Thanks for your reply.But the answer is not I except, I will show you some
examples to explain what result I except:
@contextmanagerdef cm(): print('open file') yield print('close
file')with cm(): 1/0
If I use a contextmanager ,I except it can help me to close the file
anytime,even raise an error,but if I define a function with @contextmanager
like the example which I have showed for you, it will never print('close file')
I can only modify it like this:@contextmanagerdef cm(): try:
print('open file') yield except Exception as e:
print('Error',e) finally: print('close file')
It is not friendly for us to use it, so I modify the contextlib to fix it,you
can catch it from the e-mail attachment.It's in the line 79 and line
97----------------------------------------------------------------------
发件人：[email protected]
收件人：[email protected]
主题：Python-ideas Digest, Vol 146, Issue 13
日期：2019年01月06日 01点05分
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Today's Topics:
1. Re: Make the @contextmanager of contextlib to be a real
contextmanager (Serhiy Storchaka)
2. Re: Fixed point format for numbers with locale based
separators (?ukasz Stelmach)
----------------------------------------------------------------------
Message: 1
Date: Sat, 5 Jan 2019 16:45:34 +0200
From: Serhiy Storchaka <[email protected]>
To: [email protected]
Subject: Re: [Python-ideas] Make the @contextmanager of contextlib to
be a real contextmanager
Message-ID: <[email protected]>
Content-Type: text/plain; charset=UTF-8; format=flowed
05.01.19 14:52, Moon?sun ????:
> As we know,when we import the module--'contextlib',we can use the
> decorator '@contextmanager' and keyword ?yield? to make a 'instance' of
> Class '_GeneratorContextManager' in 'contextlib' module,then we can use
> it like:
> with 'instance' as 'xx':
> ? ? 'code block'
> ? ? pass
> But there is a little bug,when the code block raise a error,the instance
> cannot run the code which after the keyword 'yield'.
This is not a bug.
Consider the following example:
@contextmanager
def cm():
try:
yield
except BaseException as err:
print('Fail:', err)
raise
else:
print('Success')
with cm():
1/0
What result would you expect? Test it with the stdlib implementation and
with your implementation.
------------------------------
Message: 2
Date: Sat, 05 Jan 2019 16:41:20 +0100
From: ?ukasz Stelmach <[email protected]>
To: Steven D'Aprano <[email protected]>
Cc: [email protected]
Subject: Re: [Python-ideas] Fixed point format for numbers with locale
based separators
Message-ID: <87y37z9knz.fsf%[email protected]>
Content-Type: text/plain; charset="utf-8"
Steven D'Aprano <[email protected]> writes:
> On Fri, Jan 04, 2019 at 03:57:53PM +0100, ?ukasz Stelmach wrote:
>> Hi,
>>
>> I would like to present two pull requests[1][2] implementing fixed point
>> presentation of numbers and ask for comments. The first is mine. I
>> learnt about the second after publishing mine.
>
> Before I look at the implementation, can you explain the functional
> requirements please?
As I stated in the original message below the table:
>> In the application I want to create I am going to present users numbers
>> ranging up to 3 orders of magnitude and I (my users) want them to be
>> presented consistently with regards to number of decimal digits AND I
>> want to conform to rules of languages of my users. And I would like to
>> avoid the exponent notation by all means.
The pint[1] library I use, implements formatting of physical quantities
using the format()/__format__ code. As far as I can tell my patch for
Python is shorter and more straightforward than a patch for pint to use
locale.format().
Because the "g" based "n" formatter has been present since the advanced
string formatting was described in PEP-3101, I think it is necessary to
add the "m" formatter based on "f". The advanced string formatting
facility in Python is very convenient and programmers shouldn't forced
to use locale.format() like this
"The total length of {} sticks is {} meters.".format(n_sticks,
locale.format(".2f", l_sticks))
instead of
"The total length of {} sticks is {:.2f} meters.".format(n_sticks, l_sticks)
> In other words, what is the new feature you hope to have excepted?
