contextlib
— Utilitários para contextos da instrução with
¶
Código-fonte: Lib/contextlib.py
Este módulo fornece utilitários para tarefas comuns envolvendo a instrução with
. Para mais informações, veja também Tipos de Gerenciador de Contexto e Gerenciadores de contexto da instrução with.
Utilitários¶
Funções e classes fornecidas:
- class contextlib.AbstractContextManager¶
Uma classe base abstrata para classes que implementam
object.__enter__()
eobject.__exit__()
. Uma implementação padrão paraobject.__enter__()
é fornecida, que retornaself
, enquantoobject.__exit__()
é um método abstrato que, por padrão, retornaNone
. Veja também a definição de Tipos de Gerenciador de Contexto.Adicionado na versão 3.6.
- class contextlib.AbstractAsyncContextManager¶
Uma classe base abstrata para classes que implementam
object.__aenter__()
eobject.__aexit__()
. Uma implementação padrão paraobject.__aenter__()
é fornecida, que retornaself
, enquantoobject.__aexit__()
é um método abstrato que, por padrão, retornaNone
. Veja também a definição de Gerenciadores de contexto assíncronosAdicionado na versão 3.7.
- @contextlib.contextmanager¶
Esta função é um decorador que pode ser usado para definir uma função de fábrica para gerenciadores de contexto de instrução
with
, sem precisar criar uma classe ou separar os métodos__enter__()
e__exit__()
.Embora muitos objetos ofereçam suporte nativo ao uso em instruções with, às vezes é necessário gerenciar um recurso que não seja um gerenciador de contexto por si só e não implemente um método
close()
para uso comcontextlib.closing
Um exemplo abstrato seria o seguinte para garantir o gerenciamento correto dos recursos:
from contextlib import contextmanager @contextmanager def managed_resource(*args, **kwds): # Códigopara obter recursos como, por exemplo: resource = acquire_resource(*args, **kwds) try: yield resource finally: # Código para liberar recursos como, por exemplo: release_resource(resource)
A função pode, então, ser usada da seguinte forma:
>>> with managed_resource(timeout=3600) as resource: ... # O recurso é liberado no final deste bloco, ... # mesmo que o código no bloco levante uma exceção
A função que está sendo decorada deve retornar um iterador gerador quando chamada. Este iterador deve produzir exatamente um valor, que será vinculado aos alvos na cláusula
as
da instruçãowith
, se houver.No ponto em que o gerador é produzido, o bloco aninhado na instrução
with
é executado. O gerador é então retomado após o bloco ser encerrado. Se uma exceção não tratada ocorrer no bloco, ela será levantada novamente dentro do gerador no ponto em que a produção ocorreu. Assim, você pode usar uma instruçãotry
…except
…finally
para capturar o erro (se houver), ou garantir que alguma limpeza ocorra. Se uma exceção for capturada apenas para registrá-la ou para executar alguma ação (em vez de suprimi-la completamente), o gerador deve levantar novamente essa exceção. Caso contrário, o gerenciador de contexto do gerador indicará à instruçãowith
que a exceção foi tratada, e a execução será retomada com a instrução imediatamente após a instruçãowith
.contextmanager()
usaContextDecorator
para que os gerenciadores de contexto que ela cria possam ser usados como decoradores, bem como em instruçõeswith
. Quando usada como um decorador, uma nova instância do gerador é implicitamente criada em cada chamada de função (isso permite que os gerenciadores de contexto criados porcontextmanager()
atendam ao requisito de que os gerenciadores de contexto ofereçam suporte a múltiplas invocações para serem usados como decoradores).Alterado na versão 3.2: Uso de
ContextDecorator
.
