Generator in Python (2)

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*Memo:

• My post explains a generator (1).
• My post explains a generator (3).
• My post explains a generator (4).

A generator comprehension can create a generator's iterator as shown below:

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

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

import copy

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

v1 = gen()

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 gen():
    yield 0
    yield 1
    yield 2
    yield 3

v = gen()

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

<yield from without a try statement>:

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

v = gen()

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

<yield with a try statement>:

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

v = gen()

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

<yield from with a try statement>:

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

v = gen()

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

v = gen()

print(next(v)) # 0
print(next(v)) # 1

v.close()

print(next(v)) # StopIteration:

<yield with a finally clause>:

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

v = gen()

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

<yield from without a finally clause>:

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

v = gen()

print(next(v)) # 0
print(next(v)) # 1

v.close()

print(next(v)) # StopIteration:

<yield from with a finally clause>:

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

v = gen()

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