> Explain the intention and the API (the interface). The implementation is
> the least important part :-)
I wish to add a new formatter "m" for float/complex/decimal numbers,
which behaves like the existing "f", but uses the decimal separator from
the locale database. There is "n" formmatter which behaves like "g" but
it does not fit my needs.
> [...]
>> Formatting 1.23456789 * n (LC_ALL=3Dpl_PL.UTF-8)
>> | n | ".2f" | ".3n" |
>> |---+----------+----------|
>> | 1 | 1.23 | 1,23 |
>> | 2 | 12.35 | 12,3 |
>> | 3 | 123.46 | 123 |
>> | 4 | 1234.57 | 1,23e+03 |
>
> I'm afraid I cannot work out what that table means. You say "Formatting
> 1.23... * n" (multiplying by n) but the results shown aren't multiplied
> by n=2, n=3, n=4 as the table suggests.
>
> Can you show what Python code you expect will produce the expected
> output?
for n in range(1,5): print("| {} | {:8.2f} | {:8.3n} |".format(n,1.23456789
* 10**n, 1.23456789 * 10**n))
[1] http://pint.readthedocs.io/
--
By?o mi bardzo mi?o. --- Rurku. --- ...
>?ukasz< --- To dobrze, ?e mnie s?uchasz.
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"""Utilities for with-statement contexts. See PEP 343."""
import abc
import sys
import _collections_abc
from collections import deque
from functools import wraps
__all__ = ["contextmanager", "closing", "AbstractContextManager",
"ContextDecorator", "ExitStack", "redirect_stdout",
"redirect_stderr", "suppress"]
class AbstractContextManager(abc.ABC):
"""An abstract base class for context managers."""
def __enter__(self):
"""Return `self` upon entering the runtime context."""
return self
@abc.abstractmethod
def __exit__(self, exc_type, exc_value, traceback):
"""Raise any exception triggered within the runtime context."""
return None
@classmethod
def __subclasshook__(cls, C):
if cls is AbstractContextManager:
return _collections_abc._check_methods(C, "__enter__", "__exit__")
return NotImplemented
class ContextDecorator(object):
"A base class or mixin that enables context managers to work as decorators."
def _recreate_cm(self):
"""Return a recreated instance of self.
Allows an otherwise one-shot context manager like
_GeneratorContextManager to support use as
a decorator via implicit recreation.
This is a private interface just for _GeneratorContextManager.
See issue #11647 for details.
"""
return self
def __call__(self, func):
@wraps(func)
def inner(*args, **kwds):
with self._recreate_cm():
return func(*args, **kwds)
return inner
class _GeneratorContextManager(ContextDecorator, AbstractContextManager):
"""Helper for @contextmanager decorator."""
def __init__(self, func, args, kwds):
self.gen = func(*args, **kwds)
self.func, self.args, self.kwds = func, args, kwds
# Issue 19330: ensure context manager instances have good docstrings
doc = getattr(func, "__doc__", None)
if doc is None:
doc = type(self).__doc__
self.__doc__ = doc
# Unfortunately, this still doesn't provide good help output when
# inspecting the created context manager instances, since pydoc
# currently bypasses the instance docstring and shows the docstring
# for the class instead.
# See http://bugs.python.org/issue19404 for more details.
def _recreate_cm(self):
# _GCM instances are one-shot context managers, so the
# CM must be recreated each time a decorated function is
# called
return self.__class__(self.func, self.args, self.kwds)
def _next(self):
# éè¿åå ä¸å±try,å¯ä»¥æè·next()å¨æ§è¡å®ä»£ç åæåºçStopIterationå¼å¸¸
try:
# éè¿æ·»å æ¤å¤ä»£ç ,å³å¯å®ç°è¿ç¨ä¸åºé®é¢æ æ³å
³éæ件çæ
åµ
next(self.gen)
except StopIteration:
return False
else:
raise RuntimeError("generator didn't stop")
def __enter__(self):
try:
return next(self.gen)
except StopIteration:
raise RuntimeError("generator didn't yield") from None
def __exit__(self, type, value, traceback):
if type is None:
self._next()
else:
# æ 论æ¯å¦ä¼ éäºé误ä¾ç¶æ§è¡next()æä½æ§è¡yieldåé¢çè¯å¥
try:
self._next()
finally:
if value is None:
# Need to force instantiation so we can reliably
# tell if we get the same exception back
value = type()
try:
self.gen.throw(type, value, traceback)
except StopIteration as exc:
