Iterator in Python (1)

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*Memo:

• My post explains an iterator (2) and the iterator with copy and sorted().
• My post explains a generator.
• My post explains a class-based iterator with __iter__() and/or __next__().
• My post explains the shallow and deep copy of an iterator.
• My post explains global and nonlocal with 2 functions or generators.
• My post explains itertools about count(), cycle() and repeat().
• My post explains a list and the list with indexing.
• My post explains a tuple.
• My post explains a set and the set with copy.
• My post explains a frozenset (1).
• My post explains a dictionary (1).
• My post explains a string.
• My post explains a bytes.
• My post explains a bytearray.
• My post explains a range (1).

An iterator:

• is the collection of zero or more elements which can return an element one by one:
  • Whether it's ordered or unordered, what type of an iterator it's, whether duplicated elements it can have and how many mixed types it can have all depend on the base type of an iterator.
• can be huge because it's the special object which always uses low memory not to get MemoryError.
• is immutable(hashable) so it cannot be changed.
• has __iter__() and __next__().
• can be iterated with a for statement.
• can be unpacked with an assignment and for statement, function and * but not with **.
• isn't False even if it's empty.
• can be checked if a specific element is in the iterator with in keyword.
• can be checked if the iterator is referred to by two variables with is keyword.
• cannot be enlarged with * and a number.
• can be created by iter() or __iter__() with an iterable, a generator, generator comprehension or itertools:
  • For iter(), the words type conversion are also suitable in addition to the word creation.
• can be read by next(), __next__() or for loop to get each element one by one from the iterator.
• cannot be read or changed by indexing or slicing.
• raises StopIteration: if there are no elements to return.
• can be continuously used through multiple variables.
• except the one created by a generator or generator comprehension can be copied(shallow-copied and deep-copied).
• can be used with len() after using list(), tuple(), set(), frozenset() or more_itertools.ilen() to get the length:
  • more-itertools must be installed with pip install more-itertools.
  • An iterator cannot be directly used with len() to get the length.

iter() or __iter__() can create an iterator, then next(), __next__() or for loop can get each element one by one from the iterator as shown below:

*Memo:

• iter():
  • The 1st argument is object(Required-Type:Iterable or Callable):
    • It's for an iterable if sentinel isn't set.
    • It's for a callable if sentinel is set.
    • Don't use object=.
  • The 2nd argument is sentinel(Optional-Type:Any):
    • It terminates object(Callable) if object(Callable) returns the same value as it:
      • The returned same value as it cannot be seen.
    • Don't use sentinel=.
• iter() has no arguments.
• next():
  • The 1st argument is iterator(Required-Type:Iterator):
    • Don't use iterator=.
  • The 2nd argument is default(Optional-Type:Any):
    • It's returned if the iterator is terminated:
      • Error occurs if it's not set and if the iterator is terminated.
    • Don't use default=.
• next() has no arguments.

v = iter([])                                       # Empty 1D iterator
v = iter([0, 1, 2, 3, 4])                          # 1D iterator
v = iter([0, 1, 2, 0, 1, 2])                       # 1D iterator
v = iter([0, 1, 2, 3, iter([4, 5, 6, 7])])         # 2D iterator
v = iter([iter([0, 1, 2, 3]), iter([4, 5, 6, 7])]) # 2D iterator
v = iter([iter([0, 1, 2, 3]),                      # 3D iterator
          iter([iter([4, 5]), iter([6, 7])])])
v = iter([iter([iter([0, 1]), iter([2, 3])]),      # 3D iterator
          iter([iter([4, 5]), iter([6, 7])])])
# No error

