A critic of Guido's blog on Python's lambda

[Carl Friedrich Bolz]

Bill Atkins wrote:
[snip]
>>Here's how one of the cells examples might look in corrupted Python
>>(this is definitely not executable):
>>
>>  class FallingRock:
>>    def __init__(self, pos):
>>      define_slot( 'velocity', lambda: self.accel * self.elapsed )
>>      define_slot( 'pos', lambda: self.accel * (self.elapsed ** 2) / 2,
>>                    initial_position = cell_initial_value( 100 ) )
>>      self.accel = -9.8
>>
>>  rock = FallingRock(100)
>>  print rock.accel, rock.velocity, rock.pos
>>  #     -9.8, 0, 100
>>
>>  rock.elapsed = 1
>>  print rock.accel, rock.velocity, rock.pos
>>  #     -9.8, -9.8, -9.8
>>
>>  rock.elapsed = 8
>>  print rock.accel, rock.velocity, rock.pos
>>  #     -9.8, -78.4, -627.2
>>
>>Make sense?  The idea is to declare what a slot's value represents
>>(with code) and then to stop worrying about keeping different things
>>synchronized.
>>
>>Here's another of the examples, also translated into my horrific
>>rendition of Python (forgive me):
>>
>>  class Menu:
>>    def __init__(self):
>>      define_slot( 'enabled', 
>>         lambda: focused_object( self ).__class__ == TextEntry and 
>>                 focused_object( self ).selection )
>>  
>>Now whenever the enabled slot is accessed, it will be calculated based
>>on what object has the focus.  Again, it frees the programmer from
>>having to keep these different dependencies updated.
>>
>>-- 
>>This is a song that took me ten years to live and two years to write.
>> - Bob Dylan
> 
> 
> Oh dear, there were a few typos:
> 
>   class FallingRock:
>     def __init__(self, pos):
>       define_slot( 'velocity', lambda: self.accel * self.elapsed )
>       define_slot( 'pos', lambda: self.accel * (self.elapsed ** 2) / 2,
>                     initial_value = cell_initial_value( 100 ) )
>       self.accel = -9.8
> 
>   rock = FallingRock(100)
>   print rock.accel, rock.velocity, rock.pos
>   #     -9.8, 0, 100
> 
>   rock.elapsed = 1
>   print rock.accel, rock.velocity, rock.pos
>   #     -9.8, -9.8, 90.2
> 
>   rock.elapsed = 8
>   print rock.accel, rock.velocity, rock.pos
>   #     -9.8, -78.4, -527.2
> 

you mean something like this? (and yes, this is executable python):


class FallingRock(object):
     def __init__(self, startpos):
         self.startpos = startpos
         self.elapsed = 0
         self.accel = -9.8

     velocity  = property(lambda self: self.accel * self.elapsed)
     pos = property(lambda self: self.startpos + self.accel *
                                 (self.elapsed ** 2) / 2)

rock = FallingRock(100)
print rock.accel, rock.velocity, rock.pos
#     -9.8, 0, 100

rock.elapsed = 1
print rock.accel, rock.velocity, rock.pos
#     -9.8, -9.8, 95.1

rock.elapsed = 8
print rock.accel, rock.velocity, rock.pos
#     -9.8, -78.4, -213.6


Cheers,

Carl Friedrich Bolz