# Examples

Here are a bunch of small example programs that use Eventlet. All of these examples can be found in the `examples` directory of a source copy of Eventlet.

## Web Crawler

`examples/webcrawler.py`

    #!/usr/bin/env python
    """
    This is a simple web "crawler" that fetches a bunch of urls using a pool to
    control the number of outbound connections. It has as many simultaneously open
    connections as coroutines in the pool.

    The prints in the body of the fetch function are there to demonstrate that the
    requests are truly made in parallel.
    """
    import eventlet
    from eventlet.green.urllib.request import urlopen

    urls = [
        "https://www.google.com/intl/en_ALL/images/logo.gif",
        "http://python.org/images/python-logo.gif",
        "http://us.i1.yimg.com/us.yimg.com/i/ww/beta/y3.gif",
    ]

    def fetch(url):
        print("opening", url)
        body = urlopen(url).read()
        print("done with", url)
        return url, body

    pool = eventlet.GreenPool(200)
    for url, body in pool.imap(fetch, urls):
        print("got body from", url, "of length", len(body))

## WSGI Server

`examples/wsgi.py`

    """This is a simple example of running a wsgi application with eventlet.
    For a more fully-featured server which supports multiple processes,
    multiple threads, and graceful code reloading, see:

    http://pypi.python.org/pypi/Spawning/
    """

    import eventlet
    from eventlet import wsgi

    def hello_world(env, start_response):
        if env['PATH_INFO'] != '/':
            start_response('404 Not Found', [('Content-Type', 'text/plain')])
            return ['Not Found\r\n']
        start_response('200 OK', [('Content-Type', 'text/plain')])
        return ['Hello, World!\r\n']

    wsgi.server(eventlet.listen(('', 8090)), hello_world)

## Echo Server

`examples/echoserver.py`

    #! /usr/bin/env python
    """\
    Simple server that listens on port 6000 and echos back every input to
    the client.  To try out the server, start it up by running this file.

    Connect to it with:
      telnet localhost 6000

    You terminate your connection by terminating telnet (typically Ctrl-]
    and then 'quit')
    """

    import eventlet

    def handle(fd):
        print("client connected")
        while True:
            # pass through every non-eof line
            x = fd.readline()
            if not x:
                break
            fd.write(x)
            fd.flush()
            print("echoed", x, end=' ')
        print("client disconnected")

    print("server socket listening on port 6000")
    server = eventlet.listen(('0.0.0.0', 6000))
    pool = eventlet.GreenPool()
    while True:
        try:
            new_sock, address = server.accept()
            print("accepted", address)
            pool.spawn_n(handle, new_sock.makefile('rw'))
        except (SystemExit, KeyboardInterrupt):
            break

## Socket Connect

`examples/connect.py`

    """Spawn multiple workers and collect their results.

    Demonstrates how to use the eventlet.green.socket module.
    """

    import eventlet
    from eventlet.green import socket

    def geturl(url):
        c = socket.socket()
        ip = socket.gethostbyname(url)
        c.connect((ip, 80))
        print('%s connected' % url)
        c.sendall(b'GET /\r\n\r\n')
        return c.recv(1024)

    urls = ['www.google.com', 'www.yandex.ru', 'www.python.org']
    pile = eventlet.GreenPile()
    for x in urls:
        pile.spawn(geturl, x)

    # note that the pile acts as a collection of return values from the functions
    # if any exceptions are raised by the function they'll get raised here
    for url, result in zip(urls, pile):
        print('%s: %s' % (url, repr(result)[:50]))

## Multi-User Chat Server

`examples/chat_server.py`

This is a little different from the echo server, in that it broadcasts the messages to all participants, not just the sender.

    import eventlet
    from eventlet.green import socket

    PORT = 3001
    participants = set()

    def read_chat_forever(writer, reader):
        line = reader.readline()
        while line:
            print("Chat:", line.strip())
            for p in participants:
                try:
                    if p is not writer:  # Don't echo
                        p.write(line)
                        p.flush()
                except OSError as e:
                    # ignore broken pipes, they just mean the participant
                    # closed its connection already
                    if e[0] != 32:
                        raise
            line = reader.readline()
        participants.remove(writer)
        print("Participant left chat.")

    try:
        print("ChatServer starting up on port %s" % PORT)
        server = eventlet.listen(('0.0.0.0', PORT))
        while True:
            new_connection, address = server.accept()
            print("Participant joined chat.")
            new_writer = new_connection.makefile('w')
            participants.add(new_writer)
            eventlet.spawn_n(read_chat_forever,
                             new_writer,
                             new_connection.makefile('r'))
    except (KeyboardInterrupt, SystemExit):
        print("ChatServer exiting.")

