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[ Write a python function to return nth item or a default value ]

Write a python function to return nth item or a default value

def nth(iterable, n, default=None):
    from itertools import islice
    return next(islice(iterable, n, None), default)

Write a python function to check whether all elements are equal to each other

def all_equal(iterable):
    from itertools import groupby
    g = groupby(iterable)
    return next(g, True) and not next(g, False)

Write a python function to count how many times the predicate is true

def quantify(iterable, pred=bool):
    return sum(map(pred, iterable))

Write a python function to emulate the behavior of built-in map() function

def pad_none(iterable):
    """Returns the sequence elements and then returns None indefinitely.

    Useful for emulating the behavior of the built-in map() function.
    from itertools import chain, repeat
    return chain(iterable, repeat(None))

Write a python function to return the sequence elements n times

def ncycles(iterable, n):
    from itertools import chain, repeat
    return chain.from_iterable(repeat(tuple(iterable), n))

Write a python function to return the dot product of two vectors

def dotproduct(vec1, vec2):
    return sum(map(operator.mul, vec1, vec2))

Write a python function to flatten one level of nesting

def flatten(list_of_lists):
    from itertools import chain
    return chain.from_iterable(list_of_lists)

Write a python function to repeat calls to function with specified arguments

def repeatfunc(func, times=None, *args):
    from itertools import starmap, repeat
    if times is None:
        return starmap(func, repeat(args))
    return starmap(func, repeat(args, times))

Write a python function to convert iterable to pairwise iterable

def pairwise(iterable):
    from itertools import tee
    a, b = tee(iterable)
    next(b, None)
    return zip(a, b)

Write a python function to collect data into fixed-length chunks or blocks

def grouper(iterable, n, fillvalue=None):
    from itertools import zip_longest
    # grouper('ABCDEFG', 3, 'x') --> ABC DEF Gxx"
    args = [iter(iterable)] * n
    return zip_longest(*args, fillvalue=fillvalue)

Write a python program to create round robin algorithm: "roundrobin('ABC', 'D', 'EF') --> A D E B F C"

def roundrobin(*iterables):    
    from itertools import islice, cycle
    # Recipe credited to George Sakkis
    num_active = len(iterables)
    nexts = cycle(iter(it).__next__ for it in iterables)
    while num_active:
            for next in nexts:
                yield next()
        except StopIteration:
            # Remove the iterator we just exhausted from the cycle.
            num_active -= 1
            nexts = cycle(islice(nexts, num_active))

Write a python function to use a predicate and return entries particition into false entries and true entries

def partition(pred, iterable):
    from itertools import filterfalse, tee
    # partition(is_odd, range(10)) --> 0 2 4 6 8   and  1 3 5 7 9
    t1, t2 = tee(iterable)
    return filterfalse(pred, t1), filter(pred, t2)

Write a python function to return powerset of iterable

def powerset(iterable):
    "powerset([1,2,3]) --> () (1,) (2,) (3,) (1,2) (1,3) (2,3) (1,2,3)"
    from itertools import chain, combinations
    s = list(iterable)
    return chain.from_iterable(combinations(s, r) for r in range(len(s)+1))


Write a python function to list all unique elements, preserving order

def unique_everseen(iterable, key=None):
    from itertools import filterfalse
    # unique_everseen('AAAABBBCCDAABBB') --> A B C D
    # unique_everseen('ABBCcAD', str.lower) --> A B C D
    seen = set()
    seen_add = seen.add
    if key is None:
        for element in filterfalse(seen.__contains__, iterable):
            yield element
        for element in iterable:
            k = key(element)
            if k not in seen:
                yield element

Write a python function to list unique elements, preserving order remembering only the element just seen.

def unique_justseen(iterable, key=None):
    import operator
    from itertools import groupby    
    # unique_justseen('AAAABBBCCDAABBB') --> A B C D A B
    # unique_justseen('ABBCcAD', str.lower) --> A B C A D
    return map(next, map(operator.itemgetter(1), groupby(iterable, key)))

Write a python function to call a function repeatedly until an exception is raised.

def iter_except(func, exception, first=None):
    """Converts a call-until-exception interface to an iterator interface.
    Like builtins.iter(func, sentinel) but uses an exception instead
    of a sentinel to end the loop.
        iter_except(s.pop, KeyError)                             # non-blocking set iterator
        if first is not None:
            yield first()            # For database APIs needing an initial cast to db.first()
        while True:
            yield func()
    except exception:

Write a python function to return random selection from itertools.product(*args, **kwds)

def random_product(*args, repeat=1):
    import random
    pools = [tuple(pool) for pool in args] * repeat
    return tuple(map(random.choice, pools))