Introduction
Welcome to this journey into the world of web servers and the Flask framework! In the previous weeks, you’ve successfully set up a web server using GitHub Pages, converting Jupyter Notebooks into Markdown for a seamless online presentation. Today, we’ll take that knowledge to the next level as we dive into creating your very own web server using Flask.
Understanding Web Servers
What is a Web Server?
Traditionally, we had librarians at libraries that would help you find books or information. Today in the digital world, thousands upon thousands of home pages, search engines, and digital archives have been built using web servers.
GitHub Pages vs. Flask
You’ve already experienced a form of web server through GitHub Pages. Think of GitHub Pages as a library that has established rules for publishing Markdown notes and Jupyter Notebooks neatly on a bookshelf.
Now, let’s introduce Flask, your personal web server. Flask can create and manage any type of content, including customizing everything according to your preferences, and even serve additional information (like a database with APIs).
The Flask Framework Flask is a micro web framework written in Python. It’s designed to be minimal and easy to use, making it perfect for building web applications, APIs, and, yes, even your web server. Today, we will start with the basics of Flask and see how it empowers you to create and manage web content.
Our Goals for Today
Here’s what we’ll accomplish in this session:
- Create a minimal Flask server.
- Explore the Python/Flask process.
- Access data from our Flask server using Python.
- Access data from our Flask server using JavaScript.
- Learn how to stop the Python/Flask process gracefully.
Note: Jupyter magic commmand %%python --bg that follows runs the server in background. This enables us to continue interacting with the subsequent Notebook cells.
# %%python --bg
from flask import Flask, jsonify
from flask_cors import CORS
# initialize a flask application (app)
app = Flask(__name__)
CORS(app, supports_credentials=True, origins='*') # Allow all origins (*)
# ... your existing Flask
# add an api endpoint to flask app
@app.route('/api/data')
def get_data():
# start a list, to be used like a information database
InfoDb = []
# add a row to list, an Info record
InfoDb.append({
"FirstName": "John",
"LastName": "Mortensen",
"DOB": "October 21",
"Residence": "San Diego",
"Email": "jmortensen@powayusd.com",
"Owns_Cars": ["2015-Fusion", "2011-Ranger", "2003-Excursion", "1997-F350", "1969-Cadillac"]
})
# add a row to list, an Info record
InfoDb.append({
"FirstName": "Shane",
"LastName": "Lopez",
"DOB": "February 27",
"Residence": "San Diego",
"Email": "slopez@powayusd.com",
"Owns_Cars": ["2021-Insight"]
})
return jsonify(InfoDb)
# add an HTML endpoint to flask app
@app.route('/')
def say_hello():
html_content = """
<html>
<head>
<title>Hellox</title>
</head>
<body>
<h2>Hello, Aditya!</h2>
</body>
</html>
"""
return html_content
if __name__ == '__main__':
# starts flask server on default port, http://127.0.0.1:5001
app.run(port=5001)
* Serving Flask app '__main__'
* Debug mode: off
INFO:werkzeug:[31m[1mWARNING: This is a development server. Do not use it in a production deployment. Use a production WSGI server instead.[0m
* Running on http://127.0.0.1:5001
INFO:werkzeug:[33mPress CTRL+C to quit[0m
INFO:werkzeug:127.0.0.1 - - [19/Sep/2023 21:26:25] "GET / HTTP/1.1" 200 -
Show Python/Flask process
This script discovers the running flask process
%%script bash
# After app.run(), see the the Python process
lsof -i :5001
# see the the Python app
lsof -i :5001 | awk '/Python/ {print $2}' | xargs ps
Access API with Python
This script extracts data from Web Server.
