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"""Client (Connection)."""
from __future__ import absolute_import, unicode_literals
import os
import socket
import sys
from collections import OrderedDict
from contextlib import contextmanager
from itertools import count, cycle
from operator import itemgetter
from ssl import CERT_NONE
ssl_available = True
except ImportError: # pragma: no cover
ssl_available = False
# jython breaks on relative import for .exceptions for some reason
# (Issue #112)
from kombu import exceptions
from .five import (
bytes_if_py2, python_2_unicode_compatible, reraise, string_t, text_t,
from .log import get_logger
from .resource import Resource
from .transport import get_transport_cls, supports_librabbitmq
from .utils.collections import HashedSeq
from .utils.functional import dictfilter, lazy, retry_over_time, shufflecycle
from .utils.objects import cached_property
from .utils.url import as_url, parse_url, quote, urlparse
__all__ = ('Connection', 'ConnectionPool', 'ChannelPool')
logger = get_logger(__name__)
roundrobin_failover = cycle
resolve_aliases = {
'pyamqp': 'amqp',
'librabbitmq': 'amqp',
failover_strategies = {
'round-robin': roundrobin_failover,
'shuffle': shufflecycle,
_log_connection = os.environ.get('KOMBU_LOG_CONNECTION', False)
_log_channel = os.environ.get('KOMBU_LOG_CHANNEL', False)
class Connection(object):
"""A connection to the broker.
>>> Connection('amqp://guest:guest@localhost:5672//')
>>> Connection('amqp://foo;amqp://bar',
... failover_strategy='round-robin')
>>> Connection('redis://', transport_options={
... 'visibility_timeout': 3000,
... })
>>> import ssl
>>> Connection('amqp://', login_method='EXTERNAL', ssl={
... 'ca_certs': '/etc/pki/tls/certs/something.crt',
... 'keyfile': '/etc/something/system.key',
... 'certfile': '/etc/something/system.cert',
... 'cert_reqs': ssl.CERT_REQUIRED,
... })
SSL currently only works with the py-amqp, and qpid
transports. For other transports you can use stunnel.
URL (str, Sequence): Broker URL, or a list of URLs.
Keyword Arguments:
ssl (bool): Use SSL to connect to the server. Default is ``False``.
May not be supported by the specified transport.
transport (Transport): Default transport if not specified in the URL.
connect_timeout (float): Timeout in seconds for connecting to the
server. May not be supported by the specified transport.
transport_options (Dict): A dict of additional connection arguments to
pass to alternate kombu channel implementations. Consult the
transport documentation for available options.
heartbeat (float): Heartbeat interval in int/float seconds.
Note that if heartbeats are enabled then the
:meth:`heartbeat_check` method must be called regularly,
around once per second.
The connection is established lazily when needed. If you need the
connection to be established, then force it by calling
>>> conn = Connection('amqp://')
>>> conn.connect()
and always remember to close the connection::
>>> conn.release()
These options have been replaced by the URL argument, but are still
supported for backwards compatibility:
:keyword hostname: Host name/address.
NOTE: You cannot specify both the URL argument and use the hostname
keyword argument at the same time.
:keyword userid: Default user name if not provided in the URL.
:keyword password: Default password if not provided in the URL.
:keyword virtual_host: Default virtual host if not provided in the URL.
:keyword port: Default port if not provided in the URL.
port = None
virtual_host = '/'
connect_timeout = 5
_closed = None
_connection = None
_default_channel = None
_transport = None
_logger = False
uri_prefix = None
#: The cache of declared entities is per connection,
#: in case the server loses data.
declared_entities = None
#: Iterator returning the next broker URL to try in the event
#: of connection failure (initialized by :attr:`failover_strategy`).
cycle = None
#: Additional transport specific options,
#: passed on to the transport instance.
transport_options = None
#: Strategy used to select new hosts when reconnecting after connection
#: failure. One of "round-robin", "shuffle" or any custom iterator
#: constantly yielding new URLs to try.
failover_strategy = 'round-robin'
#: Heartbeat value, currently only supported by the py-amqp transport.
