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