Flask is a micro framework designed to facilitate the development of web applications under the MVC pattern and provides a simple interface. The main advantage is that it doesn't require any complex preconfiguration; all we need to do is install it with a command:
>>> pip install flask
Downloading/unpacking flask
Flask can also be downloaded from the project's home page at http://flask.pocoo.org. Note that to run Flask under Python 3, you will need Python 3.3 or higher.
Among the main features of Flask, we can highlight the following:
- Open source: Flask is open source and is covered under a BSD license.
- Includes web server development: You do not need any infrastructure with a web server to test the applications, as you can simply run a web server to see the results that are obtained.
- It has a debugger and integrated support for unit tests: If we have an error in the code that is being built, we can debug that error and we can see the values of the variables. There is also the possibility of integrating unit tests.
- It is compatible with WSGI: WSGI is a protocol that uses web servers to serve web pages that are written in Python.
- Good route management: When you work with web apps that have been made in Python, you have the driver that receives all the requests that the clients make, and it has to determine which route the client is accessing to execute the necessary code.
- Build web services: It is used to build web services (such as RESTful APIs) or static content applications.
Among the main objects and methods that Flask provides for work, we can highlight the following:
- flask: This is the main object of the framework and is a way to agglutinate the callable WSGI with a set of routes. Our application is going to be an instance of this object.
- request: An object that allows us to access the data referring to the request that was made to us. It includes the GET parameters, cookies, and headers, among other things.
- response: An object that allows us to modify our responses; add headers, status codes, and cookies; and other concepts.
- render_template: This is a method that injects our context into a template and returns the answer in its complete form, ready to be returned.
- redirect: A helper that allows us to return a redirect to another URL in our code.
- abort: A helper that allows us to return an error status from our controller.
Our app is going to allow us to browse the docstrings for the Python built-in functions. An application that's built with Flask is basically an instance of the Flask object, which we will record routes in.
You can find the following code in the flaskapp_demo.py file on the GitHub repository (https://github.com/PacktPublishing/Learning-Python-Networking-Second-Edition):
#!/usr/local/bin/python3
from flask import Flask, abort
app = Flask(__name__)
app.debug = True
objs = __builtins__.__dict__.items()
docstrings = {name.lower(): obj.__doc__ for name, obj in objs if
name[0].islower() and hasattr(obj, '__name__')}
...
Flask includes a development web server, so to try it out on our application, all we need to do is run the following command:
$ python flaskapp_demo.py
* Serving Flask app "demo" (lazy loading)
* Environment: production
WARNING: Do not use the development server in a production environment.
Use a production WSGI server instead.
* Debug mode: on
* Restarting with stat
* Debugger is active!
* Debugger PIN: 190-598-045
* Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
We can see that the Flask server tells us the IP address and port it's listening on. Connect to the http://127.0.0.1:5000/ URL. It will now display in a web browser, and you should see a page with a list of Python built-in functions. Clicking on one should display a page showing the function name and its docstring. If you want to run the server on another interface or port, you can change this data in the app.run() call, for example, to app.run(host='0.0.0.0', port=5000).
Let's go through our code. From the top, we created our Flask app by creating a Flask instance, in this case giving it the name of our main module. We then set the debug mode to active, which provides nice tracebacks in the browser when something goes wrong, and also sets the development server to automatically reload code changes without needing a restart. Note that the debug mode should never be left active in a production app! This is because the debugger has an interactive element, which allows code to be executed on the server. By default, debug is off, so all we need to do is delete the app.config.debug line when we put the app into production.