How to create functors?

Robert Dailey rcdailey at gmail.com
Tue Aug 18 22:51:19 CEST 2009


On Aug 18, 3:45 pm, "Rami Chowdhury" <rami.chowdh... at gmail.com> wrote:
> Lambda expressions are, I believe, syntactically limited to a single  
> expression -- no statements, like 'print' is in Python 2.x.
>
> If you are strongly against just defining a function, you might have to  
> use a trick to get around it -- this page  
> (http://p-nand-q.com/python/stupid_lambda_tricks.html) has some  
> suggestions.
>
> On Tue, 18 Aug 2009 13:32:55 -0700, Robert Dailey <rcdai... at gmail.com>  
> wrote:
>
>
>
>
>
> > On Aug 18, 3:31 pm, Duncan Booth <duncan.bo... at invalid.invalid> wrote:
> >> Robert Dailey <rcdai... at gmail.com> wrote:
> >> > Hello,
>
> >> > I want to simply wrap a function up into an object so it can be called
> >> > with no parameters. The parameters that it would otherwise have taken
> >> > are already filled in. Like so:
>
> >> >       print1 = lambda: print( "Foobar" )
> >> >       print1()
>
> >> > However, the above code fails with:
>
> >> >   File "C:\IT\work\distro_test\distribute_radix.py", line 286
> >> >     print1 = lambda: print( "Foobar" )
> >> >                          ^
> >> > SyntaxError: invalid syntax
>
> >> > How can I get this working?
>
> >> def print1():
> >>     print "Foobar"
>
> >> It looks like in your version of Python "print" isn't a function. It  
> >> always
> >> helps if you say the exact version you are using in your question as the
> >> exact answer you need may vary.
>
> > I'm using Python 2.6. And using the legacy syntax in the lambda does
> > not work either. I want to avoid using a def if possible. Thanks.
>
> --
> Rami Chowdhury
> "Never attribute to malice that which can be attributed to stupidity" --  
> Hanlon's Razor
> 408-597-7068 (US) / 07875-841-046 (UK) / 0189-245544 (BD)

The example I gave earlier is a bit contrived, the real example
fundamentally requires a lambda since I am actually passing in local
variables into the functions the lambda is wrapping. Example:

def MyFunction():
  localVariable = 20
  CreateTask( lambda: SomeOtherFunction( localVariable ) ) # CreateTask
() executes the functor internally

This is more or less like the real scenario I'm working with. There
are other (more personal) reasons why I prefer to avoid 'def' in this
case. I want to keep the functor as central to the code that needs it
as possible to improve code readability.

Thanks for the help everyone. I guess in Python 3.0 the print()
function will not require the import from __future__ to work in this
particular case?



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