heartbeat = None
resolve_aliases = resolve_aliases
failover_strategies = failover_strategies
hostname = userid = password = ssl = login_method = None
def __init__(self, hostname='localhost', userid=None,
password=None, virtual_host=None, port=None, insist=False,
ssl=False, transport=None, connect_timeout=5,
transport_options=None, login_method=None, uri_prefix=None,
heartbeat=0, failover_strategy='round-robin',
alternates=None, **kwargs):
alt = [] if alternates is None else alternates
# have to spell the args out, just to get nice docstrings :(
params = self._initial_params = {
'hostname': hostname, 'userid': userid,
'password': password, 'virtual_host': virtual_host,
'port': port, 'insist': insist, 'ssl': ssl,
'transport': transport, 'connect_timeout': connect_timeout,
'login_method': login_method, 'heartbeat': heartbeat
if hostname and not isinstance(hostname, string_t):
hostname = alt[0]
if hostname and '://' in hostname:
if ';' in hostname:
hostname = alt[0]
if '+' in hostname[:hostname.index('://')]:
# e.g. sqla+mysql://root:masterkey@localhost/
params['transport'], params['hostname'] = \
hostname.split('+', 1)
self.uri_prefix = params['transport']
transport = transport or urlparse(hostname).scheme
if not get_transport_cls(transport).can_parse_url:
# we must parse the URL
url_params = parse_url(hostname)
params['transport'] = transport
# fallback hosts
self.alt = alt
# keep text representation for .info
# only temporary solution as this won't work when
# passing a custom object (Issue celery/celery#3320).
self._failover_strategy = failover_strategy or 'round-robin'
self.failover_strategy = self.failover_strategies.get(
self._failover_strategy) or self._failover_strategy
if self.alt:
self.cycle = self.failover_strategy(self.alt)
next(self.cycle) # skip first entry
if transport_options is None:
transport_options = {}
self.transport_options = transport_options
if _log_connection: # pragma: no cover
self._logger = True
if uri_prefix:
self.uri_prefix = uri_prefix
self.declared_entities = set()
def switch(self, conn_str):
"""Switch connection parameters to use a new URL or hostname.
Does not reconnect!
conn_str (str): either a hostname or URL.
self._closed = False
conn_params = (
parse_url(conn_str) if "://" in conn_str else {"hostname": conn_str} # noqa
self._init_params(**dict(self._initial_params, **conn_params))
def maybe_switch_next(self):
"""Switch to next URL given by the current failover strategy."""
if self.cycle:
def _init_params(self, hostname, userid, password, virtual_host, port,
insist, ssl, transport, connect_timeout,
login_method, heartbeat):
transport = transport or 'amqp'
if transport == 'amqp' and supports_librabbitmq():
transport = 'librabbitmq'
if transport == 'rediss' and ssl_available and not ssl:
'Secure redis scheme specified (rediss) with no ssl '
'options, defaulting to insecure SSL behaviour.'
ssl = {'ssl_cert_reqs': CERT_NONE}
self.hostname = hostname
self.userid = userid
self.password = password
self.login_method = login_method
self.virtual_host = virtual_host or self.virtual_host
self.port = port or self.port
self.insist = insist
self.connect_timeout = connect_timeout
self.ssl = ssl
self.transport_cls = transport
self.heartbeat = heartbeat and float(heartbeat)
def register_with_event_loop(self, loop):
self.transport.register_with_event_loop(self.connection, loop)
def _debug(self, msg, *args, **kwargs):
if self._logger: # pragma: no cover
fmt = '[Kombu connection:{id:#x}] {msg}'
logger.debug(fmt.format(id=id(self), msg=text_t(msg)),
*args, **kwargs)
def connect(self):
"""Establish connection to server immediately."""
self._closed = False
return self.connection
def channel(self):
"""Create and return a new channel."""
self._debug('create channel')
chan = self.transport.create_channel(self.connection)
if _log_channel: # pragma: no cover
from .utils.debug import Logwrapped
return Logwrapped(chan, '',
'[Kombu channel:{0.channel_id}] ')
return chan
def heartbeat_check(self, rate=2):
"""Check heartbeats.
Allow the transport to perform any periodic tasks
required to make heartbeats work. This should be called
approximately every second.
If the current transport does not support heartbeats then
this is a noop operation.
rate (int): Rate is how often the tick is called
compared to the actual heartbeat value. E.g. if
the heartbeat is set to 3 seconds, and the tick
is called every 3 / 2 seconds, then the rate is 2.
