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456 lines
16 KiB
Python
456 lines
16 KiB
Python
# -*- coding: utf-8; fill-column: 76 -*-
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"""Signals and events.
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A small implementation of signals, inspired by a snippet of Django signal
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API client code seen in a blog post. Signals are first-class objects and
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each manages its own receivers and message emission.
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The :func:`signal` function provides singleton behavior for named signals.
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"""
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from warnings import warn
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from weakref import WeakValueDictionary
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from blinker._utilities import (
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WeakTypes,
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contextmanager,
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defaultdict,
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hashable_identity,
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lazy_property,
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reference,
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symbol,
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)
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ANY = symbol('ANY')
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ANY.__doc__ = 'Token for "any sender".'
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ANY_ID = 0
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class Signal(object):
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"""A notification emitter."""
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#: An :obj:`ANY` convenience synonym, allows ``Signal.ANY``
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#: without an additional import.
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ANY = ANY
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@lazy_property
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def receiver_connected(self):
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"""Emitted after each :meth:`connect`.
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The signal sender is the signal instance, and the :meth:`connect`
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arguments are passed through: *receiver*, *sender*, and *weak*.
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.. versionadded:: 1.2
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"""
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return Signal(doc="Emitted after a receiver connects.")
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@lazy_property
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def receiver_disconnected(self):
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"""Emitted after :meth:`disconnect`.
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The sender is the signal instance, and the :meth:`disconnect` arguments
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are passed through: *receiver* and *sender*.
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Note, this signal is emitted **only** when :meth:`disconnect` is
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called explicitly.
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The disconnect signal can not be emitted by an automatic disconnect
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(due to a weakly referenced receiver or sender going out of scope),
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as the receiver and/or sender instances are no longer available for
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use at the time this signal would be emitted.
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An alternative approach is available by subscribing to
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:attr:`receiver_connected` and setting up a custom weakref cleanup
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callback on weak receivers and senders.
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.. versionadded:: 1.2
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"""
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return Signal(doc="Emitted after a receiver disconnects.")
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def __init__(self, doc=None):
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"""
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:param doc: optional. If provided, will be assigned to the signal's
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__doc__ attribute.
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"""
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if doc:
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self.__doc__ = doc
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#: A mapping of connected receivers.
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#:
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#: The values of this mapping are not meaningful outside of the
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#: internal :class:`Signal` implementation, however the boolean value
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#: of the mapping is useful as an extremely efficient check to see if
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#: any receivers are connected to the signal.
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self.receivers = {}
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self._by_receiver = defaultdict(set)
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self._by_sender = defaultdict(set)
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self._weak_senders = {}
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def connect(self, receiver, sender=ANY, weak=True):
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"""Connect *receiver* to signal events sent by *sender*.
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:param receiver: A callable. Will be invoked by :meth:`send` with
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`sender=` as a single positional argument and any \*\*kwargs that
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were provided to a call to :meth:`send`.
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:param sender: Any object or :obj:`ANY`, defaults to ``ANY``.
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Restricts notifications delivered to *receiver* to only those
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:meth:`send` emissions sent by *sender*. If ``ANY``, the receiver
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will always be notified. A *receiver* may be connected to
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multiple *sender* values on the same Signal through multiple calls
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to :meth:`connect`.
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:param weak: If true, the Signal will hold a weakref to *receiver*
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and automatically disconnect when *receiver* goes out of scope or
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is garbage collected. Defaults to True.
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"""
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receiver_id = hashable_identity(receiver)
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if weak:
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receiver_ref = reference(receiver, self._cleanup_receiver)
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receiver_ref.receiver_id = receiver_id
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else:
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receiver_ref = receiver
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if sender is ANY:
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sender_id = ANY_ID
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else:
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sender_id = hashable_identity(sender)
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self.receivers.setdefault(receiver_id, receiver_ref)
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self._by_sender[sender_id].add(receiver_id)
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self._by_receiver[receiver_id].add(sender_id)
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del receiver_ref
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if sender is not ANY and sender_id not in self._weak_senders:
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# wire together a cleanup for weakref-able senders
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try:
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sender_ref = reference(sender, self._cleanup_sender)
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sender_ref.sender_id = sender_id
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except TypeError:
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pass
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else:
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self._weak_senders.setdefault(sender_id, sender_ref)
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del sender_ref
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# broadcast this connection. if receivers raise, disconnect.
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if ('receiver_connected' in self.__dict__ and
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self.receiver_connected.receivers):
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try:
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self.receiver_connected.send(self,
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receiver=receiver,
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sender=sender,
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weak=weak)
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except:
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self.disconnect(receiver, sender)
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raise
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if receiver_connected.receivers and self is not receiver_connected:
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try:
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receiver_connected.send(self,
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receiver_arg=receiver,
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sender_arg=sender,
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weak_arg=weak)
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except:
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self.disconnect(receiver, sender)
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raise
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return receiver
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def connect_via(self, sender, weak=False):
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"""Connect the decorated function as a receiver for *sender*.