v = iter([0, 0.0, 0.0+0.0j, False])
v = iter([1, 1.0, 1.0+0.0j, True])
v = iter(['A', b'A', bytearray(b'A'), 2, 2.3, 2.3+4.5j, True,
          [2, 3], (2, 3), {2, 3}, frozenset({2, 3}), {'A':'a'},
          range(2, 3), iter([2, 3])])
for x in iter([0, 1, 2, 3, 4]): pass
for x in iter([iter([0, 1, 2, 3]), iter([4, 5, 6, 7])]): pass
for x in iter([iter([iter([0, 1]), iter([2, 3])]),
               iter([iter([4, 5]), iter([6, 7])])]): pass
v1, v2, v3 = iter([0, 1, 2])
v1, *v2, v3 = iter([0, 1, 2, 3, 4, 5])
for v1, v2, v3 in iter([iter([0, 1, 2]), iter([3, 4, 5])]): pass
for v1, *v2, v3 in iter([iter([0, 1, 2, 3, 4, 5]),
                         iter([6, 7, 8, 9, 10, 11])]): pass
print(*iter([0, 1]), 2, *iter([3, 4, *iter([5])]))
print([*iter([0, 1]), 2, *iter([3, 4, *iter([5])])])
v = (x**2 for x in [0, 1, 2, 3, 4, 5, 6, 7])
v = ((y**2 for y in x) for x in [[0, 1, 2, 3], [4, 5, 6, 7]])
v = (((z**2 for z in y) for y in x) for x in [[[0, 1], [2, 3]],
                                              [[4, 5], [6, 7]]])
# No error

v = iter([0, 1, 2, 3, 4]) * 3
# Error

<iter(object) & next(iterator)>:

v = iter([0, 1, 2, 3, 4])

print(v)
# <list_iterator object at 0x000002821F75D240>

print(type(v))
# <class 'list_iterator'>

print(next(v)) # 0
print(next(v)) # 1
print(next(v)) # 2
print(next(v)) # 3
print(next(v)) # 4
print(next(v)) # StopIteration:

<__iter__() & __next__()>:

v = [0, 1, 2, 3, 4].__iter__()

print(v)
# <list_iterator object at 0x000001FCD2883280>

print(v.__next__()) # 0
print(v.__next__()) # 1
print(v.__next__()) # 2
print(v.__next__()) # 3
print(v.__next__()) # 4
print(v.__next__()) # StopIteration: 

<iter(object) & next(iterator, default)>:

v = iter([0, 1, 2, 3, 4])

print(v)
# <list_iterator object at 0x000002821F75D240>

print(next(v, 'No value')) # 0
print(next(v, 'No value')) # 1
print(next(v, 'No value')) # 2
print(next(v, 'No value')) # 3
print(next(v, 'No value')) # 4
print(next(v, 'No value')) # No value

<iter(object, sentinel) & for loop>:

import random

def get_random_numbers():
    return random.randint(0, 9)

v = iter(get_random_numbers, 3)

print(v)
# <callable_iterator object at 0x000001FCD2F376A0>

print(next(v)) # 7
print(next(v)) # 0
print(next(v)) # 2
print(next(v)) # 9
print(next(v)) # 5
print(next(v)) StopIteration:

An iterator can be iterated with a for statement as shown below:

1D iterator:

for x in iter([0, 1, 2, 3, 4]):
    print(x)
# 0
# 1
# 2
# 3
# 4

2D iterator:

for x in iter([iter([0, 1, 2, 3]), iter([4, 5, 6, 7])]):
    for y in x:
        print(y)
# 0
# 1
# 2
# 3
# 4
# 5
# 6
# 7

3D iterator:

for x in iter([iter([iter([0, 1]), iter([2, 3])]),
               iter([iter([4, 5]), iter([6, 7])])]):
    for y in x:
        for z in y:
            print(z)
# 0
# 1
# 2
# 3
# 4
# 5
# 6
# 7

An iterator can be unpacked with an assignment and for statement, function and * but not with ** as shown below:

v1, v2, v3 = iter([0, 1, 2])

print(v1, v2, v3)
# 0 1 2

v1, *v2, v3 = iter([0, 1, 2, 3, 4, 5])

print(v1, v2, v3)  # 0 [1, 2, 3, 4] 5
print(v1, *v2, v3) # 0 1 2 3 4 5

for v1, v2, v3 in iter([iter([0, 1, 2]), iter([3, 4, 5])]):
    print(v1, v2, v3)
# 0 1 2
# 3 4 5