- @contextlib.asynccontextmanager¶
Semelhante a
contextmanager()
, mas cria um gerenciador de contexto assíncrono.Esta função é um decorador que pode ser usado para definir uma função de fábrica para gerenciadores de contexto assíncronos de instrução
async with
, sem precisar criar uma classe ou separar os métodos__aenter__()
e__aexit__()
. Ela deve ser aplicada a uma função que atua como gerador assíncrono.Um exemplo simples:
from contextlib import asynccontextmanager @asynccontextmanager async def get_connection(): conn = await acquire_db_connection() try: yield conn finally: await release_db_connection(conn) async def get_all_users(): async with get_connection() as conn: return conn.query('SELECT ...')
Adicionado na versão 3.7.
Gerenciadores de contexto definidos com
asynccontextmanager()
podem ser usados como decoradores ou com instruçõesasync with
:import time from contextlib import asynccontextmanager @asynccontextmanager async def timeit(): now = time.monotonic() try: yield finally: print(f'it took {time.monotonic() - now}s to run') @timeit() async def main(): # ... código do async ...
Quando usada como um decorador, uma nova instância do gerador é implicitamente criada em cada chamada de função. Isso permite que os gerenciadores de contexto criados por
asynccontextmanager()
atendam ao requisito de que os gerenciadores de contexto ofereçam suporte a múltiplas invocações para serem usados como decoradores.Alterado na versão 3.10: Gerenciadores de contexto assíncronos criados com
asynccontextmanager()
podem ser usados como decoradores.
- contextlib.closing(thing)¶
Retorna um gerenciador de contexto que fecha thing após a conclusão do bloco. Isso basicamente equivale a:
from contextlib import contextmanager @contextmanager def closing(thing): try: yield thing finally: thing.close()
E permite que você escreva código como isso:
from contextlib import closing from urllib.request import urlopen with closing(urlopen('https://www.python.org')) as page: for line in page: print(line)
sem precisar fechar explicitamente
page
. Mesmo se ocorrer um erro,page.close()
será chamado quando o blocowith
for encerrado.Nota
A maioria dos tipos que gerenciam recursos suporta o protocolo de gerenciador de contexto, que fecha thing ao sair da declaração
with
. Como tal,closing()
é mais útil para tipos de terceiros que não oferecem suporte a gerenciadores de contexto. Este exemplo é puramente para fins ilustrativos, poisurlopen()
normalmente seria usado em um gerenciador de contexto.
- contextlib.aclosing(thing)¶
Retorna um gerenciador de contexto async que chama o método
aclose()
de thing após a conclusão do bloco. Isso basicamente equivale a:from contextlib import asynccontextmanager @asynccontextmanager async def aclosing(thing): try: yield thing finally: await thing.aclose()
De forma significativa,
aclosing()
oferece suporte a limpeza determinística de geradores assíncronos quando eles são encerrados mais cedo porbreak
ou uma exceção. Por exemplo:from contextlib import aclosing async with aclosing(my_generator()) as values: async for value in values: if value == 42: break
Esse padrão garante que o código de saída assíncrono do gerador seja executado no mesmo contexto que suas iterações (para que exceções e variáveis de contexto funcionem conforme o esperado, e o código de saída não seja executado após o tempo de vida de alguma tarefa da qual ele depende).
Adicionado na versão 3.10.
- contextlib.nullcontext(enter_result=None)¶
Retorna um gerenciador de contexto que retorna enter_result de
__enter__
, mas não faz nada de outra forma. Ele foi criado para ser usado como um substituto para um gerenciador de contexto opcional, por exemplo:def myfunction(arg, ignore_exceptions=False): if ignore_exceptions: # Usa suppress para ignorar todas as exceções. cm = contextlib.suppress(Exception) else: # Não ignora quaisquer exceções, cm não tem efeito. cm = contextlib.nullcontext() with cm: # Faz algo
Um exemplo usando enter_result:
def process_file(file_or_path): if isinstance(file_or_path, str): # Se for uma string, abre o arquivo cm = open(file_or_path) else: # O chamador é responsável por fechar o arquivo cm = nullcontext(file_or_path) with cm as file: # Efetua um processamento no arquivo
Também pode ser usado como um substituto para gerenciadores de contexto assíncronos:
async def send_http(session=None): if not session: # Se houver nenhuma sessão http, cria-a com aiohttp cm = aiohttp.ClientSession() else: # O chamador é responsável por fechar a sessão cm = nullcontext(session) async with cm as session: # Envia requisições http com sessão
Adicionado na versão 3.7.
