added chapter map to README
This commit is contained in:
Luciano Ramalho
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18-context-mngr/README.rst
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4
18-context-mngr/README.rst
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Sample code for Chapter 15 - "Context managers and something else"
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From the book "Fluent Python" by Luciano Ramalho (O'Reilly, 2015)
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http://shop.oreilly.com/product/0636920032519.do
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92
18-context-mngr/mirror.py
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92
18-context-mngr/mirror.py
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"""
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A "mirroring" ``stdout`` context.
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While active, the context manager reverses text output to
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``stdout``::
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# BEGIN MIRROR_DEMO_1
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>>> from mirror import LookingGlass
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>>> with LookingGlass() as what: # <1>
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... print('Alice, Kitty and Snowdrop') # <2>
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... print(what)
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...
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pordwonS dna yttiK ,ecilA # <3>
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YKCOWREBBAJ
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>>> what # <4>
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'JABBERWOCKY'
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>>> print('Back to normal.') # <5>
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Back to normal.
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# END MIRROR_DEMO_1
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This exposes the context manager operation::
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# BEGIN MIRROR_DEMO_2
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>>> from mirror import LookingGlass
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>>> manager = LookingGlass() # <1>
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>>> manager
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<mirror.LookingGlass object at 0x2a578ac>
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>>> monster = manager.__enter__() # <2>
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>>> monster == 'JABBERWOCKY' # <3>
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eurT
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>>> monster
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'YKCOWREBBAJ'
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>>> manager
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>ca875a2x0 ta tcejbo ssalGgnikooL.rorrim<
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>>> manager.__exit__(None, None, None) # <4>
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>>> monster
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'JABBERWOCKY'
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# END MIRROR_DEMO_2
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The context manager can handle and "swallow" exceptions.
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# BEGIN MIRROR_DEMO_3
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>>> from mirror import LookingGlass
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>>> with LookingGlass():
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... print('Humpty Dumpty')
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... x = 1/0 # <1>
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... print('END') # <2>
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...
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ytpmuD ytpmuH
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Please DO NOT divide by zero!
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>>> with LookingGlass():
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... print('Humpty Dumpty')
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... x = no_such_name # <1>
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... print('END') # <2>
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...
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Traceback (most recent call last):
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...
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NameError: name 'no_such_name' is not defined
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# END MIRROR_DEMO_3
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"""
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# BEGIN MIRROR_EX
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class LookingGlass:
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def __enter__(self): # <1>
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import sys
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self.original_write = sys.stdout.write # <2>
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sys.stdout.write = self.reverse_write # <3>
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return 'JABBERWOCKY' # <4>
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def reverse_write(self, text): # <5>
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self.original_write(text[::-1])
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def __exit__(self, exc_type, exc_value, traceback): # <6>
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import sys # <7>
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sys.stdout.write = self.original_write # <8>
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if exc_type is ZeroDivisionError: # <9>
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print('Please DO NOT divide by zero!')
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return True # <10>
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# <11>
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# END MIRROR_EX
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64
18-context-mngr/mirror_gen.py
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64
18-context-mngr/mirror_gen.py
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"""
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A "mirroring" ``stdout`` context manager.
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While active, the context manager reverses text output to
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``stdout``::
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# BEGIN MIRROR_GEN_DEMO_1
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>>> from mirror_gen import looking_glass
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>>> with looking_glass() as what: # <1>
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... print('Alice, Kitty and Snowdrop')
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... print(what)
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...
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pordwonS dna yttiK ,ecilA
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YKCOWREBBAJ
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>>> what
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'JABBERWOCKY'
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# END MIRROR_GEN_DEMO_1
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This exposes the context manager operation::
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# BEGIN MIRROR_GEN_DEMO_2
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>>> from mirror_gen import looking_glass
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>>> manager = looking_glass() # <1>
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>>> manager # doctest: +ELLIPSIS
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<contextlib._GeneratorContextManager object at 0x...>
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>>> monster = manager.__enter__() # <2>
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>>> monster == 'JABBERWOCKY' # <3>
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eurT
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>>> monster
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'YKCOWREBBAJ'
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>>> manager # doctest: +ELLIPSIS
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>...x0 ta tcejbo reganaMtxetnoCrotareneG_.biltxetnoc<
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>>> manager.__exit__(None, None, None) # <4>
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>>> monster
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'JABBERWOCKY'
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# END MIRROR_GEN_DEMO_2
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"""
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# BEGIN MIRROR_GEN_EX
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import contextlib
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@contextlib.contextmanager # <1>
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def looking_glass():
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import sys
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original_write = sys.stdout.write # <2>
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def reverse_write(text): # <3>
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original_write(text[::-1])
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sys.stdout.write = reverse_write # <4>
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yield 'JABBERWOCKY' # <5>
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sys.stdout.write = original_write # <6>
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# END MIRROR_GEN_EX
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101
18-context-mngr/mirror_gen_exc.py
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101
18-context-mngr/mirror_gen_exc.py
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"""
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A "mirroring" ``stdout`` context manager.
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While active, the context manager reverses text output to
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``stdout``::
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# BEGIN MIRROR_GEN_DEMO_1
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>>> from mirror_gen import looking_glass
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>>> with looking_glass() as what: # <1>
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... print('Alice, Kitty and Snowdrop')
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... print(what)
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...
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pordwonS dna yttiK ,ecilA
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YKCOWREBBAJ
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>>> what
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'JABBERWOCKY'
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# END MIRROR_GEN_DEMO_1
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This exposes the context manager operation::
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# BEGIN MIRROR_GEN_DEMO_2
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>>> from mirror_gen import looking_glass
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>>> manager = looking_glass() # <1>
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>>> manager # doctest: +ELLIPSIS
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<contextlib._GeneratorContextManager object at 0x...>
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>>> monster = manager.__enter__() # <2>
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>>> monster == 'JABBERWOCKY' # <3>
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eurT
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>>> monster
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'YKCOWREBBAJ'
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>>> manager # doctest: +ELLIPSIS
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>...x0 ta tcejbo reganaMtxetnoCrotareneG_.biltxetnoc<
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>>> manager.__exit__(None, None, None) # <4>
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>>> monster
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'JABBERWOCKY'
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# END MIRROR_GEN_DEMO_2
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The context manager can handle and "swallow" exceptions.
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The following test does not pass under doctest (a
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ZeroDivisionError is reported by doctest) but passes
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if executed by hand in the Python 3 console (the exception
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is handled by the context manager):
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# BEGIN MIRROR_GEN_DEMO_3
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>>> from mirror_gen import looking_glass
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>>> with looking_glass():
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... print('Humpty Dumpty')
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... x = 1/0 # <1>
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... print('END') # <2>
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...
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ytpmuD ytpmuH
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Please DO NOT divide by zero!
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# END MIRROR_GEN_DEMO_3
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>>> with looking_glass():
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... print('Humpty Dumpty')
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... x = no_such_name # <1>
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... print('END') # <2>
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...
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Traceback (most recent call last):
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...
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NameError: name 'no_such_name' is not defined
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"""
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# BEGIN MIRROR_GEN_EXC
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import contextlib
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@contextlib.contextmanager
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def looking_glass():
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import sys
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original_write = sys.stdout.write
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def reverse_write(text):
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original_write(text[::-1])
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sys.stdout.write = reverse_write
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msg = '' # <1>
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try:
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yield 'JABBERWOCKY'
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except ZeroDivisionError: # <2>
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msg = 'Please DO NOT divide by zero!'
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finally:
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sys.stdout.write = original_write # <3>
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if msg:
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print(msg) # <4>
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# END MIRROR_GEN_EXC
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