*Memo:
- My post explains an iterator (1).
- My post explains an iterator (2).
- My post explains an iterator (3).
- My post explains an iterator (4).
- My post explains an iterator (6).
A generator can create an iterator as shown below:
def gen():
yield 0
yield 1
yield from [2, 3, 4]
print(gen) # <function func at 0x000001FCD2E3CAE0>
print(type(gen)) # <class 'function'>
v = gen()
print(v) # <generator object func at 0x00000282207E3CC0>
print(type(v)) # <class 'generator'>
for x in v:
print(x)
# 0
# 1
# 2
# 3
# 4
A generator comprehension can create a generator's iterator as shown below:
<1D iterator>:
v = (x**2 for x in [0, 1, 2, 3, 4, 5, 6, 7])
print(v) # <generator object <genexpr> at 0x0000020E06F9F440>
print(type(v)) # <class 'generator'>
for x in v:
print(x)
# 0
# 1
# 4
# 9
# 16
# 25
# 36
# 49
<2D iterator>:
sample = [[0, 1, 2, 3], [4, 5, 6, 7]]
v = ((y**2 for y in x) for x in sample)
for x in v:
for y in x:
print(y)
# 0
# 1
# 4
# 9
# 16
# 25
# 36
# 49
<3D iterator>:
sample = [[[0, 1], [2, 3]], [[4, 5], [6, 7]]]
v = (((z**2 for z in y) for y in x) for x in sample)
for x in v:
for y in x:
for z in y:
print(z)
# 0
# 1
# 4
# 9
# 16
# 25
# 36
# 49
Even a big iterator doesn't get MemoryError as shown below:
v = iter(range(100000000))
print(next(v)) # 0
print(next(v)) # 1
print(next(v)) # 2
print(next(v)) # 3
print(next(v)) # 4
# ...
v = (x for x in range(100000000))
print(next(v)) # 0
print(next(v)) # 1
print(next(v)) # 2
print(next(v)) # 3
print(next(v)) # 4
# ...
An iterator cannot be read by indexing and slicing and changed by indexing, slicing and a del statement as shown below:
*Memo:
- A
delstatement cannot remove zero or more elements from an iterator by indexing and slicing but can remove one or more variables themselves.
v = iter(['a', 'b', 'c', 'd', 'e', 'f'])
print(v[1], v[3:5])
print(v[-5], v[-3:-1])
# TypeError: 'list_iterator' object is not subscriptable
v = iter(['a', 'b', 'c', 'd', 'e', 'f'])
v[1] = 'X'
v[-5] = 'X'
v[3:5] = ['Y', 'Z']
v[-3:-1] = ['Y', 'Z']
v[1], v[3:5] = 'X', ['Y', 'Z']
v[-5], v[-3:-1] = 'X', ['Y', 'Z']
# TypeError: 'list_iterator' object does not support item assignment
v = iter(['a', 'b', 'c', 'd', 'e', 'f'])
del v[1], v[3:5]
del v[-5], v[-2:5]
# 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 an iterator by indexing and slicing or change an iterator by indexing, slicing and a del statement, use list() and iter() or __iter__() as shown below:
v = iter(['a', 'b', 'c', 'd', 'e', 'f'])
v = list(v)
print(v[1], v[3:5])
print(v[-5], v[-3:-1])
# b ['d', 'e']
v[1] = 'X'
v[-5] = 'X'
v[3:5] = ['Y', 'Z']
v[-3:-1] = ['Y', 'Z']
v[1], v[3:5] = 'X', ['Y', 'Z']
v[-5], v[-3:-1] = 'X', ['Y', 'Z']
v = iter(v)
v = v.__iter__()
for x in v:
print(x)
# a
# X
# c
# Y
# Z
# f
v = iter(['a', 'b', 'c', 'd', 'e', 'f'])
v = list(v)
del v[1], v[3:5]
# del v[-5], v[-2:5]
v = iter(v)
v = v.__iter__()
for x in v:
print(x)
# a
# c
# d
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