Alterado na versão 3.10: Suporte a gerenciador de contexto assíncrono foi adicionado.
- contextlib.suppress(*exceptions)¶
Retorna um gerenciador de contexto que suprime qualquer uma das exceções especificadas se elas ocorrerem no corpo de uma instrução
with
e então retoma a execução com a primeira instrução após o final da instruçãowith
.Como qualquer outro mecanismo que suprime completamente exceções, este gerenciador de contexto deve ser usado apenas para cobrir erros muito específicos, onde continuar silenciosamente com a execução do programa é considerado a coisa certa a fazer.
Por exemplo:
from contextlib import suppress with suppress(FileNotFoundError): os.remove('algumarquivo.tmp') with suppress(FileNotFoundError): os.remove('outroarquivo.tmp')
Este código equivale a:
try: os.remove('algumarquivo.tmp') except FileNotFoundError: pass try: os.remove('outroarquivo.tmp') except FileNotFoundError: pass
O gerenciador de contexto é reentrante.
Se o código dentro do bloco
with
levantar uma exceçãoBaseExceptionGroup
, exceções suprimidas são removidas do grupo. Quaisquer exceções do grupo que não forem suprimidas são levantadas novamente em um novo grupo que é criado usando o métododerive()
do grupo original.Adicionado na versão 3.4.
Alterado na versão 3.12:
suppress
now supports suppressing exceptions raised as part of aBaseExceptionGroup
.
- contextlib.redirect_stdout(new_target)¶
Context manager for temporarily redirecting
sys.stdout
to another file or file-like object.This tool adds flexibility to existing functions or classes whose output is hardwired to stdout.
For example, the output of
help()
normally is sent to sys.stdout. You can capture that output in a string by redirecting the output to anio.StringIO
object. The replacement stream is returned from the__enter__
method and so is available as the target of thewith
statement:with redirect_stdout(io.StringIO()) as f: help(pow) s = f.getvalue()
To send the output of
help()
to a file on disk, redirect the output to a regular file:with open('help.txt', 'w') as f: with redirect_stdout(f): help(pow)
To send the output of
help()
to sys.stderr:with redirect_stdout(sys.stderr): help(pow)
Note that the global side effect on
sys.stdout
means that this context manager is not suitable for use in library code and most threaded applications. It also has no effect on the output of subprocesses. However, it is still a useful approach for many utility scripts.O gerenciador de contexto é reentrante.
Adicionado na versão 3.4.
- contextlib.redirect_stderr(new_target)¶
Similar to
redirect_stdout()
but redirectingsys.stderr
to another file or file-like object.O gerenciador de contexto é reentrante.
Adicionado na versão 3.5.
- contextlib.chdir(path)¶
Non parallel-safe context manager to change the current working directory. As this changes a global state, the working directory, it is not suitable for use in most threaded or async contexts. It is also not suitable for most non-linear code execution, like generators, where the program execution is temporarily relinquished – unless explicitly desired, you should not yield when this context manager is active.
This is a simple wrapper around
chdir()
, it changes the current working directory upon entering and restores the old one on exit.O gerenciador de contexto é reentrante.
Adicionado na versão 3.11.
- class contextlib.ContextDecorator¶
A base class that enables a context manager to also be used as a decorator.
