On 2007-11-20, Terry Reedy <[EMAIL PROTECTED]> wrote: > "Neil Cerutti" <[EMAIL PROTECTED]> wrote in message > news:[EMAIL PROTECTED] >| s[i:j:t] = t (1) t must have the same length as the slice it is > replacing. > > This is essentially the same rule as requiring a proper length > of t for > > a,b,c = t # for whatever number of targets > > And people have made similar suggestions as below for that case > also. > >| Why? > > A mismatch could be intentional or accidental. In most cases > of this sort, Python assumes 'accident', especially when intent > can easily be indicated otherwise.
Thanks. Assignment to slices are a convenient way to insert, assign, and delete elements, but extended slices are only good for assignment. Perhaps I was searching for consistency in the wrong place, though. >| >>> def foo(): >| ... while True: >| ... yield 'a' >| ... >| >>> foo() >| >>> x = range(10) >| >>> x[::2] = foo() >| >| This is infinite loop due to Python building a sequence out of >| the iterator to check its length. >| >| I think it might be more useful for >| >| x[::2] = foo() >| >| to result in an x of >| >| ['a', 1, 'a', 3, 'a', 5, 'a', 7, 'a', 9] >| >| In other words, take (j-i)//k elements from t for abs(k) != 1. > > Use the appropriate itertools function to take the proper > number of elements. And anyway my math was quite wrong. :( >| A problem, though, arises when t is too short--the sequence >| could be corrupted before an exception is thrown if you omit the >| length check. >| >| So you'd also have to define >| >| x[::2] = 'aaa' >| >| as resulting in >| >| ['a', 1, 'a', 2, 'a', 3, 5, 7, 9] > > No, it should be defined as resulting in > > ['a', 1, 'a', 2, 'a', 3, None, 5, None, 7, None, 9] # ;-) I thought deletion of elements would be more similar to slice assignment, e.g.: x[5:] = [] --> [0, 1, 2, 3, 4] -/ /-> [0, 1, 2, 3, 4, None, None, None, None, None] > Or better yet, require the programmer to specify by modifying > either the target or source spec, as is done now. It seems a shame to accept iterators but to build a sequence out of them, if it can be avoided. But if there's too much confusion about what it should mean, I guess that kills the idea. -- Neil Cerutti -- http://mail.python.org/mailman/listinfo/python-list
