[Python-Dev] Let's just *keep* lambda

Bengt Richter bokr at oz.net
Thu Feb 9 19:24:43 CET 2006

On Thu, 09 Feb 2006 17:39:31 +0100, =?ISO-8859-1?Q?=22Martin_v=2E_L=F6wis=22?= <martin at v.loewis.de> wrote:

>Jiwon Seo wrote:
>> Apparently, simplest example is,
>> collection.visit(lambda x: print x)
>Ok. I remotely recall Guido suggesting that print should become
>a function.
Even so, that one is so trivial to define (other than the >> part):

 >>> import sys
 >>> def printfun(*args): sys.stdout.write(' '.join(map(str,args))+'\n')
 >>> lamb = lambda x: printfun(x)
 >>> lamb(123)
 >>> printfun('How', 'about', 'that?')
 How about that?

Also the quasi-C variant:
 >>> def printf(fmt, *args): sys.stdout.write(fmt%args)
 >>> (lambda x: printf('How about this: %s', x))('-- also a function\n(no \\n here ;-) ')
 How about this: -- also a function
 (no \n here ;-) >>>

>It's not a specific example though: what precise library provides
>the visit method?
>> which currently is not possible. Another example is,
>> map(lambda x: if odd(x): return 1
>>                       else: return 0,
>>         listOfNumbers)
>Hmm. What's wrong with
>map(odd, listOfNumbers)
>or, if you really need ints:
>map(lambda x:int(odd(x)), listOfNumbers)
>> Also, anything with exception handling code can't be without explicit
>> function definition.
>> collection.visit(lambda x: try: foo(x); except SomeError: error("error
>> message"))
>That's not a specific example.
 >>> (lambda : """
 ...     I will say that the multi-line part
 ...     of the argument against lambda suites
 ...     is bogus, though ;-)
 ... """)(
 ...     ).splitlines(
 ...     )[-1].split()[1].capitalize(
 ...     ).rstrip(',')+'! (though this is ridiculous ;-)'
 'Bogus! (though this is ridiculous ;-)'

And, as you know, you can abuse the heck out of lambda (obviously this is
ridiculous**2 avoidance of external def)

 >>> lamb = lambda x: eval(compile("""if 1:
 ...     def f(x):
 ...         try: return 'zero one two three'.split()[x]
 ...         except Exception,e:return 'No name for %r -- %s:%s'%(x,e.__class__.__name__, e)
 ... """,'','exec')) or locals()['f'](x)
 >>> lamb(2)
 >>> lamb(0)
 >>> lamb(4)
 'No name for 4 -- IndexError:list index out of range'
 >>> lamb('x')
 "No name for 'x' -- TypeError:list indices must be integers"

But would e.g. [1]

    collection.visit(lambda x::  # double ':' to signify suite start
       try: return 'zero one two three'.split()[x]
       except Exception,e:return 'No name for %r -- %s:%s'%(x,e.__class__.__name__, e)

be so bad an "improvement"? Search your heart for the purest answer ;-)
(requires enclosing parens, and suite ends on closing ')' and if multiline,
the first line after the :: defines the indent-one left edge, and explicit
return of value required after ::).

[1] (using the function body above just as example, not saying it makes sense for collection.visit)

Bengt Richter

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