Context managers inheriting from
ContextDecorator
have to implement__enter__
and__exit__
as normal.__exit__
retains its optional exception handling even when used as a decorator.ContextDecorator
is used bycontextmanager()
, so you get this functionality automatically.Example of
ContextDecorator
:from contextlib import ContextDecorator class mycontext(ContextDecorator): def __enter__(self): print('Starting') return self def __exit__(self, *exc): print('Finishing') return False
The class can then be used like this:
>>> @mycontext() ... def function(): ... print('The bit in the middle') ... >>> function() Starting The bit in the middle Finishing >>> with mycontext(): ... print('The bit in the middle') ... Starting The bit in the middle Finishing
This change is just syntactic sugar for any construct of the following form:
def f(): with cm(): # Do stuff
ContextDecorator
lets you instead write:@cm() def f(): # Do stuff
It makes it clear that the
cm
applies to the whole function, rather than just a piece of it (and saving an indentation level is nice, too).Existing context managers that already have a base class can be extended by using
ContextDecorator
as a mixin class:from contextlib import ContextDecorator class mycontext(ContextBaseClass, ContextDecorator): def __enter__(self): return self def __exit__(self, *exc): return False
Nota
As the decorated function must be able to be called multiple times, the underlying context manager must support use in multiple
with
statements. If this is not the case, then the original construct with the explicitwith
statement inside the function should be used.Adicionado na versão 3.2.
- class contextlib.AsyncContextDecorator¶
Similar to
ContextDecorator
but only for asynchronous functions.Example of
AsyncContextDecorator
:from asyncio import run from contextlib import AsyncContextDecorator class mycontext(AsyncContextDecorator): async def __aenter__(self): print('Starting') return self async def __aexit__(self, *exc): print('Finishing') return False
The class can then be used like this:
>>> @mycontext() ... async def function(): ... print('The bit in the middle') ... >>> run(function()) Starting The bit in the middle Finishing >>> async def function(): ... async with mycontext(): ... print('The bit in the middle') ... >>> run(function()) Starting The bit in the middle Finishing
Adicionado na versão 3.10.
- class contextlib.ExitStack¶
A context manager that is designed to make it easy to programmatically combine other context managers and cleanup functions, especially those that are optional or otherwise driven by input data.
For example, a set of files may easily be handled in a single with statement as follows:
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
The
__enter__()
method returns theExitStack
instance, and performs no additional operations.Each instance maintains a stack of registered callbacks that are called in reverse order when the instance is closed (either explicitly or implicitly at the end of a
with
statement). Note that callbacks are not invoked implicitly when the context stack instance is garbage collected.This stack model is used so that context managers that acquire their resources in their
__init__
method (such as file objects) can be handled correctly.Since registered callbacks are invoked in the reverse order of registration, this ends up behaving as if multiple nested
with
statements had been used with the registered set of callbacks. This even extends to exception handling - if an inner callback suppresses or replaces an exception, then outer callbacks will be passed arguments based on that updated state.This is a relatively low level API that takes care of the details of correctly unwinding the stack of exit callbacks. It provides a suitable foundation for higher level context managers that manipulate the exit stack in application specific ways.
Adicionado na versão 3.3.
- enter_context(cm)¶
Enters a new context manager and adds its
__exit__()
method to the callback stack. The return value is the result of the context manager’s own__enter__()
method.These context managers may suppress exceptions just as they normally would if used directly as part of a
with
statement.Alterado na versão 3.11: Raises
TypeError
instead ofAttributeError
if cm is not a context manager.
- push(exit)¶
Adds a context manager’s
__exit__()
method to the callback stack.As
__enter__
is not invoked, this method can be used to cover part of an__enter__()
implementation with a context manager’s own__exit__()
method.If passed an object that is not a context manager, this method assumes it is a callback with the same signature as a context manager’s
__exit__()
method and adds it directly to the callback stack.By returning true values, these callbacks can suppress exceptions the same way context manager
__exit__()
methods can.The passed in object is returned from the function, allowing this method to be used as a function decorator.
- callback(callback, /, *args, **kwds)¶
Accepts an arbitrary callback function and arguments and adds it to the callback stack.
Unlike the other methods, callbacks added this way cannot suppress exceptions (as they are never passed the exception details).
