*Memo:
- My post explains an iterator (2).
- My post explains an iterator (3).
- My post explains an iterator (4).
- My post explains a generator (1).
- My post explains a class-based iterator.
- My post explains iterator functions.
- My post explains itertools about count(), cycle() and repeat().
- My post explains a generator comprehension.
- My post explains an iterator shallow and deep copy.
- My post explains global and nonlocal with 2 functions or generators.
- My post explains a list (1).
- My post explains a tuple (1).
- My post explains a set (1).
- My post explains a frozenset (1).
- My post explains a dictionary (1).
- My post explains a string (1).
- My post explains a bytes (1).
- My post explains a bytearray (1).
- My post explains a range (1).
An iterator:
- is the immutable(hashable) 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.
- Immutable(Hashable) means the elements of an iterator cannot be changed.
- has
__iter__()and__next__(). - can be read by next() or
__next__()to get the elements one by one. - raises
StopIteration:if there are no elements to return. - can be used indirectly with len() after using list(), tuple(), set() and frozenset() and directly with 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.
-
more-itertools must be installed with
- isn't
Falseeven if it's empty. - can be checked if a specific element is in the iterator with
inkeyword. - can be checked if the iterator is referred to by two variables with
iskeyword. - and one or more iterators cannot be concatenated with
+and|. - cannot be enlarged with
*and a number. - can be iterated with a
forstatement. - can be created by iter() and
__iter__()with an iterable, by a generator and by a generator comprehension:- For
iter(), the words type conversion are also suitable in addition to the word creation.
- For
- can be big because it's the special object which always uses small memory not to get
MemoryError. - can be unpacked with an assignment and
forstatement, function and*but not with**. - cannot be read by indexing and slicing.
- cannot be changed by indexing, slicing and a del statement.
- can be continuously used through multiple variables.
- except the one created by a generator and by a generator comprehension can be copied(shallow-copied by copy.copy() and deep-copied by copy.deepcopy()).
Even a big iterator doesn't get MemoryError.
MemoryError.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
sentinelisn't set. - It's for a callable if
sentinelis set. - Don't use
object=.
- It's for an iterable if
- The 2nd argument is
sentinel(Optional-Type:Any):- It terminates
object(Callable)ifobject(Callable)returns the same value as it:- The returned same value as it cannot be seen.
- Don't use
sentinel=.
- It terminates
- The 1st argument is
-
__iter__()has no arguments. -
next(): -
__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
print(len(list(iter([0, 1, 2, 3, 4]))))
print(len(tuple(iter([0, 1, 2, 3, 4]))))
print(len(set(iter([0, 1, 2, 3, 4]))))
print(len(frozenset(iter([0, 1, 2, 3, 4]))))
from more_itertools import ilen
print(ilen(iter([0, 1, 2, 3, 4])))
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]]])
v = iter(range(100000000))
v = (x for x in range(100000000))
# No error
print(len(iter([0, 1, 2, 3, 4])))
v = iter([0, 1, 2]) + iter([iter([3, 4])]) + iter([iter([iter([5, 6, 7, 8])])])
v = iter([0, 1, 2]) | iter([iter([3, 4])]) | iter([iter([iter([5, 6, 7, 8])])])
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)>:
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 used indirectly with len() after using list(), tuple(), set() and frozenset() and directly with more_itertools.ilen() to get the length as shown below:
*Memo:
-
more-itertoolsmust be installed withpip 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 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
An iterator can be checked if a specific element is in the iterator with in keyword as shown below:
v = iter(['A', iter(['B', 'C'])])
print('A' in v)
# True
print('a' in v)
print('B' in v)
print('C' in v)
print(iter(['A']) in v)
print(iter(['B']) in v)
print(iter(['C']) in v)
print(iter(['B', 'C']) in v)
print(iter(['A', iter(['B', 'C'])]) in v)
print(iter(['A', ['B', 'C']]) in v)
print(['A', iter(['B', 'C'])] in v)
print(['A'] in v)
print(['B'] in v)
print(['C'] in v)
print(['B', 'C'] in v)
print(['A', ['B', 'C']] in v)
# False