Create welcome-message api example.

zulip/zulip/examples/welcome-message

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+#!/usr/bin/env python
+
+import zulip
+from typing import Any, List, Dict
+
+welcome_text = 'Hello {}, Welcome to Zulip!\n \
+* The first thing you should do is to install the development environment. \
+We recommend following the vagrant setup as it is well documented and used \
+by most of the contributors. If you face any trouble during installation \
+post it in #development help\n \
+* If you have not used Zulip before play with Zulip for a while. See how \
+is Zulip diffrent from other chat applications. We would love to hear your \
+first impressions about Zulip. Post it in #feedback . We have mobile apps \
+as well as desktop apps too. You should try them too if you have time :)\n \
+* Once you are familarized with Zulip a bit you can start contributing. Some \
+of the main projects you can contribute to are Zulip  \
+[server](https://github.com/zulip/zulip),  \
+[mobile app](https://github.com/zulip/zulip-mobile),  \
+[desktop](https://github.com/zulip/zulip-electron) app etc. We even have \
+a [bot](https://github.com/zulip/zulipbot) that you can contribute to!!\n \
+* We host our source code on GitHub. If you are not familiar with Git or \
+GitHub checkout [this](http://zulip.readthedocs.io/en/latest/git-guide.html) \
+guide. You don\'t have to learn everything but please go through it and learn \
+the basics. We are here to help you if you are having any trouble. Post your \
+questions in #git help . \
+* Once you have completed these steps you can start contributing. You \
+should start with issues labelled \
+[bite-size](https://github.com/zulip/zulip/issues?q=is%3Aissue+is%3Aopen+label%3A). \
+Bite-size issues are meant for new contributors and can be solved easily as \
+compared to other issues. Currently we have bite-size labels only in Zulip server \
+but if you want a bite size issue for mobile or electron feel free post in #mobile \
+or #electron .\n \
+* Solving the first issue can be difficult. The key is to not give up. If you spend \
+enough time on the issue you should be able to solve it no matter what.\n \
+* Use `grep` command when you can\'t figure out what files to change :) For example \
+if you want know what files to modify in order to change Invite more users to Add \
+more users which you can see below the user status list, grep for "Invite more \
+users" in terminal.\n \
+* If you are stuck with something and can\'t figure out what to do you can ask \
+for help in #development help . But make sure that you tried your best to figure \
+out the issue by yourself\n \
+* If you are here for #Outreachy 2017-2018 or #GSoC don\'t worry much about \
+whether you will get selected or not. You will learn a lot contributing to \
+Zulip in course of next few months and if you do a good job at that you \
+will get selected too :)\n \
+* Most important of all welcome to the Zulip family :octopus:'
+
+# These streams will cause the message to be sent
+streams_to_watch = ['new members']
+
+# These streams will cause anyone who sends a message there to be removed from the watchlist
+streams_to_cancel = ['development help']
+
+def get_watchlist():
+    # type: () -> List[Any]
+    storage = client.get_storage()
+    return list(storage['storage'].values())
+
+def set_watchlist(watchlist):
+    # type: (List[str]) -> None
+    client.update_storage({'storage': dict(enumerate(watchlist))})
+
+def handle_event(event):
+    # type: (Dict[str, Any]) -> None
+    try:
+        if event['type'] == 'realm_user' and event['op'] == 'add':
+            watchlist = get_watchlist()
+            watchlist.append(event['person']['email'])
+            set_watchlist(watchlist)
+            return
+        if event['type'] == 'message':
+            stream = event['message']['display_recipient']
+            if stream not in streams_to_watch and stream not in streams_to_cancel:
+                return
+            watchlist = get_watchlist()
+            if event['message']['sender_email'] in watchlist:
+                watchlist.remove(event['message']['sender_email'])
+                if stream not in streams_to_cancel:
+                    client.send_message({
+                        'type': 'private',
+                        'to': event['message']['sender_email'],
+                        'content': welcome_text.format(event['message']['sender_short_name'])
+                    })
+                set_watchlist(watchlist)
+                return
+    except Exception as err:
+        print(err)
+
+
+def start_event_handler():
+    # type: () -> None
+    print("Starting event handler...")
+    client.call_on_each_event(handle_event, event_types=['realm_user', 'message'])
+
+client = zulip.Client()
+start_event_handler()