134 lines
3.9 KiB
Python
134 lines
3.9 KiB
Python
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# -*- test-case-name: twisted.test.test_context -*-
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# Copyright (c) Twisted Matrix Laboratories.
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# See LICENSE for details.
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"""
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Dynamic pseudo-scoping for Python.
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Call functions with context.call({key: value}, func); func and
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functions that it calls will be able to use 'context.get(key)' to
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retrieve 'value'.
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This is thread-safe.
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"""
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from __future__ import division, absolute_import
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from threading import local
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defaultContextDict = {}
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setDefault = defaultContextDict.__setitem__
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class ContextTracker:
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"""
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A L{ContextTracker} provides a way to pass arbitrary key/value data up and
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down a call stack without passing them as parameters to the functions on
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that call stack.
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This can be useful when functions on the top and bottom of the call stack
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need to cooperate but the functions in between them do not allow passing the
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necessary state. For example::
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from twisted.python.context import call, get
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def handleRequest(request):
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call({'request-id': request.id}, renderRequest, request.url)
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def renderRequest(url):
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renderHeader(url)
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renderBody(url)
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def renderHeader(url):
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return "the header"
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def renderBody(url):
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return "the body (request id=%r)" % (get("request-id"),)
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This should be used sparingly, since the lack of a clear connection between
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the two halves can result in code which is difficult to understand and
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maintain.
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@ivar contexts: A C{list} of C{dict}s tracking the context state. Each new
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L{ContextTracker.callWithContext} pushes a new C{dict} onto this stack
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for the duration of the call, making the data available to the function
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called and restoring the previous data once it is complete..
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"""
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def __init__(self):
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self.contexts = [defaultContextDict]
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def callWithContext(self, newContext, func, *args, **kw):
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"""
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Call C{func(*args, **kw)} such that the contents of C{newContext} will
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be available for it to retrieve using L{getContext}.
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@param newContext: A C{dict} of data to push onto the context for the
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duration of the call to C{func}.
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@param func: A callable which will be called.
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@param *args: Any additional positional arguments to pass to C{func}.
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@param **kw: Any additional keyword arguments to pass to C{func}.
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@return: Whatever is returned by C{func}
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@raise: Whatever is raised by C{func}.
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"""
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self.contexts.append(newContext)
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try:
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return func(*args,**kw)
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finally:
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self.contexts.pop()
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def getContext(self, key, default=None):
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"""
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Retrieve the value for a key from the context.
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@param key: The key to look up in the context.
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@param default: The value to return if C{key} is not found in the
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context.
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@return: The value most recently remembered in the context for C{key}.
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"""
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for ctx in reversed(self.contexts):
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try:
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return ctx[key]
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except KeyError:
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pass
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return default
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class ThreadedContextTracker(object):
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def __init__(self):
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self.storage = local()
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def currentContext(self):
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try:
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return self.storage.ct
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except AttributeError:
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ct = self.storage.ct = ContextTracker()
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return ct
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def callWithContext(self, ctx, func, *args, **kw):
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return self.currentContext().callWithContext(ctx, func, *args, **kw)
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def getContext(self, key, default=None):
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return self.currentContext().getContext(key, default)
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def installContextTracker(ctr):
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global theContextTracker
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global call
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global get
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theContextTracker = ctr
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call = theContextTracker.callWithContext
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get = theContextTracker.getContext
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installContextTracker(ThreadedContextTracker())
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