate Excel with Python and used the COM interface.
In COM, there is a value called "Missing" which is passed to functions when
no value should be passed for an argument - similar to your DefaultArg. This
leads to code like this (C# code examples):

http://msdn2.microsoft.com/en-us/library/aa168343(office.11).aspx  (straight
from the online MSDN)

MenuBarItem = MainMenuBar.Controls.Add(
    Office.MsoControlType.msoControlPopup,
    Type.Missing, Type.Missing, Type.Missing, true);


And that was not a far-fetched example - here is an extreme example:

http://blogs.ittoolbox.com/visualbasic/evolution/archives/importing-unformatted-excel-sheets-into-sql-server-2000-using-net-part-i-7501

Excel.Application excelApp = new Excel.ApplicationClass();
Excel.Workbook thisWorkbook = excelApp.Workbooks.Open(strFileName,
Type.Missing, Type.Missing,
Type.Missing, Type.Missing,Type.Missing, Type.Missing, Type.Missing,
Type.Missing,Type.Missing, Type.Missing, Type.Missing,
Type.Missing,Type.Missing, Type.Missing);


IMO this is horrendous. Horrible. Eye-gouging.

I'd rather write machine code in hex.

- Tal
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