for v1, *v2, v3 in iter([iter([0, 1, 2, 3, 4, 5]),
                         iter([6, 7, 8, 9, 10, 11])]):
    print(v1, v2, v3)
    print(v1, *v2, v3)
# 0 [1, 2, 3, 4] 5
# 0 1 2 3 4 5
# 6 [7, 8, 9, 10] 11
# 6 7 8 9 10 11

print(*iter([0, 1]), 2, *iter([3, 4, *iter([5])]))
# 0 1 2 3 4 5

print([*iter([0, 1]), 2, *iter([3, 4, *iter([5])])])
# [0, 1, 2, 3, 4, 5]

def func(p1='a', p2='b', p3='c', p4='d', p5='e', p6='f'):
    print(p1, p2, p3, p4, p5, p6)

func()
# a b c d e f

func(*iter([0, 1, 2, 3]), *iter([4, 5]))
# 0 1 2 3 4 5

def func(p1='a', p2='b', *args):
    print(p1, p2, args)
    print(p1, p2, *args)
    print(p1, p2, ['A', 'B', *args, 'C', 'D'])

func()
# a b ()
# a b
# a b ['A', 'B', 'C', 'D']

func(*iter([0, 1, 2, 3]), *iter([4, 5]))
# 0 1 (2, 3, 4, 5)
# 0 1 2 3 4 5
# 0 1 ['A', 'B', 2, 3, 4, 5, 'C', 'D']

An empty iterator isn't False as shown below:

print(bool(iter([])))         # Empty iterator
# True

print(bool(iter([0])))        # iterator
print(bool(iter([iter([])]))) # iterator(Empty iterator)
# True

A iterator can be checked if a specific element is in the iterator with in keyword as shown below:

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

print('B' in v)
# True

print('C' in v)
print(['C', 'D'] in v)
print(iter(['C', 'D']) in v)
# False

A iterator cannot be enlarged with * and a number as shown below:

v = iter([0, 1, 2, 3, 4]) * 3
# TypeError: unsupported operand type(s) for *: 'list_iterator' and 'int'

An iterator can be used with len() after using list(), tuple(), set(), frozenset() or more_itertools.ilen() to get the length as shown below:

*Memo:

• more-itertools must be installed with pip install more-itertools.
• An iterator cannot be directly used with len() to get the length.

from copy import copy
from more_itertools import ilen

v1 = iter([0, 1, 2, 3, 4])

v2 = copy(v1)
print(len(list(v2)))
# 5

v2 = copy(v1)
print(len(tuple(v2)))
# 5

v2 = copy(v1)
print(len(set(v2)))
# 5

v2 = copy(v1)
print(len(frozenset(v2)))
# 5

v2 = copy(v1)
print(ilen(v2))
# 5

v2 = copy(v1)
print(len(v2))
# TypeError: object of type 'list_iterator' has no len()

An iterator cannot be read or changed by indexing or slicing as shown below:

*Memo:

• A del statement can still be used to remove one or more variables themselves.

v = iter(['a', 'b', 'c', 'd', 'e', 'f'])

print(v[0], v[2:6])
# TypeError: 'list_iterator' object is not subscriptable

v = iter(['a', 'b', 'c', 'd', 'e', 'f'])

v[0] = 'X'
v[2:6] = ['Y', 'Z']
# TypeError: 'list_iterator' object does not support item assignment

v = iter(['a', 'b', 'c', 'd', 'e', 'f'])

del v[0], v[3:5]
# TypeError: 'list_iterator' object does not support item deletion

v = iter(['a', 'b', 'c', 'd', 'e', 'f'])

del v

print(v)
# NameError: name 'v' is not defined

If you really want to read and change an iterator, use list() and iter() or __iter__() as shown below:

v = iter(['a', 'b', 'c', 'd', 'e', 'f'])

v = list(v)

v[0] = 'X'
v[2:6] = ['Y', 'Z']

v = iter(v)
v = v.__iter__()

for x in v:
    print(x)
# X
# b
# Y
# Z

v = iter(['a', 'b', 'c', 'd', 'e', 'f'])

v = list(v)

del v[0], v[3:5]

v = iter(v)
v = v.__iter__()

for x in v:
    print(x)
# b
# c
# d