The passed in callback is returned from the function, allowing this method to be used as a function decorator.
- pop_all()¶
Transfers the callback stack to a fresh
ExitStack
instance and returns it. No callbacks are invoked by this operation - instead, they will now be invoked when the new stack is closed (either explicitly or implicitly at the end of awith
statement).For example, a group of files can be opened as an “all or nothing” operation as follows:
with ExitStack() as stack: files = [stack.enter_context(open(fname)) for fname in filenames] # Hold onto the close method, but don't call it yet. close_files = stack.pop_all().close # If opening any file fails, all previously opened files will be # closed automatically. If all files are opened successfully, # they will remain open even after the with statement ends. # close_files() can then be invoked explicitly to close them all.
- close()¶
Immediately unwinds the callback stack, invoking callbacks in the reverse order of registration. For any context managers and exit callbacks registered, the arguments passed in will indicate that no exception occurred.
- class contextlib.AsyncExitStack¶
An asynchronous context manager, similar to
ExitStack
, that supports combining both synchronous and asynchronous context managers, as well as having coroutines for cleanup logic.The
close()
method is not implemented;aclose()
must be used instead.- coroutine enter_async_context(cm)¶
Similar to
ExitStack.enter_context()
but expects an asynchronous context manager.Alterado na versão 3.11: Raises
TypeError
instead ofAttributeError
if cm is not an asynchronous context manager.
- push_async_exit(exit)¶
Similar to
ExitStack.push()
but expects either an asynchronous context manager or a coroutine function.
- push_async_callback(callback, /, *args, **kwds)¶
Similar to
ExitStack.callback()
but expects a coroutine function.
- coroutine aclose()¶
Similar to
ExitStack.close()
but properly handles awaitables.
Continuing the example for
asynccontextmanager()
:async with AsyncExitStack() as stack: connections = [await stack.enter_async_context(get_connection()) for i in range(5)] # All opened connections will automatically be released at the end of # the async with statement, even if attempts to open a connection # later in the list raise an exception.
Adicionado na versão 3.7.
Exemplos e receitas¶
This section describes some examples and recipes for making effective use of
the tools provided by contextlib
.
Supporting a variable number of context managers¶
The primary use case for ExitStack
is the one given in the class
documentation: supporting a variable number of context managers and other
cleanup operations in a single with
statement. The variability
may come from the number of context managers needed being driven by user
input (such as opening a user specified collection of files), or from
some of the context managers being optional:
with ExitStack() as stack:
for resource in resources:
stack.enter_context(resource)
if need_special_resource():
special = acquire_special_resource()
stack.callback(release_special_resource, special)
# Perform operations that use the acquired resources
As shown, ExitStack
also makes it quite easy to use with
statements to manage arbitrary resources that don’t natively support the
context management protocol.
Catching exceptions from __enter__
methods¶
It is occasionally desirable to catch exceptions from an __enter__
method implementation, without inadvertently catching exceptions from
the with
statement body or the context manager’s __exit__
method. By using ExitStack
the steps in the context management
protocol can be separated slightly in order to allow this:
stack = ExitStack()
try:
x = stack.enter_context(cm)
except Exception:
# handle __enter__ exception
else:
with stack:
# Handle normal case
Actually needing to do this is likely to indicate that the underlying API
should be providing a direct resource management interface for use with
try
/except
/finally
statements, but not
all APIs are well designed in that regard. When a context manager is the
only resource management API provided, then ExitStack
can make it
easier to handle various situations that can’t be handled directly in a
with
statement.
Cleaning up in an __enter__
implementation¶
As noted in the documentation of ExitStack.push()
, this
method can be useful in cleaning up an already allocated resource if later
steps in the __enter__()
implementation fail.
