"""
Created on Mon Mar 11 18:39:13 2013
@author Vaclav Petras <wenzeslaus gmail.com>
"""
from grass.pydispatch import dispatcher
def _islambda(function):
"""
Tests if object is a lambda function.
Should work on the most of Python implementations where name of lambda
function is not unique.
>>> mylambda = lambda x: x*x
>>> _islambda(mylambda)
True
>>> _islambda(_islambda)
False
"""
return (
isinstance(function, type(lambda: None))
and function.__name__ == (lambda: None).__name__
)
[docs]class Signal(object):
"""
The signal object is created usually as a instance attribute.
However, it can be created anywhere.
>>> signal1 = Signal('signal1')
The function has to be connected to a signal in order to be called when
the signal is emitted. The connection can be done where the function is
defined (e. g., a class) but also on some other place, typically,
user of a class connects some signal to the method of some other class.
>>> def handler1():
... print "from handler1"
>>> signal1.connect(handler1)
Emitting of the signal is done usually only in the class which has the
signal as a instance attribute. Again, generally, it can be emitted
anywhere.
>>> signal1.emit()
from handler1
The signal can have parameters. These parameters are specified when
emitting but should be documented together with the signal (e.g., in the
class documentation). Parameters should be keyword arguments and handlers
must use these names (if the names cannot be the same, lambda function
can be used to overcome this problem).
>>> signal2 = Signal('signal2')
>>> def handler2(text):
... print "handler2: %s" % text
>>> signal2.connect(handler2)
>>> signal2.emit(text="Hello")
handler2: Hello
Do not emit the same signal with different parameters when emitting at
different places.
A handler is the standard function, lambda function, method or any other
callable object.
>>> import sys
>>> signal2.connect(lambda text:
... sys.stdout.write('lambda handler: %s\\n' % text))
>>> signal2.emit(text="Hi")
handler2: Hi
lambda handler: Hi
The handler function can have only some of the signal parameters or no
parameters at all even if the signal has some.
>>> def handler3():
... print "from handler3"
>>> signal2.connect(handler3)
>>> signal2.emit(text="Ciao")
handler2: Ciao
lambda handler: Ciao
from handler3
It is possible to use signal as a handler. By this, signals can be
forwarded from one object to another. In other words, one object can
expose signal of some object.
>>> signal3 = Signal('signal3')
>>> signal3.connect(handler3)
>>> signal1.connect(signal3)
>>> signal1.emit()
from handler1
from handler3
It is possible to disconnect a particular handler.
>>> signal3.disconnect(handler3)
>>> signal1.emit()
from handler1
>>> signal2.disconnect(handler2)
>>> signal2.disconnect(handler3)
>>> signal2.emit(text='Hello')
lambda handler: Hello
"""
# TODO: use the name for debugging
def __init__(self, name):
"""Creates a signal object.
The parameter name is used for debugging.
"""
self._name = name
[docs] def connect(self, handler, weak=None):
"""
Connects handler to a signal.
Typically, a signal is defined in some class and the user of this
class connects to the signal::
from module import SomeClass
...
self.someObject = SomeClass()
self.someObject.connect(self.someMethod)
Usually, it is not needed to set the weak parameter. This method
creates weak references for all handlers but for lambda functions, it
automatically creates (standard) references (otherwise, lambdas would be
garbage collected. If you want to force some behaviour, specify the
weak parameter.
>>> signal1 = Signal('signal1')
>>> import sys
>>> signal1.connect(lambda: sys.stdout.write('will print\\n'))
>>> signal1.connect(lambda: sys.stdout.write('will print\\n'), weak=False)
>>> signal1.connect(lambda: sys.stdout.write('will not print'), weak=True)
>>> signal1.emit()
will print
will print
"""
if weak is None:
if _islambda(handler):
weak = False
else:
weak = True
dispatcher.connect(receiver=handler, signal=self, weak=weak)
[docs] def disconnect(self, handler, weak=True):
"""
Disconnects a specified handler.
It is not necessary to disconnect object when it is deleted.
Underlying PyDispatcher will take care of connections to deleted
objects.
>>> signal1 = Signal('signal1')
>>> import sys
>>> signal1.connect(sys.stdout.write)
>>> signal1.disconnect(sys.stdout.write)
The weak parameter of must have the same value as for connection.
If you not specified the parameter when connecting,
you don't have to specify it when disconnecting.
Disconnecting the not-connected handler will result in error.
>>> signal1.disconnect(sys.stdout.flush) #doctest: +ELLIPSIS
Traceback (most recent call last):
DispatcherKeyError: 'No receivers found for signal ...'
Disconnecting the non-exiting or unknown handler will result in error.
>>> signal1.disconnect(some_function)
Traceback (most recent call last):
NameError: name 'some_function' is not defined
>>> signal1.emit()
"""
dispatcher.disconnect(receiver=handler, signal=self, weak=weak)
# TODO: remove args?, make it work for args?
# TODO: where to put documentation
[docs] def emit(self, *args, **kwargs):
"""
Emits the signal which means that all connected handlers will be
called.
It is advised to have signals as instance attributes and emit signals
only in the class which owns the signal::
class Abc(object):
def __init__(self):
self.colorChanged = Signal('Abc.colorChanged')
...
def setColor(self, color):
...
self.colorChanged.emit(oldColor=self.Color, newColor=color)
...
Documentation of an signal should be placed to the class documentation
or to the code (this need to be more specified).
Calling a signal from outside the class is usually not good
practice. The only case when it is permitted is when signal is the part
of some globaly shared object and permission to emit is stayed in the
documentation.
The parameters of the emit function must be the same as the parameters
of the handlers. However, handler can omit some parameters.
The associated parameters shall be documented for each Signal instance.
Use only keyword arguments when emitting.
>>> signal1 = Signal('signal1')
>>> def mywrite(text):
... print text
>>> signal1.connect(mywrite)
>>> signal1.emit(text='Hello')
Hello
>>> signal1.emit()
Traceback (most recent call last):
TypeError: mywrite() takes exactly 1 argument (0 given)
>>> signal1.emit('Hello')
Traceback (most recent call last):
TypeError: send() got multiple values for keyword argument 'signal'
"""
dispatcher.send(signal=self, *args, **kwargs)
# TODO: remove args?
def __call__(self, *args, **kwargs):
"""Allows emitting signal with function call syntax.
It allows handling signal as a function or other callable object.
So, the signal can be in the list of functions or can be connected as
a handler for another signal.
However, it is strongly recommended to use emit method for direct
signal emitting.
The use of emit method is more explicit than the function call
and thus it it clear that we are using signal.
>>> signal1 = Signal('signal1')
>>> def mywrite(text):
... print text
>>> signal1.connect(mywrite)
>>> functions = [signal1, lambda text: mywrite(text + '!')]
>>> for function in functions:
... function(text='text')
text
text!
The other reason why the function call should not by used when it is
possible to use emit method is that this function does ugly hack to
enable calling as a signal handler. The signal parameter is deleted
when it is in named keyword arguments. As a consequence, when the
signal is emitted with the signal parameter (which is a very bad
name for parameter when using signals), the error is much more readable
when using emit than function call. Concluding remark is that
emit behaves more predictable.
>>> signal1.emit(signal='Hello')
Traceback (most recent call last):
TypeError: send() got multiple values for keyword argument 'signal'
>>> signal1(signal='Hello')
Traceback (most recent call last):
TypeError: mywrite() takes exactly 1 argument (0 given)
"""
if "signal" in kwargs:
del kwargs["signal"]
self.emit(*args, **kwargs)
if __name__ == "__main__":
import doctest
doctest.testmod()