# Suppress StopIteration *unless* it's the same exception that
# was passed to throw(). This prevents a StopIteration
# raised inside the "with" statement from being suppressed.
return exc is not value
except RuntimeError as exc:
# Don't re-raise the passed in exception. (issue27122)
if exc is value:
return False
# Likewise, avoid suppressing if a StopIteration exception
# was passed to throw() and later wrapped into a RuntimeError
# (see PEP 479).
if type is StopIteration and exc.__cause__ is value:
return False
raise
except:
# only re-raise if it's *not* the exception that was
# passed to throw(), because __exit__() must not raise
# an exception unless __exit__() itself failed. But throw()
# has to raise the exception to signal propagation, so this
# fixes the impedance mismatch between the throw() protocol
# and the __exit__() protocol.
#
if sys.exc_info()[1] is value:
return False
raise
raise RuntimeError("generator didn't stop after throw()")
def contextmanager(func):
"""@contextmanager decorator.
Typical usage:
@contextmanager
def some_generator(<arguments>):
<setup>
try:
yield <value>
finally:
<cleanup>
This makes this:
with some_generator(<arguments>) as <variable>:
<body>
equivalent to this:
<setup>
try:
<variable> = <value>
<body>
finally:
<cleanup>
"""
@wraps(func)
def helper(*args, **kwds):
return _GeneratorContextManager(func, args, kwds)
return helper
class closing(AbstractContextManager):
"""Context to automatically close something at the end of a block.
Code like this:
with closing(<module>.open(<arguments>)) as f:
<block>
is equivalent to this:
f = <module>.open(<arguments>)
try:
<block>
finally:
f.close()
"""
def __init__(self, thing):
self.thing = thing
def __enter__(self):
return self.thing
def __exit__(self, *exc_info):
self.thing.close()
class _RedirectStream(AbstractContextManager):
_stream = None
def __init__(self, new_target):
self._new_target = new_target
# We use a list of old targets to make this CM re-entrant
self._old_targets = []
def __enter__(self):
self._old_targets.append(getattr(sys, self._stream))
setattr(sys, self._stream, self._new_target)
return self._new_target
def __exit__(self, exctype, excinst, exctb):
setattr(sys, self._stream, self._old_targets.pop())
class redirect_stdout(_RedirectStream):
"""Context manager for temporarily redirecting stdout to another file.
# How to send help() to stderr
with redirect_stdout(sys.stderr):
help(dir)
# How to write help() to a file
with open('help.txt', 'w') as f:
with redirect_stdout(f):
help(pow)
"""
_stream = "stdout"
class redirect_stderr(_RedirectStream):
"""Context manager for temporarily redirecting stderr to another file."""
_stream = "stderr"
class suppress(AbstractContextManager):
"""Context manager to suppress specified exceptions
After the exception is suppressed, execution proceeds with the next
statement following the with statement.
with suppress(FileNotFoundError):
os.remove(somefile)
# Execution still resumes here if the file was already removed
"""
def __init__(self, *exceptions):
self._exceptions = exceptions
def __enter__(self):
pass
def __exit__(self, exctype, excinst, exctb):
