subexpressions

Steve Holden steve at holdenweb.com
Fri Jun 1 13:09:05 CEST 2007


Sergey Dorofeev wrote:
> "Peter Otten" <__peter__ at web.de> wrote in message 
> news:f3ok60$vp7$03$1 at news.t-online.com...
>> Sergey Dorofeev wrote:
>>
>>> Please help, is there way to use sub-expressions in lambda?
>>> For example, if I want to calculate sin(x^2)+cos(x^2) I must code:
>>> lambda x: sin(x*x)+cos(x*x)
>>> How to make x*x to be evaluated once?
>>>>> (lambda x: [sin(x2) + cos(x2) for x2 in [x*x]][0])(.5) == sin(.5*.5) +
>> cos(.5*.5)
>> True
>>
>> The real answer is of course: Use a function.
> 
> But what about something like
> 
> lambda x: sin(y)+cos(y) where y=x*x
> 
> ?
> May be this could be a PEP? If there is no straight way to do this. 
> 
> 
Or maybe it could be made a part of some other language. When 
straightforward mechanisms (in rhis case, function definitins) exist to 
avoid repeated computations it's very unlikely that such mangled 
constructions will be made a part of Python.

If it *were* considered, you should at least change the "where" to 
"for", and extend it to unpacking assignment to allow

lambda x, y: (sin(xx+yy) + cos(xx+yy) for xx, yy = x*x, y*y

regards
  Steve
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