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*Memos:

• My post explains an iterator (2) and copy.
• 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 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 copy.
• My post explains a dictionary, the dictionary with keying and copy.
• My post explains variable assignment.

An iterator:

• is the object which can return an element one by one to use huge data with low memory usage so it doesn't get MemoryError.
• is immutable so it cannot be changed.
• has __iter__() and __next__().
• can be created by iter() or __iter__() with a list, tuple, set, dictionary, iterator, string or range(), by a generator or generator comprehension or by itertools: *Memos:
  • A generator is the function with one or more yield statements.
  • A yield statement is yield or yield from.
  • A generator comprehension is an expression.
• can be accessed by next(), next() or for loop to get each value.
• cannot be accessed and changed by indexing or slicing.
• raises StopIteration: if there is no element to return.
• can be continuously used through multiple variables.
• except the one created by a generator or generator comprehension can be copied to refer to the same iterator.
• can be used with len() after using list(), tuple() or set() or with more_itertools.ilen() to get the length. *more-itertools must be installed with pip install more-itertools.
• cannot be directly used with len() to get the length.

An iterator is for huge data so it doesn't get MemoryError.

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

*Memos:

• iter(): *Memos:
  • The 1st argument is object(Required-Type:iterable or callable) for an iterator if sentinel isn't set or a callable if sentinel is set. *Don't use object=.
  • The 2nd argument is sentinel(Optional) to raise error if object returns the same element as sentinel. *Don't use sentinel=.
• __iter__() has no arguments.
• next(): *Memos:
  • The 1st argument is iterator(Required-Type:iterable) for an iterator. *Don't use iterator=.
  • The 2nd argument is default(Optional) to return its value if a iterator is terminated. *Don't use default=.
  • Error occurs if default isn't set and if an iterator is terminated.
• __next__() has no arguments.

<iter(object) & next(iterator)>

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

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

print(next(v)) # a
print(next(v)) # b
print(next(v)) # c
print(next(v)) # d
print(next(v)) # e
print(next(v)) # StopIteration:

<__iter__() & __next__()>

v = ['a', 'b', 'c', 'd', 'e'].__iter__()

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

print(v.__next__()) # a
print(v.__next__()) # b
print(v.__next__()) # c
print(v.__next__()) # d
print(v.__next__()) # e
print(v.__next__()) # StopIteration: 

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

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

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

print(next(v, 'No value')) # a
print(next(v, 'No value')) # b
print(next(v, 'No value')) # c
print(next(v, 'No value')) # d
print(next(v, 'No value')) # e
print(next(v, 'No value')) # No value

<iter(object) & for loop>

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

print(v)
# <list_iterator object at 0x000001FCD2F90790>

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

<iter(object, sentinel) & for loop>

import random

def get_random_numbers():
    return random.randint(1, 10)

v = iter(get_random_numbers, 3)

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

for x in v:
    print(x)
# 10
# 6
# 2
# 1
# 6

An iterator can be used with len() after using list(), tuple() or set() or with more_itertools.ilen() to get the length as shown below. *more-itertools must be installed with pip install more-itertools:

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

print(len(list(v))) # 5

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

print(len(tuple(v))) # 5

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

print(len(set(v)))

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

print(ilen(v))

An iterator cannot be directly used with len() to get the length as shown below:

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

print(len(v))
# TypeError: object of type 'list_iterator' has no len()

You cannot access and change an iterator by indexing or slicing because it's immutable as shown below. *A del statement can still be used to remove a variable itself:

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

v[0] = 'X'
# v[-6] = '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]
# del v[-6]
del 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 access and change an iterator, use list() and iter() or __iter__() as shown below:

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

v = list(v) # Here

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

v = iter(v) # Here
v = v.__iter__() # Here

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

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

v = list(v) # Here

del v[0]
# del v[-6]
del v[3:5]

v = iter(v) # Here
v = v.__iter__() # Here

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