On Wed, Nov 7, 2012 at 8:13 PM, Andrew Robinson <andr...@r3dsolutions.com> wrote: > OK, and is this a main use case? (I'm not saying it isn't I'm asking.)
I have no idea what is a "main" use case. > There is a special keyword which signals the new type of comprehension; A > normal comprehension would say eg: '[ foo for i in xrange ]'; but when the > 'for i in' is reduced to a specific keyword such as 'ini' (instead of > problematic 'in') the caching form of list comprehension would start. FYI, the Python devs are not very fond of adding new keywords. Any time a new keyword is added, existing code that uses that word as a name is broken. 'ini' is particularly bad, because 1) it's not a word, and 2) it's the name of a common type of configuration file and is probably frequently used as a variable name in relation to such files. > So, then, just like a comprehension -- the interpreter will begin to > evaluate the code from the opening bracket '['; But anything other than a > function/method will raise a type error (people might want to change that, > but it's safe). > > The interpreter then caches all functions/initialiser methods it comes into > contact with. > Since every function/method has a parameter list (even if empty); The > interpreter would evaluate the parameter list on the first pass through the > comprehension, and cache each parameter list with it's respective function. > > When the 'ini' keyword is parsed a second time, Python would then evaluate > each cached function on its cached parameter list; and the result would be > stored in the created list. > This cached execution would be repeated as many times as is needed. > > Now, for your example: > > values = zip(samples, times * num_groups) > if len(values) < len(times) * num_groups: > # raise an error > > Might be done with: > > values = zip( samples, [ lambda:times, ini xrange(num_groups) ] ) > > if len(values) < len(times) * num_groups > > The comma after the lambda is questionable, and this construction would be > slower since lambda automatically invokes the interpreter; but it's correct. How is this any better than the ordinary list comprehension I already suggested as a replacement? For that matter, how is this any better than list multiplication? Your basic complaint about list multiplication as I understand it is that the non-copying semantics are unintuitive. Well, the above is even less intuitive. It is excessively complicated and almost completely opaque. If I were to come across it outside the context of this thread, I would have no idea what it is meant to be doing. > As an aside, how would you do the lambda inside a list comprehension? As a general rule, I wouldn't. I would use map instead. > [lambda:6 for i in xrange(10) ] # Nope. Thak constructs a list of 10 functions and never calls them. If you want to actually call the lambda, then: [(lambda: 6)() for i in range(10)] or: map(lambda i: 6, range(10)) But note that the former creates equivalent 10 functions and calls each of them once, whereas the latter creates one function and calls it ten times. >> Of course you got an integer. You took an index of the range object, not a >> slice. The rule is that taking an index of a sequence returns an element; >> taking a slice of a sequence returns a sub-sequence. You still have not >> shown any inconsistency here. > > > Because it's an arbitrary rule which operates differently than the > traditional idea shown in python docs? > > slice.indices() is *for* (QUOTE)"representing the set of indices specified > by range(start, stop, step)" > http://docs.python.org/2/library/functions.html#slice slice.indices() has nothing to do with it. Indexing a sequence and calling the .indices() method on a slice are entirely different operations. The slice.indices method is a utility method meant to be called by __getitem__ implementations when doing slicing, not an implementation of indexing. When a sequence is indexed, there is no slice. That method is not related in any way to the semantics of indexing a sequence. -- http://mail.python.org/mailman/listinfo/python-list