Generators & Iterators

What does this print? ``` gen = (x*x for x in range(3)) print(sum(gen)) print(sum(gen)) ```

What does calling this line do? ``` def squares(n): for i in range(n): yield i*i g = squares(3) ```

What is the key difference in behavior between [x for x in big] and (x for x in big)?

What does this print? ``` i = iter([10, 20]) print(next(i, 'done')) print(next(i, 'done')) print(next(i, 'done')) ```

Which methods must an iterator implement, and what ends the iteration?

An iterator's __next__ has run out of items. How does it signal "no more"?

What does this print? ``` def ab(): yield from ['a', 'b'] yield from ['c'] print(list(ab())) ```

What does this print? ``` def g(): yield 1 return 99 yield 2 print(list(g())) ```

What does this print? ``` import itertools c = itertools.count(10, 2) out = list(itertools.islice(c, 3)) print(out) ```

What does this print? ``` from itertools import groupby data = [1, 1, 2, 1, 1] out = [k for k, _ in groupby(data)] print(out) ```

What does this print? ``` from itertools import groupby g = list(groupby('AAB')) print([list(grp) for k, grp in g]) ```

What does this print? ``` from itertools import combinations print(list(combinations('ABC', 2))) ```

What does this print? ``` from itertools import permutations print(len(list(permutations('ABC', 2)))) ```

What does this print? ``` from itertools import product out = list(product('AB', repeat=2)) print(out) ```

What does this print? ``` from itertools import accumulate print(list(accumulate([1, 2, 3, 4]))) ```

What does this print? ``` from itertools import cycle, islice c = cycle([1, 2, 3]) print(list(islice(c, 5))) ```

What does this print? ``` from itertools import chain print(list(chain('AB', 'CD'))) ```

What does this print? ``` z = zip([1, 2], ['a', 'b']) print(list(z)) print(list(z)) ```

What does this print? ``` z = zip([1, 2, 3], ['a', 'b']) print(list(z)) ```

What does this print? ``` l = [1, 2, 3] print(iter(l) is iter(l)) a = iter(l) print(a is iter(a)) ```

What does this print? ``` import itertools as it print(list(it.islice('ABCDEFG', 1, 5, 2))) ```

What does the two-argument form iter(callable, sentinel) do? (e.g. iter(f.readline, ''))

What does this print? from itertools import compress print(list(compress('ABCDEF', [1, 0, 1, 0, 1, 1])))

What does this print? from itertools import dropwhile, takewhile data = [1, 2, 3, 1, 2] pred = lambda x: x < 3 print(list(takewhile(pred, data))) print(list(dropwhile(pred, data)))

What does this print? from itertools import filterfalse print(list(filterfalse(lambda x: x % 2, [1, 2, 3, 4, 5])))

What does this print? def g(): x = 10 yield x x += 5 yield x it = g() print(next(it)) print(next(it))

What does this print? def sub(): yield 1 return 99 def main(): result = yield from sub() print('sub returned', result) print(list(main()))

When may you drop the parentheses of a generator expression, as in sum(i*i for i in range(1, 11))?

What does this print, and why is defining iteration this way convenient? class Countdown: def __init__(self, n): self.n = n def __iter__(self): for i in range(self.n, 0, -1): yield i print(list(Countdown(3)))

What does this print? class Count: def __init__(self, n): self.n, self.i = n, 0 def __iter__(self): return self def __next__(self): if self.i >= self.n: raise StopIteration self.i += 1 return self.i c = Count(2) for a in c: for b in c: print(a, b)

What is the difference between repeat(obj, times) and repeat(obj) in itertools?

What does this print? from itertools import starmap print(list(starmap(pow, [(2, 3), (2, 4), (3, 2)])))

What does itertools.tee(iterable, n) give you, and what must you avoid doing afterward?

What does this print, and what one-line mistake would break it? def printer(): while True: value = (yield) print('got', value) p = printer() next(p) p.send('hi')

What does a generator's throw(exc) method do, and where does the exception surface?

In a consuming generator, one of these lines does NOT do what it looks like. Which, and why? a = yield # (1) a = yield 5 # (2) a = (yield) + 10 # (3) a = yield + 10 # (4)

How can a class be made iterable without defining __iter__ or __next__ at all?