Data Science

Describing URL Routing

Modern web applications use meaningful URLs to help users
A server binds a socket to a listening web service
The route decorator binds a function to a URL
The following is an example:

>>> @app.route('/')
... def index():
...     return 'Index page'

>>> @app.route('/hello')
... def hello():
...     return 'Hello world'

We can also attach multiple rules to a function
This makes part of the URL dynamic
Specifically, these are called variable rules

Describing Variable Rules

We can add variable sections to a URL by marking sections
We can mark secions with <variable_name>
Then, a function receives the <variable_name> as a keyword argument
The following is an example:

>>> from markupsafe import escape
>>> @app.route(/user/<username>')
... def show_user_profile(username):
...     return 'User %s' % escape(username)

Describing Redirection Behavior

The following two rules differ in their behavior:

>>> @app.route('/projects/')
... def projects():
...     return 'The project page'

>>> @app.route('/about')
... def about():
...     return 'The about page'

If we access the projects URL:
- Flask successfully accepts /projects/
- Flask redirects /projects to /projects/
If we access the about URL:
- Flask successfully accepts /about
- Flask produces a $404$ error for /about/
This approach helps keep URLs unique
This helps search engines avoid indexing the same page twice

Describing URL Building

The url_for function builds a URL to a specific function
Its first argument is a function
Typically, we'll prefer building URLs instead of hard-coding them
These are some reasons:
1. The url_for function is more descriptive
2. Related URLs can be captured by one url_for function
3. Related URLs can be changed in one place
4. URL building handles escaping special characters
5. URL building handles unicode data well
6. The generated paths are absolute

Describing HTTP Methods

Web applications use different HTTP methods when accessing URLs
A route only answers to GET requests by default
We can specify additional HTTP methods using the methods argument
The following provides an example of this:

>>> from flask import request
>>> @app.route('/login', methods=['GET', 'POST'])
>>> def login():
...     if request.method == 'POST':
...         return do_the_login()
...     else:
...         return show_the_login_form()

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