I need a loop containing range(3,666,2) and 2 (for the sieve of Eratosthenes, by the way). This doesn't work ("AttributeError: 'range' object has no attribute 'extend'" ... or "append"):
primes = range(3,limit,2)
primes.extend(2)
How can I do it in the simple intuitive pythonesque way?
range() in Python 3 returns a dedicated immutable sequence object. You'll have to turn it into a list to extend it:
primes = list(range(3, limit, 2))
primes.append(2)
Note that I used list.append(), not list.extend() (which expects a sequence of values, not one integer).
However, you probably want to start your loop with 2, not end it. Moreover, materializing the whole range into a list requires some memory and kills the efficiency of the object. Use iterator chaining instead:
from itertools import chain
primes = chain([2], range(3, limit, 2))
Now you can loop over primes without materializing a whole list in memory, and still include 2 at the start of the loop.
range(3,ThisIsSuchABigNumber,2) ? Will I do a list of billions just for a range?
chain and range are lazy functions. chain will lazily loop over its arguments. So, the entire list will not be created in memory, when invoked. range will return a range object, not the entire list.chain solution wasn't added yet. Now it's hard to choose an answer..extend() doesn't work on a range() object (assuming you tried to use list methods on it), before showing how it should be done instead. Jon focused on the latter first. Such is the nature of Stack Overflow sometimes, if the answer is reasonably obvious for the regulars, it comes down to the faster typer.If you're only looping and don't want to materialise, then:
from itertools import chain
primes = chain([2], range(3, limit, 2))
I think the two makes more sense at the start though...
AttributeError: 'range' object has no attribute 'extend'.