## Feed Scraper

`examples/feedscraper.py`

This example requires Feedparser to be installed or on the PYTHONPATH.

    """A simple web server that accepts POSTS containing a list of feed urls,
    and returns the titles of those feeds.
    """
    import eventlet
    feedparser = eventlet.import_patched('feedparser')

    # the pool provides a safety limit on our concurrency
    pool = eventlet.GreenPool()

    def fetch_title(url):
        d = feedparser.parse(url)
        return d.feed.get('title', '')

    def app(environ, start_response):
        if environ['REQUEST_METHOD'] != 'POST':
            start_response('403 Forbidden', [])
            return []

        # the pile collects the result of a concurrent operation -- in this case,
        # the collection of feed titles
        pile = eventlet.GreenPile(pool)
        for line in environ['wsgi.input'].readlines():
            url = line.strip()
            if url:
                pile.spawn(fetch_title, url)
        # since the pile is an iterator over the results,
        # you can use it in all sorts of great Pythonic ways
        titles = '\n'.join(pile)
        start_response('200 OK', [('Content-type', 'text/plain')])
        return [titles]

    if __name__ == '__main__':
        from eventlet import wsgi
        wsgi.server(eventlet.listen(('localhost', 9010)), app)

## Port Forwarder

`examples/forwarder.py`

    """ This is an incredibly simple port forwarder from port 7000 to 22 on
    localhost.  It calls a callback function when the socket is closed, to
    demonstrate one way that you could start to do interesting things by
    starting from a simple framework like this.
    """

    import eventlet

    def closed_callback():
        print("called back")

    def forward(source, dest, cb=lambda: None):
        """Forwards bytes unidirectionally from source to dest"""
        while True:
            d = source.recv(32384)
            if d == '':
                cb()
                break
            dest.sendall(d)

    listener = eventlet.listen(('localhost', 7000))
    while True:
        client, addr = listener.accept()
        server = eventlet.connect(('localhost', 22))
        # two unidirectional forwarders make a bidirectional one
        eventlet.spawn_n(forward, client, server, closed_callback)
        eventlet.spawn_n(forward, server, client)

## Recursive Web Crawler

`examples/recursive_crawler.py`

This is an example recursive web crawler that fetches linked pages from a seed url.

    """This is a recursive web crawler.  Don't go pointing this at random sites;
    it doesn't respect robots.txt and it is pretty brutal about how quickly it
    fetches pages.

    The code for this is very short; this is perhaps a good indication
    that this is making the most effective use of the primitves at hand.
    The fetch function does all the work of making http requests,
    searching for new urls, and dispatching new fetches.  The GreenPool
    acts as sort of a job coordinator (and concurrency controller of
    course).
    """
    from eventlet.green.urllib.request import urlopen
    import eventlet
    import re

    # http://daringfireball.net/2009/11/liberal_regex_for_matching_urls
    url_regex = re.compile(r'\b(([\w-]+://?|www[.])[^\s()<>]+(?:\([\w\d]+\)|([^[:punct:]\s]|/)))')

    def fetch(url, seen, pool):
        """Fetch a url, stick any found urls into the seen set, and
        dispatch any new ones to the pool."""
        print("fetching", url)
        data = ''
        with eventlet.Timeout(5, False):
            data = urlopen(url).read().decode()
        for url_match in url_regex.finditer(data):
            new_url = url_match.group(0)
            # only send requests to eventlet.net so as not to destroy the internet
            if new_url not in seen and 'eventlet.net' in new_url:
                seen.add(new_url)
                # while this seems stack-recursive, it's actually not:
                # spawned greenthreads start their own stacks
                pool.spawn_n(fetch, new_url, seen, pool)

    def crawl(start_url):
        """Recursively crawl starting from *start_url*.  Returns a set of
        urls that were found."""
        pool = eventlet.GreenPool()
        seen = set()
        fetch(start_url, seen, pool)
        pool.waitall()
        return seen

    seen = crawl("http://eventlet.net")
    print("I saw these urls:")
    print("\n".join(seen))

## Producer Consumer Web Crawler

`examples/producer_consumer.py`

This is an example implementation of the producer/consumer pattern as well as being identical in functionality to the recursive web crawler.

