The use of :

[Nick Coghlan]

BJörn Lindqvist wrote:
> Because it contains more non-significant symbols (, ), { and } that
> "steal" the programmers attention. But consider
> 
> def f(x, y, z)
>     print x, y, z
> 
> to
> 
> def f(x, y, z):
>     print x, y, z
> 
> IMHO, the colon-less variant is more readable than the one with the colon.  

Except that it is quite acceptable to do the following:

   def f(x, y, z,
         long_func_arg_name):
     long_func_arg_name(x, y, z)


   def f(x, y, z,
         long_func_arg_name)
     long_func_arg_name(x, y, z)

The colons do a decent job of flagging the beginning of suites, mainly because 
of Python general lack of *other* punctuation (e.g. the colon would be entirely 
ineffective at improving readability if every line ended with a semi-colon).

Cheers,
Nick.