[Python-ideas] while conditional in list comprehension ?? (original) (raw)

Oscar Benjamin oscar.j.benjamin at gmail.com
Tue Jan 29 02:34:46 CET 2013

On 29 January 2013 01:12, Ian Cordasco <graffatcolmingov at gmail.com> wrote:

On Mon, Jan 28, 2013 at 8:02 PM, Oscar Benjamin <oscar.j.benjamin at gmail.com> wrote:

Although dicts and sets should be considered unordered they may still be constructed from a naturally ordered iterable. There are still cases where it makes sense to define the construction of such an object in terms of an order-dependent rule on the underlying iterator. They may be, but they may also be constructed from an unordered iterable. How so? Let d be a non-empty dictionary, and f a function that defines some mutation of it's input such that there doesn't exist x such that x = f(x). e = {k: f(v) for k, v in d.items()} You're taking an unordered object (a dictionary) and making a new one from it. An order dependent rule here would not make sense. Likewise, if we were to do: e = [(k, f(v)) for k, v in d.items()] We're creating order from an object in which there is none. How could the while statement be useful there? An if statement works fine. A while statement as suggested wouldn't.

I was referring to the case of constructing an object that does not preserve order by iterating over an object that does. Clearly a while clause would be a lot less useful if you were iterating over an object whose order was arbitrary: so don't use it in that case.

A (contrived) example - caching Fibonacci numbers:

Fibonacci number generator

def fib(): a = b = 1 while True: yield a a, b = b, a+b

Cache the first N fibonacci numbers

fib_cache = {n: x for n, x in zip(range(N), fib())}

Alternative

fib_cache = {n: x for n, x in enumerate(fib()) while n < N}

Cache the Fibonacci numbers less than X

fib_cache = {} for n, x in enumerate(fib()): if x > X: break fib_cache[n] = x

Alternative 1

fib_cache = {n: x for n, x in enumerate(takewhile(lambda x: x < X, fib()))}

Alternative 2

fib_cache = {n: x for n, x in enumerate(fib()) while x < X}

Oscar

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