    """This is a recursive web crawler.  Don't go pointing this at random sites;
    it doesn't respect robots.txt and it is pretty brutal about how quickly it
    fetches pages.

    This is a kind of "producer/consumer" example; the fetch function produces
    jobs, and the GreenPool itself is the consumer, farming out work concurrently.
    It's easier to write it this way rather than writing a standard consumer loop;
    GreenPool handles any exceptions raised and arranges so that there's a set
    number of "workers", so you don't have to write that tedious management code
    yourself.
    """
    from eventlet.green.urllib.request import urlopen
    from urllib.request import urlopen
    import eventlet
    import re

    # http://daringfireball.net/2009/11/liberal_regex_for_matching_urls
    url_regex = re.compile(r'\b(([\w-]+://?|www[.])[^\s()<>]+(?:\([\w\d]+\)|([^[:punct:]\s]|/)))')

    def fetch(url, outq):
        """Fetch a url and push any urls found into a queue."""
        print("fetching", url)
        data = ''
        with eventlet.Timeout(5, False):
            data = urlopen(url).read().decode()
        for url_match in url_regex.finditer(data):
            new_url = url_match.group(0)
            outq.put(new_url)

    def producer(start_url):
        """Recursively crawl starting from *start_url*.  Returns a set of
        urls that were found."""
        pool = eventlet.GreenPool()
        seen = set()
        q = eventlet.Queue()
        q.put(start_url)
        # keep looping if there are new urls, or workers that may produce more urls
        while True:
            while not q.empty():
                url = q.get()
                # limit requests to eventlet.net so we don't crash all over the internet
                if url not in seen and 'eventlet.net' in url:
                    seen.add(url)
                    pool.spawn_n(fetch, url, q)
            pool.waitall()
            if q.empty():
                break

        return seen

    seen = producer("http://eventlet.net")
    print("I saw these urls:")
    print("\n".join(seen))

## Websocket Server Example

`examples/websocket.py`

This exercises some of the features of the websocket server implementation.

    import eventlet
    from eventlet import wsgi
    from eventlet import websocket

    # demo app
    import os
    import random

    @websocket.WebSocketWSGI
    def handle(ws):
        """  This is the websocket handler function.  Note that we
        can dispatch based on path in here, too."""
        if ws.path == '/echo':
            while True:
                m = ws.wait()
                if m is None:
                    break
                ws.send(m)

        elif ws.path == '/data':
            for i in range(10000):
                ws.send("0 %s %s\n" % (i, random.random()))
                eventlet.sleep(0.1)

    def dispatch(environ, start_response):
        """ This resolves to the web page or the websocket depending on
        the path."""
        if environ['PATH_INFO'] == '/data':
            return handle(environ, start_response)
        else:
            start_response('200 OK', [('content-type', 'text/html')])
            return [open(os.path.join(
                         os.path.dirname(__file__),
                         'websocket.html')).read()]

    if __name__ == "__main__":
        # run an example app from the command line
        listener = eventlet.listen(('127.0.0.1', 7000))
        print("\nVisit http://localhost:7000/ in your websocket-capable browser.\n")
        wsgi.server(listener, dispatch)

## Websocket Multi-User Chat Example

`examples/websocket_chat.py`

This is a mashup of the websocket example and the multi-user chat example, showing how you can do the same sorts of things with websockets that you can do with regular sockets.

    import os

    import eventlet
    from eventlet import wsgi
    from eventlet import websocket

    PORT = 7000

    participants = set()

    @websocket.WebSocketWSGI
    def handle(ws):
        participants.add(ws)
        try:
            while True:
                m = ws.wait()
                if m is None:
                    break
                for p in participants:
                    p.send(m)
        finally:
            participants.remove(ws)

    def dispatch(environ, start_response):
        """Resolves to the web page or the websocket depending on the path."""
        if environ['PATH_INFO'] == '/chat':
            return handle(environ, start_response)
        else:
            start_response('200 OK', [('content-type', 'text/html')])
            html_path = os.path.join(os.path.dirname(__file__), 'websocket_chat.html')
            return [open(html_path).read() % {'port': PORT}]

    if __name__ == "__main__":
        # run an example app from the command line
        listener = eventlet.listen(('127.0.0.1', PORT))
        print("\nVisit http://localhost:7000/ in your websocket-capable browser.\n")
        wsgi.server(listener, dispatch)