This value is currently unused by any transports.
return self.transport.heartbeat_check(self.connection, rate=rate)
def drain_events(self, **kwargs):
"""Wait for a single event from the server.
timeout (float): Timeout in seconds before we give up.
socket.timeout: if the timeout is exceeded.
return self.transport.drain_events(self.connection, **kwargs)
def maybe_close_channel(self, channel):
"""Close given channel, but ignore connection and channel errors."""
except (self.connection_errors + self.channel_errors):
def _do_close_self(self):
# Close only connection and channel(s), but not transport.
if self._default_channel:
if self._connection:
except self.connection_errors + (AttributeError, socket.error):
self._connection = None
def _close(self):
"""Really close connection, even if part of a connection pool."""
self._closed = True
def _do_close_transport(self):
if self._transport:
self._transport.client = None
self._transport = None
def collect(self, socket_timeout=None):
# amqp requires communication to close, we don't need that just
# to clear out references, Transport._collect can also be implemented
# by other transports that want fast after fork
gc_transport = self._transport._collect
except AttributeError:
_timeo = socket.getdefaulttimeout()
except socket.timeout:
self._connection = None
def release(self):
"""Close the connection (if open)."""
close = release
def ensure_connection(self, errback=None, max_retries=None,
interval_start=2, interval_step=2, interval_max=30,
callback=None, reraise_as_library_errors=True,
"""Ensure we have a connection to the server.
If not retry establishing the connection with the settings
errback (Callable): Optional callback called each time the
connection can't be established. Arguments provided are
the exception raised and the interval that will be
slept ``(exc, interval)``.
max_retries (int): Maximum number of times to retry.
If this limit is exceeded the connection error
will be re-raised.
interval_start (float): The number of seconds we start
sleeping for.
interval_step (float): How many seconds added to the interval
for each retry.
interval_max (float): Maximum number of seconds to sleep between
each retry.
callback (Callable): Optional callback that is called for every
internal iteration (1 s).
timeout (int): Maximum amount of time in seconds to spend
waiting for connection
def on_error(exc, intervals, retries, interval=0):
round = self.completes_cycle(retries)
if round:
interval = next(intervals)
if errback:
errback(exc, interval)
self.maybe_switch_next() # select next host
return interval if round else 0
ctx = self._reraise_as_library_errors
if not reraise_as_library_errors:
ctx = self._dummy_context
with ctx():
retry_over_time(self.connect, self.recoverable_connection_errors,
(), {}, on_error, max_retries,
interval_start, interval_step, interval_max,
callback, timeout=timeout)
return self
def _reraise_as_library_errors(
except (ConnectionError, ChannelError):
except self.recoverable_connection_errors as exc:
reraise(ConnectionError, ConnectionError(text_t(exc)),
except self.recoverable_channel_errors as exc:
reraise(ChannelError, ChannelError(text_t(exc)),
def _dummy_context(self):
def completes_cycle(self, retries):
"""Return true if the cycle is complete after number of `retries`."""
return not (retries + 1) % len(self.alt) if self.alt else True
def revive(self, new_channel):
"""Revive connection after connection re-established."""
if self._default_channel and new_channel is not self._default_channel:
self._default_channel = None
def ensure(self, obj, fun, errback=None, max_retries=None,
interval_start=1, interval_step=1, interval_max=1,
"""Ensure operation completes.
Regardless of any channel/connection errors occurring.
Retries by establishing the connection, and reapplying
the function.
obj: The object to ensure an action on.
fun (Callable): Method to apply.
errback (Callable): Optional callback called each time the
connection can't be established. Arguments provided are
the exception raised and the interval that will
be slept ``(exc, interval)``.
max_retries (int): Maximum number of times to retry.
