PEP308: Yet another syntax proposal

[holger krekel]

David Eppstein wrote:
> On 2/10/03 11:28 PM +0100 holger krekel <pyth at devel.trillke.net> wrote:
> >
> > When i had situations like this I rewrote it to get the special "select
> > all"  value not interfere with usual list/tuple-index semantics.  E.g.
> > using a  dummy-class for "select_all" rather than reusing an otherwise
> > valid key index.
> >
> > Sure.  But I'd rather have my co-coders think about easier ways rather
> > having to understand ternary operations often.  Had i seen your above
> > code
> >
> >     cols[i] if i >= 0 else None
> >
> > just on its own i would be puzzled (which i was).  May i ask
> > you to show the complete function where this came from?
> 
> Not that I make any claim of having polished this to jewel-like perfection, 
> but here it is.  It's part of the controller for the Find dialog in a 
> table-editing application.  The input data structures are:
> self.doc.columns() -- a list of strings used as dict keys
> self.doc.rows() -- a list of dicts keyed by the columns
> contains, beginsWith, endsWith, exactMatch -- localized string compare 
> routines
> self.table -- the controller for the table display
> self.column, self.match, self.text -- components of the Find dialog
> Note that self.match and self.columns are both pop-up buttons, but accessed 
> differently because one is static and the other has to be updated when the 
> self.doc.columns() changes.
> 
>     def search(self, row, increment):
>         def matches(row):
>             if col is not None:
>                 return match(row.get(col, ''), target)
>             for c in cols:
>                 if match(row.get(c, ''), target):
>                     return 1
>             return 0
> 
>         cols = self.doc.columns()
>         col = self.column.indexOfSelectedItem() - 1
>         if col < 0:
>             col = None
>         else:
>             col = cols[col]
> 
>         match = self.match.selectedItem().tag()
>         match = (contains, beginsWith, endsWith, exactMatch)[match]
>         target = self.text.stringValue()
>         rows = self.doc.rows()
>         while 1:
>             row += increment
>             if row < 0 or row >= len(rows):
>                 return 0
>             if matches(rows[row]):
>                 self.table.setSelectedLine_(row)
>                 return 1

thanks ...
 
> Looking at it again, it is possible to simplify this, in a way that gets 
> rid of the implicit ternary, allowing the subroutine to be more easily 
> folded into the main routine:

interesting ...
 
>     def search(self, row, increment):
>         cols = self.doc.columns()
>         col = self.column.indexOfSelectedItem() - 1
>         if col >= 0:
>             cols = [cols[col]]

this was exactly what i hoped for when i asked:  avoiding
the special cased index (-1) and constructing the list of
all columns (or just with one item) from the start.  Thus
you adapt the Cocoa-semantic right where you invoked it
and doesn't let it interfere with python semantics. 

>         match = self.match.selectedItem().tag()
>         match = (contains, beginsWith, endsWith, exactMatch)[match]
>         target = self.text.stringValue()
>         rows = self.doc.rows()
>         while 1:
>             row += increment
>             if row < 0 or row >= len(rows):
>                 return 0
>             for c in cols:
>                 if match(rows[row].get(c, ''), target):
>                     self.table.setSelectedLine_(row)
>                     return 1
> 
> But, clearly, the existence or nonexistence of a ternary operator had 
> nothing to do with my ability to find this simplification -- I already had 
> working code for this problem, so had little motivation to work harder on 
> simplifying it until you asked me to post it.

sure, but 

    a) would you have reimplemented it with PEP-308 syntax? 

    b) it doesn't exactly disprove my point that ternary operators
       can often be avoided for great justice :-)

cheers,

    holger