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:param sender: Any object or :obj:`ANY`. The decorated function
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will only receive :meth:`send` emissions sent by *sender*. If
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``ANY``, the receiver will always be notified. A function may be
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decorated multiple times with differing *sender* values.
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:param weak: If true, the Signal will hold a weakref to the
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decorated function and automatically disconnect when *receiver*
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goes out of scope or is garbage collected. Unlike
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:meth:`connect`, this defaults to False.
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The decorated function will be invoked by :meth:`send` with
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`sender=` as a single positional argument and any \*\*kwargs that
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were provided to the call to :meth:`send`.
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.. versionadded:: 1.1
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"""
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def decorator(fn):
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self.connect(fn, sender, weak)
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return fn
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return decorator
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@contextmanager
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def connected_to(self, receiver, sender=ANY):
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"""Execute a block with the signal temporarily connected to *receiver*.
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:param receiver: a receiver callable
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:param sender: optional, a sender to filter on
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This is a context manager for use in the ``with`` statement. It can
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be useful in unit tests. *receiver* is connected to the signal for
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the duration of the ``with`` block, and will be disconnected
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automatically when exiting the block:
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.. testsetup::
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from __future__ import with_statement
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from blinker import Signal
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on_ready = Signal()
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receiver = lambda sender: None
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.. testcode::
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with on_ready.connected_to(receiver):
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# do stuff
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on_ready.send(123)
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.. versionadded:: 1.1
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"""
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self.connect(receiver, sender=sender, weak=False)
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try:
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yield None
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except:
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self.disconnect(receiver)
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raise
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else:
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self.disconnect(receiver)
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def temporarily_connected_to(self, receiver, sender=ANY):
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"""An alias for :meth:`connected_to`.
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:param receiver: a receiver callable
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:param sender: optional, a sender to filter on
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.. versionadded:: 0.9
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.. versionchanged:: 1.1
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Renamed to :meth:`connected_to`. ``temporarily_connected_to`` was
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deprecated in 1.2 and will be removed in a subsequent version.
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"""
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warn("temporarily_connected_to is deprecated; "
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"use connected_to instead.",
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DeprecationWarning)
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return self.connected_to(receiver, sender)
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def send(self, *sender, **kwargs):
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"""Emit this signal on behalf of *sender*, passing on \*\*kwargs.
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Returns a list of 2-tuples, pairing receivers with their return
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value. The ordering of receiver notification is undefined.
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:param \*sender: Any object or ``None``. If omitted, synonymous
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with ``None``. Only accepts one positional argument.
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:param \*\*kwargs: Data to be sent to receivers.
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"""
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# Using '*sender' rather than 'sender=None' allows 'sender' to be
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# used as a keyword argument- i.e. it's an invisible name in the
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# function signature.
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if len(sender) == 0:
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sender = None
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elif len(sender) > 1:
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raise TypeError('send() accepts only one positional argument, '
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'%s given' % len(sender))
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else:
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sender = sender[0]
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if not self.receivers:
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return []
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else:
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return [(receiver, receiver(sender, **kwargs))
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for receiver in self.receivers_for(sender)]
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def has_receivers_for(self, sender):
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"""True if there is probably a receiver for *sender*.
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Performs an optimistic check only. Does not guarantee that all
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weakly referenced receivers are still alive. See
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:meth:`receivers_for` for a stronger search.
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"""
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if not self.receivers:
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return False
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if self._by_sender[ANY_ID]:
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return True
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if sender is ANY:
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return False
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return hashable_identity(sender) in self._by_sender
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def receivers_for(self, sender):
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"""Iterate all live receivers listening for *sender*."""
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# TODO: test receivers_for(ANY)
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if self.receivers:
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sender_id = hashable_identity(sender)
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if sender_id in self._by_sender:
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ids = (self._by_sender[ANY_ID] |
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self._by_sender[sender_id])
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else:
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ids = self._by_sender[ANY_ID].copy()
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for receiver_id in ids:
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receiver = self.receivers.get(receiver_id)
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if receiver is None:
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continue
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if isinstance(receiver, WeakTypes):
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strong = receiver()
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if strong is None:
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self._disconnect(receiver_id, ANY_ID)
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continue
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receiver = strong
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yield receiver
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def disconnect(self, receiver, sender=ANY):
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"""Disconnect *receiver* from this signal's events.