If this limit is exceeded the connection error
will be re-raised.
interval_start (float): The number of seconds we start
sleeping for.
interval_step (float): How many seconds added to the interval
for each retry.
interval_max (float): Maximum number of seconds to sleep between
each retry.
on_revive (Callable): Optional callback called whenever
revival completes successfully
>>> from kombu import Connection, Producer
>>> conn = Connection('amqp://')
>>> producer = Producer(conn)
>>> def errback(exc, interval):
... logger.error('Error: %r', exc, exc_info=1)
...'Retry in %s seconds.', interval)
>>> publish = conn.ensure(producer, producer.publish,
... errback=errback, max_retries=3)
>>> publish({'hello': 'world'}, routing_key='dest')
def _ensured(*args, **kwargs):
got_connection = 0
conn_errors = self.recoverable_connection_errors
chan_errors = self.recoverable_channel_errors
has_modern_errors = hasattr(
self.transport, 'recoverable_connection_errors',
with self._reraise_as_library_errors():
for retries in count(0): # for infinity
return fun(*args, **kwargs)
except conn_errors as exc:
if got_connection and not has_modern_errors:
# transport can not distinguish between
# recoverable/irrecoverable errors, so we propagate
# the error if it persists after a new connection
# was successfully established.
if max_retries is not None and retries > max_retries:
self._debug('ensure connection error: %r',
exc, exc_info=1)
errback and errback(exc, 0)
remaining_retries = None
if max_retries is not None:
remaining_retries = max(max_retries - retries, 1)
interval_start, interval_step, interval_max,
channel = self.default_channel
if on_revive:
got_connection += 1
except chan_errors as exc:
if max_retries is not None and retries > max_retries:
self._debug('ensure channel error: %r',
exc, exc_info=1)
errback and errback(exc, 0)
_ensured.__name__ = bytes_if_py2('{0}(ensured)'.format(fun.__name__))
_ensured.__doc__ = fun.__doc__
_ensured.__module__ = fun.__module__
return _ensured
def autoretry(self, fun, channel=None, **ensure_options):
"""Decorator for functions supporting a ``channel`` keyword argument.
The resulting callable will retry calling the function if
it raises connection or channel related errors.
The return value will be a tuple of ``(retval, last_created_channel)``.
If a ``channel`` is not provided, then one will be automatically
acquired (remember to close it afterwards).
See Also:
:meth:`ensure` for the full list of supported keyword arguments.
>>> channel =
>>> try:
... ret, channel = connection.autoretry(
... publish_messages, channel)
... finally:
... channel.close()
channels = [channel]
class Revival(object):
__name__ = getattr(fun, '__name__', None)
__module__ = getattr(fun, '__module__', None)
__doc__ = getattr(fun, '__doc__', None)
def __init__(self, connection):
self.connection = connection
def revive(self, channel):
channels[0] = channel
def __call__(self, *args, **kwargs):
if channels[0] is None:
return fun(*args, channel=channels[0], **kwargs), channels[0]
revive = Revival(self)
return self.ensure(revive, revive, **ensure_options)
def create_transport(self):
return self.get_transport_cls()(client=self)
def get_transport_cls(self):
"""Get the currently used transport class."""
transport_cls = self.transport_cls
if not transport_cls or isinstance(transport_cls, string_t):
transport_cls = get_transport_cls(transport_cls)
return transport_cls
def clone(self, **kwargs):
"""Create a copy of the connection with same settings."""
return self.__class__(**dict(self._info(resolve=False), **kwargs))
def get_heartbeat_interval(self):
return self.transport.get_heartbeat_interval(self.connection)
def _info(self, resolve=True):
transport_cls = self.transport_cls
if resolve:
transport_cls = self.resolve_aliases.get(
transport_cls, transport_cls)
D = self.transport.default_connection_params
hostname = self.hostname or D.get('hostname')
if self.uri_prefix:
hostname = '%s+%s' % (self.uri_prefix, hostname)
info = (
('hostname', hostname),
('userid', self.userid or D.get('userid')),
('password', self.password or D.get('password')),
('virtual_host', self.virtual_host or D.get('virtual_host')),
('port', self.port or D.get('port')),
('insist', self.insist),
('ssl', self.ssl),
('transport', transport_cls),
('connect_timeout', self.connect_timeout),
('transport_options', self.transport_options),
('login_method', self.login_method or D.get('login_method')),
('uri_prefix', self.uri_prefix),
('heartbeat', self.heartbeat),
('failover_strategy', self._failover_strategy),
('alternates', self.alt),
return info
def info(self):
"""Get connection info."""
return OrderedDict(self._info())
def __eqhash__(self):
return HashedSeq(self.transport_cls, self.hostname, self.userid,
self.password, self.virtual_host, self.port,
def as_uri(self, include_password=False, mask='**',
getfields=itemgetter('port', 'userid', 'password',
'virtual_host', 'transport')):
"""Convert connection parameters to URL form."""
hostname = self.hostname or 'localhost'
if self.transport.can_parse_url:
if self.uri_prefix:
return '%s+%s' % (self.uri_prefix, hostname)
return self.hostname
if self.uri_prefix:
return '%s+%s' % (self.uri_prefix, hostname)
fields =
port, userid, password, vhost, transport = getfields(fields)
return as_url(
transport, hostname, port, userid, password, quote(vhost),
sanitize=not include_password, mask=mask,
def Pool(self, limit=None, **kwargs):
"""Pool of connections.
