Iterator in Python (3)

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*Memo:

• My post explains an iterator (1).
• My post explains an iterator (2) and the iterator with copy and sorted().
• My post explains a class-based iterator with __iter__() and/or __next__().
• My post explains a generator comprehension.

A generator:

• is the function with one or more yield statements:
  • A yield statement is a yield or yield from.
• can return an iterator.
• 's iterator can be created by a generator comprehension:
  • A generator comprehension is an expression.
• terminates if there is no element to return, if close() is called or of course if error occurs.
• 's iterator cannot be copied.

A generator can be created with a function and one or more yield statements and read with next() as shown below:

*Memo:

• __next__() can also be used to read a generator.
• A yield statements is a yield or yield from.
• A yield can return any type of element.
• A yield from can only return an iterable.

<yield>:

def func():
    yield 0
    yield 1
    yield 2

print(func)       # <function func at 0x000001FCD2FF93A0>
print(type(func)) # <class 'function'>

gen = func()

print(gen)       # <generator object func at 0x000001FCD3015220>
print(type(gen)) # <class 'generator'>

print(next(gen)) # 0
print(next(gen)) # 1
print(next(gen)) # 2
print(next(gen)) # StopIteration:

def func():
    yield [0, 1, 2]
    yield [3, 4, 5]

gen = func()

print(next(gen)) # [0, 1, 2]
print(next(gen)) # [3, 4, 5]
print(next(gen)) # StopIteration:

<yield from>:

def func():
    yield from [0, 1, 2]
    yield from [3, 4, 5]

print(func)       # <function func at 0x000001FCD640B1A0>
print(type(func)) # <class 'function'>

gen = func()

print(gen)       # <generator object func at 0x000001FCD661DD80>
print(type(gen)) # <class 'generator'>

print(next(gen)) # 0
print(next(gen)) # 1
print(next(gen)) # 2
print(next(gen)) # 3
print(next(gen)) # 4
print(next(gen)) # 5
print(next(gen)) # StopIteration:

def func():
    yield from 0
    yield from 1
    yield from 2

gen = func()

print(next(gen))
# TypeError: 'int' object is not iterable

<yield & yield from>:

def func():
    yield [0, 1, 2]
    yield from [3, 4, 5]

gen = func()

print(next(gen)) # [0, 1, 2]
print(next(gen)) # 3
print(next(gen)) # 4
print(next(gen)) # 5
print(next(gen)) # StopIteration:

This is how a generator works as shown below:

*Memo:

• next() starts or resumes a generator, executes a yield statement to return a value and pauses the generator at a yield statement or raises StopIteration if a generator is terminated:
• A generator is always paused at a yield statement.
• __next__() also does the same things.

<yield>:

def func():
    print("func() starts.")
    yield "func() pauses."

    print("func() resumes.")
    yield "func() pauses again."

    print("func() resumes again.")
    yield "func() terminates."

gen = func() # `func()` doesn't start yet.

print(next(gen))
# func() starts.
# func() pauses.

print(next(gen))
# func() resumes.
# func() pauses again.

print(next(gen))
# func() resumes again.
# func() terminates.

print(next(gen)) # StopIteration:

<yield from>:

def func():
    print("func() starts.")
    yield from ["func() pauses.",
                "func() resumes and pauses."]

    print("func() resumes.")
    yield from ["func() pauses.",
                "func() resumes and terminates."]

gen = func() # `func()` doesn't start yet.

print(next(gen))
# func() starts.
# func() pauses.

print(next(gen))
# func() resumes and pauses.

print(next(gen))
# func() resumes.
# func() pauses.

print(next(gen))
# func() resumes and terminates.

print(next(gen)) # StopIteration: 

This is the generator with a for statement as shown below:

<yield>:

def func():
    for x in [0, 1, 2]:
        yield x

gen = func()

print(next(gen)) # 0
print(next(gen)) # 1
print(next(gen)) # 2
print(next(gen)) # StopIteration:

def func():
    for x in [[0, 1, 2], [3, 4, 5]]:
        yield x

gen = func()

print(next(gen)) # [0, 1, 2]
print(next(gen)) # [3, 4, 5]
print(next(gen)) # StopIteration:

<yield from>:

def func():
    for x in [[0, 1, 2], [3, 4, 5]]:
        yield from x

gen = func()

print(next(gen)) # 0
print(next(gen)) # 1
print(next(gen)) # 2
print(next(gen)) # 3
print(next(gen)) # 4
print(next(gen)) # 5
print(next(gen)) # StopIteration:

def func():
    for x in [0, 1, 2]:
        yield from x

gen = func()

print(next(gen))
# TypeError: 'int' object is not iterable

<yield & yield from>:

def func():
    for x in [[0, 1, 2], [3, 4, 5]]:
        yield x
        yield from x

gen = func()

print(next(gen)) # [0, 1, 2]
print(next(gen)) # 0
print(next(gen)) # 1
print(next(gen)) # 2
print(next(gen)) # [3, 4, 5]
print(next(gen)) # 3
print(next(gen)) # 4
print(next(gen)) # 5
print(next(gen)) # StopIteration:

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])

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

A generator's iterator cannot be copied as shown below:

import copy

def func():
    yield 0
    yield 1
    yield 2
    # yield from [0, 1, 2]

v1 = func()

v2 = copy.copy(v1)
v2 = copy.deepcopy(v1)
# TypeError: cannot pickle 'generator' object

throw() can raise an exception at the point where the generator is paused as shown below:

*Memo:

• The 1st argument is value(Required-Type:BaseException):
  • Don't use value=.