import requests
res = requests.get('http://127.0.0.1:5001/api/data')
res.json()
---------------------------------------------------------------------------
ConnectionRefusedError Traceback (most recent call last)
~/.local/lib/python3.10/site-packages/urllib3/connection.py in _new_conn(self)
202 try:
--> 203 sock = connection.create_connection(
204 (self._dns_host, self.port),
~/.local/lib/python3.10/site-packages/urllib3/util/connection.py in create_connection(address, timeout, source_address, socket_options)
84 try:
---> 85 raise err
86 finally:
~/.local/lib/python3.10/site-packages/urllib3/util/connection.py in create_connection(address, timeout, source_address, socket_options)
72 sock.bind(source_address)
---> 73 sock.connect(sa)
74 # Break explicitly a reference cycle
ConnectionRefusedError: [Errno 111] Connection refused
The above exception was the direct cause of the following exception:
NewConnectionError Traceback (most recent call last)
~/.local/lib/python3.10/site-packages/urllib3/connectionpool.py in urlopen(self, method, url, body, headers, retries, redirect, assert_same_host, timeout, pool_timeout, release_conn, chunked, body_pos, preload_content, decode_content, **response_kw)
789 # Make the request on the HTTPConnection object
--> 790 response = self._make_request(
791 conn,
~/.local/lib/python3.10/site-packages/urllib3/connectionpool.py in _make_request(self, conn, method, url, body, headers, retries, timeout, chunked, response_conn, preload_content, decode_content, enforce_content_length)
495 try:
--> 496 conn.request(
497 method,
~/.local/lib/python3.10/site-packages/urllib3/connection.py in request(self, method, url, body, headers, chunked, preload_content, decode_content, enforce_content_length)
394 self.putheader(header, value)
--> 395 self.endheaders()
396
/usr/lib/python3.10/http/client.py in endheaders(self, message_body, encode_chunked)
1277 raise CannotSendHeader()
-> 1278 self._send_output(message_body, encode_chunked=encode_chunked)
1279
/usr/lib/python3.10/http/client.py in _send_output(self, message_body, encode_chunked)
1037 del self._buffer[:]
-> 1038 self.send(msg)
1039
/usr/lib/python3.10/http/client.py in send(self, data)
975 if self.auto_open:
--> 976 self.connect()
977 else:
~/.local/lib/python3.10/site-packages/urllib3/connection.py in connect(self)
242 def connect(self) -> None:
--> 243 self.sock = self._new_conn()
244 if self._tunnel_host:
~/.local/lib/python3.10/site-packages/urllib3/connection.py in _new_conn(self)
217 except OSError as e:
--> 218 raise NewConnectionError(
219 self, f"Failed to establish a new connection: {e}"
NewConnectionError: <urllib3.connection.HTTPConnection object at 0x7f42f8b05d80>: Failed to establish a new connection: [Errno 111] Connection refused
The above exception was the direct cause of the following exception:
MaxRetryError Traceback (most recent call last)
~/.local/lib/python3.10/site-packages/requests/adapters.py in send(self, request, stream, timeout, verify, cert, proxies)
485 try:
--> 486 resp = conn.urlopen(
487 method=request.method,
~/.local/lib/python3.10/site-packages/urllib3/connectionpool.py in urlopen(self, method, url, body, headers, retries, redirect, assert_same_host, timeout, pool_timeout, release_conn, chunked, body_pos, preload_content, decode_content, **response_kw)
843
--> 844 retries = retries.increment(
845 method, url, error=new_e, _pool=self, _stacktrace=sys.exc_info()[2]
~/.local/lib/python3.10/site-packages/urllib3/util/retry.py in increment(self, method, url, response, error, _pool, _stacktrace)
514 reason = error or ResponseError(cause)
--> 515 raise MaxRetryError(_pool, url, reason) from reason # type: ignore[arg-type]
516
MaxRetryError: HTTPConnectionPool(host='127.0.0.1', port=5001): Max retries exceeded with url: /api/data (Caused by NewConnectionError('<urllib3.connection.HTTPConnection object at 0x7f42f8b05d80>: Failed to establish a new connection: [Errno 111] Connection refused'))
During handling of the above exception, another exception occurred:
ConnectionError Traceback (most recent call last)
/tmp/ipykernel_92726/2595936190.py in <module>
1 import requests
----> 2 res = requests.get('http://127.0.0.1:5001/api/data')
3 res.json()
~/.local/lib/python3.10/site-packages/requests/api.py in get(url, params, **kwargs)
71 """
72
---> 73 return request("get", url, params=params, **kwargs)
74
75
~/.local/lib/python3.10/site-packages/requests/api.py in request(method, url, **kwargs)
57 # cases, and look like a memory leak in others.