Here’s an example of doing this for a context manager that accepts resource acquisition and release functions, along with an optional validation function, and maps them to the context management protocol:
from contextlib import contextmanager, AbstractContextManager, ExitStack
class ResourceManager(AbstractContextManager):
def __init__(self, acquire_resource, release_resource, check_resource_ok=None):
self.acquire_resource = acquire_resource
self.release_resource = release_resource
if check_resource_ok is None:
def check_resource_ok(resource):
return True
self.check_resource_ok = check_resource_ok
@contextmanager
def _cleanup_on_error(self):
with ExitStack() as stack:
stack.push(self)
yield
# The validation check passed and didn't raise an exception
# Accordingly, we want to keep the resource, and pass it
# back to our caller
stack.pop_all()
def __enter__(self):
resource = self.acquire_resource()
with self._cleanup_on_error():
if not self.check_resource_ok(resource):
msg = "Failed validation for {!r}"
raise RuntimeError(msg.format(resource))
return resource
def __exit__(self, *exc_details):
# We don't need to duplicate any of our resource release logic
self.release_resource()
Replacing any use of try-finally
and flag variables¶
A pattern you will sometimes see is a try-finally
statement with a flag
variable to indicate whether or not the body of the finally
clause should
be executed. In its simplest form (that can’t already be handled just by
using an except
clause instead), it looks something like this:
cleanup_needed = True
try:
result = perform_operation()
if result:
cleanup_needed = False
finally:
if cleanup_needed:
cleanup_resources()
As with any try
statement based code, this can cause problems for
development and review, because the setup code and the cleanup code can end
up being separated by arbitrarily long sections of code.
ExitStack
makes it possible to instead register a callback for
execution at the end of a with
statement, and then later decide to skip
executing that callback:
from contextlib import ExitStack
with ExitStack() as stack:
stack.callback(cleanup_resources)
result = perform_operation()
if result:
stack.pop_all()
This allows the intended cleanup behaviour to be made explicit up front, rather than requiring a separate flag variable.
If a particular application uses this pattern a lot, it can be simplified even further by means of a small helper class:
from contextlib import ExitStack
class Callback(ExitStack):
def __init__(self, callback, /, *args, **kwds):
super().__init__()
self.callback(callback, *args, **kwds)
def cancel(self):
self.pop_all()
with Callback(cleanup_resources) as cb:
result = perform_operation()
if result:
cb.cancel()
If the resource cleanup isn’t already neatly bundled into a standalone
function, then it is still possible to use the decorator form of
ExitStack.callback()
to declare the resource cleanup in
advance:
from contextlib import ExitStack
with ExitStack() as stack:
@stack.callback
def cleanup_resources():
...
result = perform_operation()
if result:
stack.pop_all()
Devido à maneira como o protocolo decorador funciona, uma função de retorno de chamada declarada dessa forma não pode receber nenhum parâmetro. Em vez disso, quaisquer recursos a serem liberados devem ser acessados como variáveis de clausura.
Using a context manager as a function decorator¶
ContextDecorator
makes it possible to use a context manager in
both an ordinary with
statement and also as a function decorator.
For example, it is sometimes useful to wrap functions or groups of statements
with a logger that can track the time of entry and time of exit. Rather than
writing both a function decorator and a context manager for the task,
inheriting from ContextDecorator
provides both capabilities in a
single definition:
from contextlib import ContextDecorator
import logging
logging.basicConfig(level=logging.INFO)
class track_entry_and_exit(ContextDecorator):
def __init__(self, name):
self.name = name
def __enter__(self):
logging.info('Entering: %s', self.name)
def __exit__(self, exc_type, exc, exc_tb):
logging.info('Exiting: %s', self.name)
Instances of this class can be used as both a context manager:
with track_entry_and_exit('widget loader'):
print('Some time consuming activity goes here')
load_widget()
And also as a function decorator:
@track_entry_and_exit('widget loader')
def activity():
print('Some time consuming activity goes here')
load_widget()
Note that there is one additional limitation when using context managers
as function decorators: there’s no way to access the return value of
__enter__()
. If that value is needed, then it is still necessary to use
an explicit with
statement.