# Unlike isinstance and issubclass, CPython exception handling
# currently only looks at the concrete type hierarchy (ignoring
# the instance and subclass checking hooks). While Guido considers
# that a bug rather than a feature, it's a fairly hard one to fix
# due to various internal implementation details. suppress provides
# the simpler issubclass based semantics, rather than trying to
# exactly reproduce the limitations of the CPython interpreter.
#
# See http://bugs.python.org/issue12029 for more details
return exctype is not None and issubclass(exctype, self._exceptions)
# Inspired by discussions on http://bugs.python.org/issue13585
class ExitStack(AbstractContextManager):
"""Context manager for dynamic management of a stack of exit callbacks
For example:
with ExitStack() as stack:
files = [stack.enter_context(open(fname)) for fname in filenames]
# All opened files will automatically be closed at the end of
# the with statement, even if attempts to open files later
# in the list raise an exception
"""
def __init__(self):
self._exit_callbacks = deque()
def pop_all(self):
"""Preserve the context stack by transferring it to a new instance"""
new_stack = type(self)()
new_stack._exit_callbacks = self._exit_callbacks
self._exit_callbacks = deque()
return new_stack
def _push_cm_exit(self, cm, cm_exit):
"""Helper to correctly register callbacks to __exit__ methods"""
def _exit_wrapper(*exc_details):
return cm_exit(cm, *exc_details)
_exit_wrapper.__self__ = cm
self.push(_exit_wrapper)
def push(self, exit):
"""Registers a callback with the standard __exit__ method signature
Can suppress exceptions the same way __exit__ methods can.
Also accepts any object with an __exit__ method (registering a call
to the method instead of the object itself)
"""
# We use an unbound method rather than a bound method to follow
# the standard lookup behaviour for special methods
_cb_type = type(exit)
try:
exit_method = _cb_type.__exit__
except AttributeError:
# Not a context manager, so assume its a callable
self._exit_callbacks.append(exit)
else:
self._push_cm_exit(exit, exit_method)
return exit # Allow use as a decorator
def callback(self, callback, *args, **kwds):
"""Registers an arbitrary callback and arguments.
Cannot suppress exceptions.
"""
def _exit_wrapper(exc_type, exc, tb):
callback(*args, **kwds)
# We changed the signature, so using @wraps is not appropriate, but
# setting __wrapped__ may still help with introspection
_exit_wrapper.__wrapped__ = callback
self.push(_exit_wrapper)
return callback # Allow use as a decorator
def enter_context(self, cm):
"""Enters the supplied context manager
If successful, also pushes its __exit__ method as a callback and
returns the result of the __enter__ method.
"""
# We look up the special methods on the type to match the with statement
_cm_type = type(cm)
_exit = _cm_type.__exit__
result = _cm_type.__enter__(cm)
self._push_cm_exit(cm, _exit)
return result
def close(self):
"""Immediately unwind the context stack"""
self.__exit__(None, None, None)
def __exit__(self, *exc_details):
received_exc = exc_details[0] is not None
# We manipulate the exception state so it behaves as though
# we were actually nesting multiple with statements
frame_exc = sys.exc_info()[1]
def _fix_exception_context(new_exc, old_exc):
# Context may not be correct, so find the end of the chain
while 1:
exc_context = new_exc.__context__
if exc_context is old_exc:
# Context is already set correctly (see issue 20317)
return
if exc_context is None or exc_context is frame_exc:
break
new_exc = exc_context
# Change the end of the chain to point to the exception
# we expect it to reference
new_exc.__context__ = old_exc
# Callbacks are invoked in LIFO order to match the behaviour of
# nested context managers
suppressed_exc = False
pending_raise = False
while self._exit_callbacks:
cb = self._exit_callbacks.pop()
try:
if cb(*exc_details):
suppressed_exc = True
pending_raise = False
exc_details = (None, None, None)
except:
new_exc_details = sys.exc_info()
# simulate the stack of exceptions by setting the context
_fix_exception_context(new_exc_details[1], exc_details[1])
pending_raise = True
exc_details = new_exc_details
if pending_raise:
try:
# bare "raise exc_details[1]" replaces our carefully
# set-up context
fixed_ctx = exc_details[1].__context__
raise exc_details[1]
except BaseException:
exc_details[1].__context__ = fixed_ctx
raise
return received_exc and suppressed_exc
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