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:param receiver: a previously :meth:`connected<connect>` callable
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:param sender: a specific sender to disconnect from, or :obj:`ANY`
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to disconnect from all senders. Defaults to ``ANY``.
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"""
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if sender is ANY:
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sender_id = ANY_ID
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else:
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sender_id = hashable_identity(sender)
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receiver_id = hashable_identity(receiver)
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self._disconnect(receiver_id, sender_id)
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if ('receiver_disconnected' in self.__dict__ and
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self.receiver_disconnected.receivers):
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self.receiver_disconnected.send(self,
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receiver=receiver,
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sender=sender)
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def _disconnect(self, receiver_id, sender_id):
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if sender_id == ANY_ID:
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if self._by_receiver.pop(receiver_id, False):
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for bucket in self._by_sender.values():
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bucket.discard(receiver_id)
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self.receivers.pop(receiver_id, None)
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else:
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self._by_sender[sender_id].discard(receiver_id)
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self._by_receiver[receiver_id].discard(sender_id)
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def _cleanup_receiver(self, receiver_ref):
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"""Disconnect a receiver from all senders."""
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self._disconnect(receiver_ref.receiver_id, ANY_ID)
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def _cleanup_sender(self, sender_ref):
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"""Disconnect all receivers from a sender."""
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sender_id = sender_ref.sender_id
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assert sender_id != ANY_ID
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self._weak_senders.pop(sender_id, None)
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for receiver_id in self._by_sender.pop(sender_id, ()):
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self._by_receiver[receiver_id].discard(sender_id)
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def _cleanup_bookkeeping(self):
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"""Prune unused sender/receiver bookeeping. Not threadsafe.
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Connecting & disconnecting leave behind a small amount of bookeeping
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for the receiver and sender values. Typical workloads using Blinker,
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for example in most web apps, Flask, CLI scripts, etc., are not
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adversely affected by this bookkeeping.
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With a long-running Python process performing dynamic signal routing
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with high volume- e.g. connecting to function closures, "senders" are
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all unique object instances, and doing all of this over and over- you
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may see memory usage will grow due to extraneous bookeeping. (An empty
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set() for each stale sender/receiver pair.)
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This method will prune that bookeeping away, with the caveat that such
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pruning is not threadsafe. The risk is that cleanup of a fully
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disconnected receiver/sender pair occurs while another thread is
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connecting that same pair. If you are in the highly dynamic, unique
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receiver/sender situation that has lead you to this method, that
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failure mode is perhaps not a big deal for you.
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"""
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for mapping in (self._by_sender, self._by_receiver):
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for _id, bucket in list(mapping.items()):
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if not bucket:
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mapping.pop(_id, None)
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def _clear_state(self):
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"""Throw away all signal state. Useful for unit tests."""
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self._weak_senders.clear()
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self.receivers.clear()
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self._by_sender.clear()
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self._by_receiver.clear()
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receiver_connected = Signal("""\
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Sent by a :class:`Signal` after a receiver connects.
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:argument: the Signal that was connected to
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:keyword receiver_arg: the connected receiver
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:keyword sender_arg: the sender to connect to
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:keyword weak_arg: true if the connection to receiver_arg is a weak reference
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.. deprecated:: 1.2
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As of 1.2, individual signals have their own private
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:attr:`~Signal.receiver_connected` and
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:attr:`~Signal.receiver_disconnected` signals with a slightly simplified
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call signature. This global signal is planned to be removed in 1.6.
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""")
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class NamedSignal(Signal):
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"""A named generic notification emitter."""
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def __init__(self, name, doc=None):
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Signal.__init__(self, doc)
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#: The name of this signal.
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self.name = name
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def __repr__(self):
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base = Signal.__repr__(self)
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return "%s; %r>" % (base[:-1], self.name)
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class Namespace(dict):
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"""A mapping of signal names to signals."""
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def signal(self, name, doc=None):
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"""Return the :class:`NamedSignal` *name*, creating it if required.
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Repeated calls to this function will return the same signal object.
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"""
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try:
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return self[name]
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except KeyError:
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return self.setdefault(name, NamedSignal(name, doc))
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class WeakNamespace(WeakValueDictionary):
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"""A weak mapping of signal names to signals.
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Automatically cleans up unused Signals when the last reference goes out
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of scope. This namespace implementation exists for a measure of legacy
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compatibility with Blinker <= 1.2, and may be dropped in the future.
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.. versionadded:: 1.3
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"""
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def signal(self, name, doc=None):
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"""Return the :class:`NamedSignal` *name*, creating it if required.
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Repeated calls to this function will return the same signal object.
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"""
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try:
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return self[name]
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except KeyError:
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return self.setdefault(name, NamedSignal(name, doc))
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signal = Namespace().signal
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