Single use, reusable and reentrant context managers¶
Most context managers are written in a way that means they can only be
used effectively in a with
statement once. These single use
context managers must be created afresh each time they’re used -
attempting to use them a second time will trigger an exception or
otherwise not work correctly.
This common limitation means that it is generally advisable to create
context managers directly in the header of the with
statement
where they are used (as shown in all of the usage examples above).
Files are an example of effectively single use context managers, since
the first with
statement will close the file, preventing any
further IO operations using that file object.
Context managers created using contextmanager()
are also single use
context managers, and will complain about the underlying generator failing
to yield if an attempt is made to use them a second time:
>>> from contextlib import contextmanager
>>> @contextmanager
... def singleuse():
... print("Before")
... yield
... print("After")
...
>>> cm = singleuse()
>>> with cm:
... pass
...
Before
After
>>> with cm:
... pass
...
Traceback (most recent call last):
...
RuntimeError: generator didn't yield
Reentrant context managers¶
More sophisticated context managers may be “reentrant”. These context
managers can not only be used in multiple with
statements,
but may also be used inside a with
statement that is already
using the same context manager.
threading.RLock
is an example of a reentrant context manager, as are
suppress()
, redirect_stdout()
, and chdir()
. Here’s a very
simple example of reentrant use:
>>> from contextlib import redirect_stdout
>>> from io import StringIO
>>> stream = StringIO()
>>> write_to_stream = redirect_stdout(stream)
>>> with write_to_stream:
... print("This is written to the stream rather than stdout")
... with write_to_stream:
... print("This is also written to the stream")
...
>>> print("This is written directly to stdout")
This is written directly to stdout
>>> print(stream.getvalue())
This is written to the stream rather than stdout
This is also written to the stream
Real world examples of reentrancy are more likely to involve multiple functions calling each other and hence be far more complicated than this example.
Note also that being reentrant is not the same thing as being thread safe.
redirect_stdout()
, for example, is definitely not thread safe, as it
makes a global modification to the system state by binding sys.stdout
to a different stream.
Gerenciadores de contexto reutilizáveis¶
Distinct from both single use and reentrant context managers are “reusable” context managers (or, to be completely explicit, “reusable, but not reentrant” context managers, since reentrant context managers are also reusable). These context managers support being used multiple times, but will fail (or otherwise not work correctly) if the specific context manager instance has already been used in a containing with statement.
threading.Lock
is an example of a reusable, but not reentrant,
context manager (for a reentrant lock, it is necessary to use
threading.RLock
instead).
Another example of a reusable, but not reentrant, context manager is
ExitStack
, as it invokes all currently registered callbacks
when leaving any with statement, regardless of where those callbacks
were added:
>>> from contextlib import ExitStack
>>> stack = ExitStack()
>>> with stack:
... stack.callback(print, "Callback: from first context")
... print("Leaving first context")
...
Leaving first context
Callback: from first context
>>> with stack:
... stack.callback(print, "Callback: from second context")
... print("Leaving second context")
...
Leaving second context
Callback: from second context
>>> with stack:
... stack.callback(print, "Callback: from outer context")
... with stack:
... stack.callback(print, "Callback: from inner context")
... print("Leaving inner context")
... print("Leaving outer context")
...
Leaving inner context
Callback: from inner context
Callback: from outer context
Leaving outer context
As the output from the example shows, reusing a single stack object across multiple with statements works correctly, but attempting to nest them will cause the stack to be cleared at the end of the innermost with statement, which is unlikely to be desirable behaviour.
Using separate ExitStack
instances instead of reusing a single
instance avoids that problem:
>>> from contextlib import ExitStack
>>> with ExitStack() as outer_stack:
... outer_stack.callback(print, "Callback: from outer context")
... with ExitStack() as inner_stack:
... inner_stack.callback(print, "Callback: from inner context")
... print("Leaving inner context")
... print("Leaving outer context")
...
Leaving inner context
Callback: from inner context
Leaving outer context
Callback: from outer context