5.14.2 Examples

The following examples show common uses for each tool and demonstrate ways they can be combined.

>>> amounts = [120.15, 764.05, 823.14]
>>> for checknum, amount in izip(count(1200), amounts):
...     print 'Check %d is for $%.2f' % (checknum, amount)
Check 1200 is for $120.15
Check 1201 is for $764.05
Check 1202 is for $823.14

>>> import operator
>>> for cube in imap(operator.pow, xrange(1,4), repeat(3)):
...    print cube

>>> reportlines = ['EuroPython', 'Roster', '', 'alex', '', 'laura',
                  '', 'martin', '', 'walter', '', 'samuele']
>>> for name in islice(reportlines, 3, None, 2):
...    print name.title()

This section shows how itertools can be combined to create other more powerful itertools. Note that enumerate() and iteritems() already have efficient implementations in Python. They are only included here to illustrate how higher level tools can be created from building blocks.

def take(n, seq):
    return list(islice(seq, n))

def enumerate(iterable):
    return izip(count(), iterable)

def tabulate(function):
    "Return function(0), function(1), ..."
    return imap(function, count())

def iteritems(mapping):
    return izip(mapping.iterkeys(), mapping.itervalues())

def nth(iterable, n):
    "Returns the nth item"
    return list(islice(iterable, n, n+1))

def all(seq, pred=bool):
    "Returns True if pred(x) is True for every element in the iterable"
    return False not in imap(pred, seq)

def any(seq, pred=bool):
    "Returns True if pred(x) is True at least one element in the iterable"
    return True in imap(pred, seq)

def no(seq, pred=bool):
    "Returns True if pred(x) is False for every element in the iterable"
    return True not in imap(pred, seq)

def quantify(seq, pred=bool):
    "Count how many times the predicate is True in the sequence"
    return sum(imap(pred, seq))

def padnone(seq):
    "Returns the sequence elements and then returns None indefinitely"
    return chain(seq, repeat(None))

def ncycles(seq, n):
    "Returns the sequence elements n times"
    return chain(*repeat(seq, n))

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

def window(seq, n=2):
    "Returns a sliding window (of width n) over data from the iterable"
    "   s -> (s0,s1,...s[n-1]), (s1,s2,...,sn), ...                   "
    it = iter(seq)
    result = tuple(islice(it, n))
    if len(result) == n:
        yield result    
    for elem in it:
        result = result[1:] + (elem,)
        yield result

def tee(iterable):
    "Return two independent iterators from a single iterable"
    def gen(next, data={}, cnt=[0]):
        dpop = data.pop
        for i in count():
            if i == cnt[0]:
                item = data[i] = next()
                cnt[0] += 1
                item = dpop(i)
            yield item
    next = iter(iterable).next
    return (gen(next), gen(next))
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