<yield without a try statement>:

def func():
    yield 0
    yield 1
    yield 2
    yield 3

gen = func()

print(next(gen))            # 0
print(next(gen))            # 1
print(gen.throw(Exception)) # Exception:

<yield from without a try statement>:

def func():
    yield from [0, 1, 2, 3]

gen = func()

print(next(gen))            # 0
print(next(gen))            # 1
print(gen.throw(Exception)) # Exception:

<yield with a try statement>:

def func():
    yield 0
    try:
        yield 1
    except Exception:
        pass
    yield 2
    yield 3

gen = func()

print(next(gen))            # 0
print(next(gen))            # 1
print(gen.throw(Exception)) # 2
print(next(gen))            # 3

<yield from with a try statement>:

def func():
    try:
        yield from [0, 1]
    except Exception:
        pass
    yield from [2, 3]

gen = func()

print(next(gen))            # 0
print(next(gen))            # 1
print(gen.throw(Exception)) # 2
print(next(gen))            # 3

close() can terminate a generator as shown below:

*Memo:

• It has no arguments.
• It should be used in a finally clause to terminate a generator for if error occurs.

<yield without a finally clause>:

def func():
    yield 0
    yield 1
    yield 2
    yield 3

gen = func()

print(next(gen)) # 0
print(next(gen)) # 1

gen.close()

print(next(gen)) # StopIteration:

<yield with a finally clause>:

def func():
    yield 0
    yield 1
    yield 2
    yield 3

gen = func()

try:
    print(next(gen)) # 0
    print(next(gen)) # 1
    print(next(gen)) # 2
    print(next(gen)) # 3
finally:
    gen.close()

<yield from without a finally clause>:

def func():
    yield from [0, 1, 2, 3]

gen = func()

print(next(gen)) # 0
print(next(gen)) # 1

gen.close()

print(next(gen)) # StopIteration:

<yield from with a finally clause>:

def func():
    yield from [0, 1, 2, 3]

gen = func()

try:
    print(next(gen)) # 0
    print(next(gen)) # 1
    print(next(gen)) # 2
    print(next(gen)) # 3
finally:
    gen.close()

gi_yieldfrom can return an iterator if the generator is resumed at yield from otherwise it returns None as shown below:

def func():
    yield from [0, 1]
    yield 2
    yield from [3, 4]
    yield 5

gen = func()

while True:
    try:
        print(gen.gi_yieldfrom, next(gen.gi_yieldfrom))
    except:
        print(gen.gi_yieldfrom, next(gen))
# None 0
# <list_iterator object at 0x000001C66EBFE410> 1
# <list_iterator object at 0x000001C66EBFE410> 2
# None 3
# <list_iterator object at 0x000001C66EBFE410> 4
# <list_iterator object at 0x000001C66EBFE410> 5

gi_running can check if the generator is currently running as shown below:

<yield>:

def func():
    print(gen.gi_running, "func") # True func
    yield 0
    print(gen.gi_running, "func") # True func
    yield 1
    print(gen.gi_running, "func") # True func
    yield 2
    print(gen.gi_running, "func") # True func

gen = func()

print(gen.gi_running) # False
print(next(gen))      # 0
print(gen.gi_running) # False
print(next(gen))      # 1
print(gen.gi_running) # False
print(next(gen))      # 2
print(gen.gi_running) # False
print(next(gen))      # StopIteration:

<yield from>:

def func():
    print(gen.gi_running, "func") # True func
    yield from [0, 1]
    print(gen.gi_running, "func") # True func
    yield from [2, 3]
    print(gen.gi_running, "func") # True func
    yield from [4, 5]
    print(gen.gi_running, "func") # True func

gen = func()

print(gen.gi_running) # False
print(next(gen))      # 0
print(gen.gi_running) # False
print(next(gen))      # 1
print(gen.gi_running) # False
print(next(gen))      # 2
print(gen.gi_running) # False
print(next(gen))      # 3
print(gen.gi_running) # False
print(next(gen))      # 4
print(gen.gi_running) # False
print(next(gen))      # 5
print(gen.gi_running) # False
print(next(gen))      # StopIteration:

gi_suspended can check if the generator is currently suspended(paused) as shown below:

<yield>:

def func():
    print(gen.gi_suspended, "func") # False func
    yield 0
    print(gen.gi_suspended, "func") # False func
    yield 1
    print(gen.gi_suspended, "func") # False func
    yield 2
    print(gen.gi_suspended, "func") # False func

gen = func()

print(gen.gi_suspended) # False
print(next(gen))        # 0
print(gen.gi_suspended) # True
print(next(gen))        # 1
print(gen.gi_suspended) # True
print(next(gen))        # 2
print(gen.gi_suspended) # True
print(next(gen))        # StopIteration:

<yield from>:

def func():
    print(gen.gi_suspended, "func") # False func
    yield from [0, 1]
    print(gen.gi_suspended, "func") # False func
    yield from [2, 3]
    print(gen.gi_suspended, "func") # False func
    yield from [4, 5]
    print(gen.gi_suspended, "func") # False func

gen = func()

print(gen.gi_suspended) # False
print(next(gen))        # 0
print(gen.gi_suspended) # True
print(next(gen))        # 1
print(gen.gi_suspended) # True
print(next(gen))        # 2
print(gen.gi_suspended) # True
print(next(gen))        # 3
print(gen.gi_suspended) # True
print(next(gen))        # 4
print(gen.gi_suspended) # True
print(next(gen))        # 5
print(gen.gi_suspended) # True
print(next(gen))        # StopIteration:

A yield statement can be assigned to a variable to be used with or without send() as shown below:

*Memo:

• send() starts or resumes the generator, sends a value into the variable assigned a yield statement only when resuming the generator, executes a yield statement to return a value and pauses the generator or raises StopIteration if the generator is terminated:
  • The 1st argument is value(Required-Type:Any):
    • It must be None when starting the generator.
    • Don't use value=.
• The variable assigned a yield statement has None by default.
• yield from only with a generator(yield from generator) works with send() properly.

<yield without send()>:

def func():
    v1 = yield 0
    print(v1, 'func')
    v2 = yield 1
    print(v2, 'func')
    v3 = yield 2
    print(v3, 'func')

gen = func()

print(next(gen)) # 0
print(next(gen)) # None func
                 # 1
print(next(gen)) # None func
                 # 2
print(next(gen)) # StopIteration:

<yield with send()>:

def func():
    v1 = yield 0
    print(v1, 'func')
    v2 = yield 1
    print(v2, 'func')
    v3 = yield 2
    print(v3, 'func')

gen = func()

print(gen.send(None)) # 0
print(gen.send('A'))  # A func
                      # 1
print(gen.send('B'))  # B func
                      # 2
print(gen.send('C'))  # C func
print(gen.send('D'))  # StopIteration:

def func():
    v1 = yield 0
    print(v1, 'func')
    v2 = yield 1
    print(v2, 'func')
    v3 = yield 2
    print(v3, 'func')

gen = func()

print(gen.send('A'))
# TypeError: can't send non-None value to a just-started generator

<yield from without send()>:

def func():
    v1 = yield from [0, 1]
    print(v1, 'func')
    v2 = yield from [2, 3]
    print(v2, 'func')
    v3 = yield from [4, 5]
    print(v3, 'func')

gen = func()

print(next(gen)) # 0
print(next(gen)) # 1
                 # None func
print(next(gen)) # 2
print(next(gen)) # 3
                 # None func
print(next(gen)) # 4
print(next(gen)) # 5
                 # None func
print(next(gen)) # StopIteration:

<yield from with send()>:

def sub_func():
    v1 = yield 0
    print(v1, 'sub_func')
    v2 = yield 1
    print(v2, 'sub_func')
    v3 = yield 2
    print(v3, 'sub_func')

def func():
    v = yield from sub_func()
    print(v, 'func')

gen = func()

print(gen.send(None)) # 0
print(gen.send('A'))  # A sub_func
                      # 1
print(gen.send('B'))  # B sub_func
                      # 2
print(gen.send('C'))  # C sub_func
                      # None func
                      # StopIteration:

def func():
    v1 = yield from [0, 1]
    print(v1, 'func')
    v2 = yield from [2, 3]
    print(v2, 'func')
    v3 = yield from [4, 5]
    print(v3, 'func')

gen = func()

print(gen.send('A'))
# TypeError: can't send non-None value to a just-started generator

def func():
    v1 = yield from [0, 1]
    print(v1, 'func')
    v2 = yield from [2, 3]
    print(v2, 'func')
    v3 = yield from [4, 5]
    print(v3, 'func')

gen = func()

print(gen.send(None)) # 0
print(gen.send('A'))
# AttributeError: 'list_iterator' object has no attribute 'send'

A yield statement cannot be assigned to a for statement as shown below:

def func():
    for v in yield [0, 1, 2]:
    # for v in yield from [0, 1, 2]:
        print(v)
# SyntaxError: invalid syntax