transpub/deferred.py

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from functools import partial
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class Deferred(object):
"""
Represents a delayed computation. This is a more elegant way to deal with
callbacks.
A Deferred object can be thought of as a computation whose value is yet to
be determined. We can manipulate the Deferred as if it where a regular
value by using the then method. Computations dependent on the Deferred will
only proceed when the run method is called.
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Attributes of a Deferred can be accessed directly as methods. The result of
calling these functions will be Deferred.
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Example:
image = Deferred()
getImageWithCallback(image.run)
image.then(displayFunc)
colors = Deferred()
colors.append('blue')
colors.then(print)
colors.run(['red', 'green']) #=> ['red', 'green', 'blue']
"""
def __init__(self):
self.subscribers = []
self.computed = False
self.args = None
self.kwargs = None
def run(self, *args, **kwargs):
"""
Give a value to the deferred. Calling this method more than once will
result in a DeferredHasValue exception to be raised.
"""
if self.computed:
raise DeferredHasValue("Deferred object already has a value.")
else:
self.args = args
self.kwargs = kwargs
for func, deferred in self.subscribers:
deferred.run(func(*args, **kwargs))
self.computed = True
def then(self, func):
"""
Apply func to Deferred value. Returns a Deferred whose value will be
the result of applying func.
"""
result = Deferred()
if self.computed:
result.run(func(*self.args, **self.kwargs))
else:
self.subscribers.append((func, result))
return result
def arg(self, n):
"""
Returns the nth positional argument of a deferred as a deferred
Args:
n - the index of the positional argument
"""
def helper(*args, **kwargs):
return args[n]
return self.then(helper)
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def when(self, func, *args, **kwargs):
""" Calls when func(*args, **kwargs) when deferred gets a value """
def helper(*args2, **kwargs2):
func(*args, **kwargs)
self.then(helper)
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def __getattr__(self, method_name):
return getattr(Then(self), method_name)
class Then(object):
"""
Allows you to call methods on a Deferred.
Example:
colors = Deferred()
Then(colors).append('blue')
colors.run(['red', 'green'])
colors.then(print) #=> ['red', 'green', 'blue']
"""
def __init__(self, deferred):
self.deferred = deferred
def __getattr__(self, name):
def tryCall(obj, *args, **kwargs):
if callable(obj):
return obj(*args, **kwargs)
else:
return obj
def helper(*args, **kwargs):
func = (lambda x: tryCall(getattr(x, name), *args, **kwargs))
return self.deferred.then(func)
return helper
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def call(func, *args, **kwargs):
"""
Call a function with deferred arguments
Example:
colors = Deferred()
colors.append('blue')
colors.run(['red', 'green'])
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call(print, colors) #=> ['red', 'green', 'blue']
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call(print, 'hi', colors) #=> hi ['red', 'green', 'blue']
"""
for i, c in enumerate(args):
if isinstance(c, Deferred):
# Function without deferred arguments
normalfunc = partial(func, *args[:i])
# Function with deferred and possibly deferred arguments
def restfunc(*arg2, **kwarg2):
apply_deferred = partial(normalfunc, *arg2, **kwarg2)
return call(apply_deferred, *args[i + 1:], **kwargs)
return c.then(restfunc)
items = kwargs.items()
for i, (k, v) in enumerate(items):
if isinstance(v, Deferred):
# Function without deferred arguments
normalfunc = partial(func, *args, **dict(items[:i]))
# Function with deferred and possibly deferred arguments
def restfunc2(*arg2, **kwarg2):
apply_deferred = partial(normalfunc, *arg2, **kwarg2)
return call(apply_deferred, **dict(items[i + 1:]))
return c.then(restfunc2)
# No items deferred
return func(*args, **kwargs)
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class DeferredHasValue(Exception):
def __init__(self, string):
super(DeferredHasValue, self).__init__(string)