58 with sessions.Session() as session:
---> 59 return session.request(method=method, url=url, **kwargs)
60
61
~/.local/lib/python3.10/site-packages/requests/sessions.py in request(self, method, url, params, data, headers, cookies, files, auth, timeout, allow_redirects, proxies, hooks, stream, verify, cert, json)
587 }
588 send_kwargs.update(settings)
--> 589 resp = self.send(prep, **send_kwargs)
590
591 return resp
~/.local/lib/python3.10/site-packages/requests/sessions.py in send(self, request, **kwargs)
701
702 # Send the request
--> 703 r = adapter.send(request, **kwargs)
704
705 # Total elapsed time of the request (approximately)
~/.local/lib/python3.10/site-packages/requests/adapters.py in send(self, request, stream, timeout, verify, cert, proxies)
517 raise SSLError(e, request=request)
518
--> 519 raise ConnectionError(e, request=request)
520
521 except ClosedPoolError as e:
ConnectionError: HTTPConnectionPool(host='127.0.0.1', port=5001): Max retries exceeded with url: /api/data (Caused by NewConnectionError('<urllib3.connection.HTTPConnection object at 0x7f42f8b05d80>: Failed to establish a new connection: [Errno 111] Connection refused'))
Access API with JavaScript
This code extracts data “live” from a local Web Server with JavaScript fetch. Additionally, it formats the data into a table.
| First Name | Last Name | Residence |
|---|
Kill Python/Flask process
This script ends Python/Flask process
%%script bash
lsof -i :5001 | awk '/Python/ {print $2}' | xargs kill -9
Usage:
kill [options] <pid> [...]
Options:
<pid> [...] send signal to every <pid> listed
-<signal>, -s, --signal <signal>
specify the <signal> to be sent
-q, --queue <value> integer value to be sent with the signal
-l, --list=[<signal>] list all signal names, or convert one to a name
-L, --table list all signal names in a nice table
-h, --help display this help and exit
-V, --version output version information and exit
For more details see kill(1).
---------------------------------------------------------------------------
CalledProcessError Traceback (most recent call last)
/tmp/ipykernel_92726/1062927812.py in <module>
----> 1 get_ipython().run_cell_magic('script', 'bash', "\nlsof -i :5001 | awk '/Python/ {print $2}' | xargs kill -9\n")
/usr/lib/python3/dist-packages/IPython/core/interactiveshell.py in run_cell_magic(self, magic_name, line, cell)
2417 with self.builtin_trap:
2418 args = (magic_arg_s, cell)
-> 2419 result = fn(*args, **kwargs)
2420 return result
2421
<decorator-gen-103> in shebang(self, line, cell)
/usr/lib/python3/dist-packages/IPython/core/magic.py in <lambda>(f, *a, **k)
185 # but it's overkill for just that one bit of state.
186 def magic_deco(arg):
--> 187 call = lambda f, *a, **k: f(*a, **k)
188
189 if callable(arg):
/usr/lib/python3/dist-packages/IPython/core/magics/script.py in shebang(self, line, cell)
243 sys.stderr.flush()
244 if args.raise_error and p.returncode!=0:
--> 245 raise CalledProcessError(p.returncode, cell, output=out, stderr=err)
246
247 def _run_script(self, p, cell, to_close):
CalledProcessError: Command 'b"\nlsof -i :5001 | awk '/Python/ {print $2}' | xargs kill -9\n"' returned non-zero exit status 123.
Hacks
Edit, stop and start the web server.
- Add to the Home Page
- Add your own information to the Web API
- Use from Template to start your own Team Flask project https://github.com/nighthawkcoders/flask_portfolio