itertools in Python (3)

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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 groupby() and islice().
• My post explains itertools about pairwise(), starmap(), tee() and zip_longest().
• 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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compress() can return the iterator which returns the elements of data one by one if the corresponding elements of selectors are True as shown below:

*Memo:

• The 1st argument is data(Required-Type:Iterable).
• The 2nd argument is selectors(Required-Type:Iterable):
  • It selects elements from data.

from itertools import compress

v = compress(data='', selectors=[])
v = compress(data=[], selectors=[])

print(v)
# <itertools.compress object at 0x0000026905F8CF10>

print(next(v))
# StopIteration:

from itertools import compress

v = compress(data='ABCDE', selectors=[1, 0, 1, 1, 0])
v = compress(data=['A', 'B', 'C', 'D', 'E'], selectors=[1, 0, 1, 1, 0])

print(next(v)) # A
print(next(v)) # C
print(next(v)) # D
print(next(v)) # StopIteration:

from itertools import compress

for x in compress(data='ABCDE', selectors=[1, 0, 1, 1, 0]):
# for x in compress(data=['A', 'B', 'C', 'D', 'E'],
#                   selectors=[1, 0, 1, 1, 0]):
    print(x)
# A
# C
# D

from itertools import compress

for x in compress(data='ABCDE', selectors=[1, 0, 1, 1, 0, 1, 1, 1]):
# for x in compress(data=['A', 'B', 'C', 'D', 'E'],
#                   selectors=[1, 0, 1, 1, 0, 1, 1, 1]):
    print(x)
# A
# C
# D

from itertools import compress

for x in compress(data='ABCDE', selectors=[1, 0, 1]):
# for x in compress(data=['A', 'B', 'C', 'D', 'E'], selectors=[1, 0, 1]):
    print(x)
# A
# C

filterfalse() can return the iterator which returns the elements of iterable one by one if predicate returns False as shown below:

*Memo:

• The 1st argument is predicate(Required-Type:Callable or NoneType):
  • If it's None, False elements are returned from iterable.
  • Don't use predicate=.
• The 2nd argument is iterable(Required-Type:Iterable):
  • Don't use iterable=.

from itertools import filterfalse

v = filterfalse(lambda: False, [])

print(v)
# <itertools.filterfalse object at 0x0000026906CE8910>

print(next(v))
# StopIteration:

from itertools import filterfalse

v = filterfalse(lambda a: a < 2, [0, -1, 2, -3, 4])

print(next(v)) # 2
print(next(v)) # 4
print(next(v)) # StopIteration:

from itertools import filterfalse

v = filterfalse(None, [0, -1, False, 2, None, -3, '', 4, [], -5, (), 6])

print(next(v)) # 0
print(next(v)) # False
print(next(v)) # None
print(next(v)) # Nothing
print(next(v)) # []
print(next(v)) # ()
print(next(v)) # StopIteration:

from itertools import filterfalse

for x in filterfalse(lambda a: a < 2, [0, -1, 2, -3, 4]):
    print(x)
# 2
# 4

from itertools import filterfalse

for x in filterfalse(None, [0, -1, False, 2, None, -3, '', 4, [], -5, (), 6]):
    print(x)
# 0
# False
# None
# Nothing
# []
# ()

takewhile() can return the iterator which takes(returns) the elements of iterable one by one while predicate returns True, then drops the rest of elements of iterable once predicate returns False as shown below:

*Memo:

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

from itertools import takewhile

v = takewhile(lambda: True, [])

print(v)
# <itertools.takewhile object at 0x0000026906F68C40>

print(next(v))
# StopIteration:

from itertools import takewhile

v = takewhile(lambda a: a < 2, [0, -1, 2, -3, 4])

print(next(v)) # 0
print(next(v)) # -1
print(next(v)) # StopIteration:

from itertools import takewhile

for x in takewhile(lambda a: a < 2, [0, -1, 2, -3, 4]):
    print(x)
# 0
# -1

dropwhile() can return the iterator which drops the elements of iterable while predicate returns True, then returns the rest of elements of iterable one by one once predicate returns False as shown below:

*Memo:

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

from itertools import dropwhile

v = dropwhile(lambda: False, [])

print(v)
# <itertools.dropwhile object at 0x0000026906F696C0>

print(next(v))
# StopIteration:

from itertools import dropwhile

v = dropwhile(lambda a: a < 2, [0, -1, 2, -3, 4])

print(next(v)) # 2
print(next(v)) # -3
print(next(v)) # 4
print(next(v)) # StopIteration:

from itertools import dropwhile

for x in dropwhile(lambda a: a < 2, [0, -1, 2, -3, 4]):
    print(x)
# 2
# -3
# 4