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These regular expressions ( regexes from now on) match an HTML href s URL and the text contained in <h1> and <h2> header tags (Regular expressions are covered in 13; understanding them is not essential to understanding this example)
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receiver = reporter() matchers = (regex_matcher(receiver, URL_RE), regex_matcher(receiver, H1_RE), regex_matcher(receiver, H2_RE))
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Since coroutines always have a yield expression, they are generators So although here we create a tuple of matcher coroutines, in effect we are creating a tuple of generators Each regex_matcher() is a coroutine that takes a receiver function (itself a coroutine) and a regex to match Whenever the matcher matches it sends the match to the receiver
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@coroutine def regex_matcher(receiver, regex): while True: text = (yield) for match in regexfinditer(text): receiversend(match)
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The matcher starts by entering an in nite loop and immediately suspends execution waiting for the yield expression to return a text to apply the regex to Once the text is received, the matcher iterates over every match it makes, sending each one to the receiver Once the matching has nished the coroutine loops back to the yield and again suspends waiting for more text There is one tiny problem with the (undecorated) matcher when it is rst created it should commence execution so that it advances to the yield ready to receive its rst text We could do this by calling the built-in next() function on each coroutine we create before sending it any data But for convenience we have created the @coroutine decorator to do this for us
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def coroutine(function): @functoolswraps(function) def wrapper(*args, **kwargs): generator = function(*args, **kwargs) next(generator) return generator return wrapper
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The @coroutine decorator takes a coroutine function, and calls the built-in next() function on it this causes the function to be executed up to the rst yield expression, ready to receive data
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Now that we have seen the matcher coroutine we will look at how the matchers are used, and then we will look at the reporter() coroutine that receives the matchers outputs
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try: for file in sysargv[1:]: print(file) html = open(file, encoding="utf8")read() for matcher in matchers: matchersend(html) finally: for matcher in matchers: matcherclose() receiverclose()
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The program reads the lenames listed on the command line, and for each one prints the lename and then reads the le s entire text into the html variable using the UTF-8 encoding Then the program iterates over all the matchers (three in this case), and sends the text to each of them Each matcher then proceeds independently, sending each match it makes to the reporter coroutine At the end we call close() on each matcher and on the reporter this terminates them, since otherwise they would continue (suspended) waiting for text (or matches in the case of the reporter) since they contain in nite loops
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@coroutine def reporter(): ignore = frozenset({"stylecss", "faviconpng", "indexhtml"}) while True: match = (yield) if match is not None: groups = matchgroupdict() if "url" in groups and groups["url"] not in ignore: print(" URL:", groups["url"]) elif "h1" in groups: print(" H1: ", groups["h1"]) elif "h2" in groups: print(" H2: ", groups["h2"])
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The reporter() coroutine is used to output results It was created by the statement receiver = reporter() which we saw earlier, and passed as the receiver argument to each of the matchers The reporter() waits (is suspended) until a match is sent to it, then it prints the match s details, and then it waits again, in an endless loop stopping only if close() is called on it Using coroutines like this may produce performance bene ts, but does require us to adopt a somewhat different way of thinking about processing
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Sometimes it is useful to create data processing pipelines A pipeline is simply the composition of one or more functions where data items are sent to the rst function, which then either discards the item ( lters it out) or passes it on to the next function (either as is or transformed in some way) The second function receives the item from the rst function and repeats the process, discarding or passing on the item (possibly transformed in a different way) to the next function, and so on Items that reach the end are then output in some way
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