See Also:
limit (int): Maximum number of active connections.
Default is no limit.
>>> connection = Connection('amqp://')
>>> pool = connection.Pool(2)
>>> c1 = pool.acquire()
>>> c2 = pool.acquire()
>>> c3 = pool.acquire()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "kombu/", line 354, in acquire
raise ConnectionLimitExceeded(self.limit)
kombu.exceptions.ConnectionLimitExceeded: 2
>>> c1.release()
>>> c3 = pool.acquire()
return ConnectionPool(self, limit, **kwargs)
def ChannelPool(self, limit=None, **kwargs):
"""Pool of channels.
See Also:
limit (int): Maximum number of active channels.
Default is no limit.
>>> connection = Connection('amqp://')
>>> pool = connection.ChannelPool(2)
>>> c1 = pool.acquire()
>>> c2 = pool.acquire()
>>> c3 = pool.acquire()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "kombu/", line 354, in acquire
raise ChannelLimitExceeded(self.limit)
kombu.connection.ChannelLimitExceeded: 2
>>> c1.release()
>>> c3 = pool.acquire()
return ChannelPool(self, limit, **kwargs)
def Producer(self, channel=None, *args, **kwargs):
"""Create new :class:`kombu.Producer` instance."""
from .messaging import Producer
return Producer(channel or self, *args, **kwargs)
def Consumer(self, queues=None, channel=None, *args, **kwargs):
"""Create new :class:`kombu.Consumer` instance."""
from .messaging import Consumer
return Consumer(channel or self, queues, *args, **kwargs)
def SimpleQueue(self, name, no_ack=None, queue_opts=None,
exchange_opts=None, channel=None, **kwargs):
"""Simple persistent queue API.
Create new :class:`~kombu.simple.SimpleQueue`, using a channel
from this connection.
If ``name`` is a string, a queue and exchange will be automatically
created using that name as the name of the queue and exchange,
also it will be used as the default routing key.
name (str, kombu.Queue): Name of the queue/or a queue.
no_ack (bool): Disable acknowledgments. Default is false.
queue_opts (Dict): Additional keyword arguments passed to the
constructor of the automatically created :class:`~kombu.Queue`.
queue_args (Dict): Additional keyword arguments passed to the
constructor of the automatically created :class:`~kombu.Queue`
for setting implementation extensions (e.g., in RabbitMQ).
exchange_opts (Dict): Additional keyword arguments passed to the
constructor of the automatically created
channel (ChannelT): Custom channel to use. If not specified the
connection default channel is used.
from .simple import SimpleQueue
return SimpleQueue(channel or self, name, no_ack, queue_opts,
exchange_opts, **kwargs)
def SimpleBuffer(self, name, no_ack=None, queue_opts=None,
exchange_opts=None, channel=None, **kwargs):
"""Simple ephemeral queue API.
Create new :class:`~kombu.simple.SimpleQueue` using a channel
from this connection.
See Also:
Same as :meth:`SimpleQueue`, but configured with buffering
semantics. The resulting queue and exchange will not be durable,
also auto delete is enabled. Messages will be transient (not
persistent), and acknowledgments are disabled (``no_ack``).
from .simple import SimpleBuffer
return SimpleBuffer(channel or self, name, no_ack, queue_opts,
exchange_opts, **kwargs)
def _establish_connection(self):
self._debug('establishing connection...')
conn = self.transport.establish_connection()
self._debug('connection established: %r', self)
return conn
def supports_exchange_type(self, exchange_type):
return exchange_type in self.transport.implements.exchange_type
def __repr__(self):
return '<Connection: {0} at {1:#x}>'.format(self.as_uri(), id(self))
def __copy__(self):
return self.clone()
def __reduce__(self):
return self.__class__, tuple(, None
def __enter__(self):
return self
def __exit__(self, *args):
def qos_semantics_matches_spec(self):
return self.transport.qos_semantics_matches_spec(self.connection)
def connected(self):
"""Return true if the connection has been established."""
return (not self._closed and
self._connection is not None and
def connection(self):
"""The underlying connection object.
