Iterators and Generators

def countdown(n: int):
    while n > 0:
        yield n
        n -= 1
list(countdown(3))   # [3, 2, 1]

def flat(xss):
    for xs in xss:
        yield from xs
list(flat([[1, 2], [3]]))   # [1, 2, 3]

from itertools import islice, count
list(islice(count(10), 3))   # [10, 11, 12]

nums = [1, -2, 3]
sum(x * x for x in nums if x > 0)   # 10
type(x * x for x in nums)           # <class 'generator'>

def echo():
    x = yield "ready"
    yield f"got {x}"
g = echo()
next(g)        # 'ready'
g.send("hi")   # 'got hi'

next(iter([10, 20]), None)   # 10
next(iter([]), None)         # None
next(iter([]), 0)            # 0

from itertools import tee
a, b = tee(iter([1, 2, 3]), 2)
list(a), list(b)   # ([1, 2, 3], [1, 2, 3])

from itertools import chain
list(chain([1, 2], [3], [4, 5]))   # [1, 2, 3, 4, 5]

from itertools import groupby
rows = [("a", 1), ("a", 2), ("b", 3)]
{k: list(g) for k, g in groupby(rows, key=lambda r: r[0])}
# {
#   'a': [('a', 1), ('a', 2)],
#   'b': [('b', 3)],
# }

from itertools import cycle, islice, repeat
list(islice(cycle(["r", "g", "b"]), 5))  # ['r', 'g', 'b', 'r', 'g']
list(repeat("x", 3))                     # ['x', 'x', 'x']

from itertools import batched
list(batched(range(7), 3))
# [(0, 1, 2), (3, 4, 5), (6,)]

class Counter:
    def __init__(self, n: int) -> None:
        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
list(Counter(3))   # [1, 2, 3]

from contextlib import closing
class R:
    def read(self): return b"ok"
    def close(self): self.closed = True
with closing(R()) as r:
    r.read()    # b'ok'

from itertools import compress, filterfalse
list(compress("abcd", [1, 0, 1, 0]))          # ['a', 'c']
list(filterfalse(lambda x: x % 2, range(5)))  # [0, 2, 4]

lines = ["  a ", "", " b"]
pipeline = (l.strip() for l in lines)
pipeline = (l for l in pipeline if l)
list(pipeline)   # ['a', 'b']