itertools in Python (5)

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*Memo:

• My post explains itertools about count(), cycle() and repeat().
• My post explains itertools about accumulate(), batched(), chain() and chain.from_iterable().
• My post explains itertools about compress(), filterfalse(), takewhile() and dropwhile().
• My post explains itertools about groupby() and islice().
• My post explains itertools about product() and permutations().
• My post explains itertools about combinations() and combinations_with_replacement().
• My post explains an iterator (1).

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itertools has the functions to create iterators.

*more-itertools has more functions by installing with pip install more-itertools.

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pairwise() can return the iterator which returns a successive overlapping tuple of the two elements of iterable one by one as shown below:

*Memo:

• The 1st argument is iterable(Required-Type:Iterable):
  • Don't use iterable=.

from itertools import pairwise

v = pairwise('')
v = pairwise([])

print(v)
# <itertools.pairwise object at 0x0000026B3E697730>

print(next(v))
# StopIteration:

from itertools import pairwise

v = pairwise('ABCD')
v = pairwise(['A', 'B', 'C', 'D'])

print(next(v)) # ('A', 'B')
print(next(v)) # ('B', 'C')
print(next(v)) # ('C', 'D')
print(next(v)) # StopIteration:

from itertools import pairwise

for x in pairwise('ABCD'):
# for x in pairwise(['A', 'B', 'C', 'D']):
    print(x)
# ('A', 'B')
# ('B', 'C')
# ('C', 'D')

starmap() can return the iterator which does function with the elements of iterable one by one to returns the result one by one as shown below:

*Memo:

• The 1st argument is function(Required-Type:Callable):
  • Don't use function=.
• The 2nd argument is iterable(Required-Type:Iterable):
  • Don't use iterable=.

from itertools import starmap

x = starmap(lambda: None, [])

print(x)
# <itertools.starmap object at 0x000001BE9A289CF0>

print(next(x))
# StopIteration:

from itertools import starmap
from operator import add

x = starmap(lambda a, b: a+b, [(2, 5), (3, 2), (10, 3)])
x = starmap(add, [(2, 5), (3, 2), (10, 3)])

print(next(x)) # 7
print(next(x)) # 5
print(next(x)) # 13
print(next(x)) # StopIteration:

from itertools import starmap
from operator import mul

for x in starmap(lambda a, b: a*b, [(2, 5), (3, 2), (10, 3)]):
# for x in starmap(mul, [(2, 5), (3, 2), (10, 3)]):
    print(x)
# 10
# 6
# 30

from itertools import starmap

for x in starmap(lambda a, b: a**b, [(2, 5), (3, 2), (10, 3)]):
# for x in starmap(pow, [(2, 5), (3, 2), (10, 3)]):
    print(x)
# 32
# 9
# 1000

tee() can return a tuple of the iterators which each return the elements of iterable one by one as shown below:

*Memo:

• The 1st argument is iterable(Required-Type:Iterable):
  • Don't use iterable=.
• The 2nd argument is n(Optional-Default:2-Type:int):
  • It's the number of iterators.
  • It must be 0 <= x.
  • Don't use n=.

from itertools import tee

v = tee('ABC')
v = tee('ABC', 2)
v = tee(['A', 'B', 'C'])

print(v)
# (<itertools._tee object at 0x000001BE99D6E3C0>,
#  <itertools._tee object at 0x000001BE99F85440>)

print(next(v[0])) # A
print(next(v[0])) # B
print(next(v[0])) # C
print(next(v[1])) # A
print(next(v[1])) # B
print(next(v[1])) # C

print(next(v[0])) # StopIteration:
print(next(v[1])) # StopIteration:

from itertools import tee

for x in tee('ABC'):
# for x in tee(['A', 'B', 'C']):
    for y in x:
        print(y)
# A
# B
# C
# A
# B
# C

from itertools import tee

for x in tee('ABC', 3):
# for x in tee(['A', 'B', 'C'], 3):
    for y in x:
        print(y)
# A
# B
# C
# A
# B
# C
# A
# B
# C

zip_longest() can return the iterator which aggregates the elements of *iterables one by one to returns a tuple of one or more elements one by one as shown below:

*Memo:

• The 1st arguments are *iterables(Optional-Default:()-Type:Iterable):
  • Don't use any keywords like *iterables=, iterables=, etc.
• The 2nd argument is fillvalue(Optional-Default:None-Type:Any or NoneType):
  • It's the value to fill the zero or more missing elements of a returned tuple.
  • fillvalue= must be used.

from itertools import zip_longest

v = zip_longest()
v = zip_longest((), fillvalue=None)

print(v)
# <itertools.zip_longest object at 0x000001BE99F29FD0>

print(next(v))
# StopIteration:

from itertools import zip_longest

v = zip_longest('ABC')
v = zip_longest(['A', 'B', 'C'])

print(next(v)) # ('A',)
print(next(v)) # ('B',)
print(next(v)) # ('C',)
print(next(v)) # StopIteration:

from itertools import zip_longest

v = zip_longest('ABC', 'vwxyz')
v = zip_longest(['A', 'B', 'C'], ['v', 'w', 'x', 'y', 'z'])

print(next(v)) # ('A', 'v')
print(next(v)) # ('B', 'w')
print(next(v)) # ('C', 'x')
print(next(v)) # (None, 'y')
print(next(v)) # (None, 'z')
print(next(v)) # StopIteration:

from itertools import zip_longest

v = zip_longest('ABC', 'vwxyz', fillvalue='Abscent')
v = zip_longest(['A', 'B', 'C'], ['v', 'w', 'x', 'y', 'z'],
                fillvalue='Abscent')

print(next(v)) # ('A', 'v')
print(next(v)) # ('B', 'w')
print(next(v)) # ('C', 'x')
print(next(v)) # ('Abscent', 'y')
print(next(v)) # ('Abscent', 'z')
print(next(v)) # StopIteration:

from itertools import zip_longest

for x in zip_longest('ABC', 'vwxyz', [10, 20], fillvalue='Abscent'):
# for x in zip_longest(['A', 'B', 'C'], ['v', 'w', 'x', 'y', 'z'],
#                      [10, 20], fillvalue='Abscent'):
    print(x)
# ('A', 'v', 10)
# ('B', 'w', 20)
# ('C', 'x', 'Abscent')
# ('Abscent', 'y', 'Abscent')
# ('Abscent', 'z', 'Abscent')

from itertools import zip_longest

for x in zip_longest('ABC', 'vwxyz', [10, 20], fillvalue='Abscent'):
# for x in zip_longest(['A', 'B', 'C'], ['v', 'w', 'x', 'y', 'z'],
#                      [10, 20], fillvalue='Abscent'):
    for y in x:
        print(y)
# A
# v
# 10
# B
# w
# 20
# C
# x
# Abscent
# Abscent
# y
# Abscent
# Abscent
# z
# Abscent