This instance is transport specific, so do not
depend on the interface of this object.
if not self._closed:
if not self.connected:
self._default_channel = None
self._connection = self._establish_connection()
self._closed = False
return self._connection
def default_channel(self):
"""Default channel.
Created upon access and closed when the connection is closed.
Can be used for automatic channel handling when you only need one
channel, and also it is the channel implicitly used if
a connection is passed instead of a channel, to functions that
require a channel.
conn_opts = {}
transport_opts = self.transport_options
if transport_opts:
if 'max_retries' in transport_opts:
conn_opts['max_retries'] = transport_opts['max_retries']
if 'interval_start' in transport_opts:
conn_opts['interval_start'] = transport_opts['interval_start']
if 'interval_step' in transport_opts:
conn_opts['interval_step'] = transport_opts['interval_step']
if 'interval_max' in transport_opts:
conn_opts['interval_max'] = transport_opts['interval_max']
# make sure we're still connected, and if not refresh.
if self._default_channel is None:
self._default_channel =
return self._default_channel
def host(self):
"""The host as a host name/port pair separated by colon."""
return ':'.join([self.hostname, str(self.port)])
def transport(self):
if self._transport is None:
self._transport = self.create_transport()
return self._transport
def manager(self):
"""AMQP Management API.
Experimental manager that can be used to manage/monitor the broker
Not available for all transports.
return self.transport.manager
def get_manager(self, *args, **kwargs):
return self.transport.get_manager(*args, **kwargs)
def recoverable_connection_errors(self):
"""Recoverable connection errors.
List of connection related exceptions that can be recovered from,
but where the connection must be closed and re-established first.
return self.transport.recoverable_connection_errors
except AttributeError:
# There were no such classification before,
# and all errors were assumed to be recoverable,
# so this is a fallback for transports that do
# not support the new recoverable/irrecoverable classes.
return self.connection_errors + self.channel_errors
def recoverable_channel_errors(self):
"""Recoverable channel errors.
List of channel related exceptions that can be automatically
recovered from without re-establishing the connection.
return self.transport.recoverable_channel_errors
except AttributeError:
return ()
def connection_errors(self):
"""List of exceptions that may be raised by the connection."""
return self.transport.connection_errors
def channel_errors(self):
"""List of exceptions that may be raised by the channel."""
return self.transport.channel_errors
def supports_heartbeats(self):
return self.transport.implements.heartbeats
def is_evented(self):
return self.transport.implements.asynchronous
BrokerConnection = Connection # noqa: E305
class ConnectionPool(Resource):
"""Pool of connections."""
LimitExceeded = exceptions.ConnectionLimitExceeded
close_after_fork = True
def __init__(self, connection, limit=None, **kwargs):
self.connection = connection
super(ConnectionPool, self).__init__(limit=limit)
def new(self):
return self.connection.clone()
def release_resource(self, resource):
except AttributeError:
def close_resource(self, resource):
def collect_resource(self, resource, socket_timeout=0.1):
if not isinstance(resource, lazy):
return resource.collect(socket_timeout)
def acquire_channel(self, block=False):
with self.acquire(block=block) as connection:
yield connection, connection.default_channel
def setup(self):
if self.limit:
q = self._resource.queue
while len(q) < self.limit:
def prepare(self, resource):
if callable(resource):
resource = resource()
return resource
class ChannelPool(Resource):
"""Pool of channels."""
LimitExceeded = exceptions.ChannelLimitExceeded
def __init__(self, connection, limit=None, **kwargs):
self.connection = connection
super(ChannelPool, self).__init__(limit=limit)
def new(self):
return lazy(
def setup(self):
channel =
if self.limit:
q = self._resource.queue
while len(q) < self.limit:
def prepare(self, channel):
if callable(channel):
channel = channel()
return channel
def maybe_channel(channel):
"""Get channel from object.
Return the default channel if argument is a connection instance,
otherwise just return the channel given.
if is_connection(channel):
return channel.default_channel
return channel
def is_connection(obj):
return isinstance(obj, Connection)
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