Lintel's Blog

FlaskAPI

Fri 12 June 2015 by Godson

Overview: Flask API is an implementation of the same web browsable APIs that Django Rest Framework provides. It gives you properly content negotiated responses and smart request parsing.

Installation

Requirements:

Python 2.7+ or 3.3+

Flask 0.10+

install using pip 

pip install Flask-API

Import and initialize your application.

from flask.ext.api import FlaskAPI
app = FlaskAPI(__name__)

Requests and Responses: Requests: Access the parsed request data using request.data. This will handle JSON or form data by default. 

@app.route('/example/')

def example():

    return {'request data': request.data}

Responses: Return any valid response object as normal, or return a list or dict.

@app.route('/example/')
def example():
    return {'hello': 'world'}

Sample file: save it as app.py

from flask import Flask
app = Flask(__name__)
@app.route('/')
def index():
    return "Hello, World!"
if __name__ == '__main__':
    app.run(debug=True)

To run this application we have to execute app.py:

$ chmod a+x app.py
$ ./app.py
 * Running on http://127.0.0.1:8000/
 * Restarting with reloader

And now you can launch your web browser and type http://localhost:8000 to see this tiny application in action. Routing Modern web applications have beautiful URLs. This helps people remember the URLs, which is especially handy for applications that are used from mobile devices with slower network connections. If the user can directly go to the desired page without having to hit the index page it is more likely they will like the page and come back next time. As you have seen above, the route() decorator is used to bind a function to a URL. Here are some basic examples:

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

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

Variable Rules To add variable parts to a URL you can mark these special sections as . Such a part is then passed as a keyword argument to your function. Optionally a converter can be used by specifying a rule with . Here are some nice examples:

@app.route('/user/<username>')
def show_user_profile(username):
    # show the user profile for that user
    return 'User %s' % username

@app.route('/post/<int:post_id>')
def show_post(post_id):
    # show the post with the given id, the id is an integer
    return 'Post %d' % post_id

The following converters exist: int accept integers float like int but for floating point values path like the default but also accepts slashes

Unique URLs / Redirection Behavior Flask’s URL rules are based on Werkzeug’s routing module. The idea behind that module is to ensure beautiful and unique URLs based on precedents laid down by Apache and earlier HTTP servers. Take these two rules:

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

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

Though they look rather similar, they differ in their use of the trailing slash in the URLdefinition. In the first case, the canonical URL for the projects endpoint has a trailing slash. In that sense, it is similar to a folder on a file system. Accessing it without a trailing slash will cause Flask to redirect to the canonical URL with the trailing slash. In the second case, however, the URL is defined without a trailing slash, rather like the pathname of a file on UNIX-like systems. Accessing the URL with a trailing slash will produce a 404 “Not Found” error. This behavior allows relative URLs to continue working even if the trailing slash is ommited, consistent with how Apache and other servers work. Also, the URLs will stay unique, which helps search engines avoid indexing the same page twice.

URL Building If it can match URLs, can Flask also generate them? Of course it can. To build a URL to a specific function you can use the url_for() function. It accepts the name of the function as first argument and a number of keyword arguments, each corresponding to the variable part of the URL rule. Unknown variable parts are appended to the URL as query parameters. Here are some examples:

>>> from flask import Flask, url_for
>>> app = Flask(__name__)
>>> @app.route('/')
... def index(): pass
...
>>> @app.route('/login')
... def login(): pass
...
>>> @app.route('/user/<username>')
... def profile(username): pass
...
>>> with app.test_request_context():
...  print url_for('index')
...  print url_for('login')
...  print url_for('login', next='/')
...  print url_for('profile', username='John Doe')
...
/
/login
/login?next=/
/user/John%20Doe

Why would you want to build URLs instead of hard-coding them into your templates? There are three good reasons for this:

  1. Reversing is often more descriptive than hard-coding the URLs. More importantly, it allows you to change URLs in one go, without having to remember to change URLs all over the place.

  2. URL building will handle escaping of special characters and Unicode data transparently for you, so you don’t have to deal with them.

  3. If your application is placed outside the URL root (say, in /myapplication instead of/), url_for() will handle that properly for you.

HTTP Methods

HTTP (the protocol web applications are speaking) knows different methods for accessing URLs. By default, a route only answers to GET requests, but that can be changed by providing the methods argument to the route() decorator. Here are some examples:

The HTTP method (also often called “the verb”) tells the server what the clients wants to do with the requested page. The following methods are very common:

GET

The browser tells the server to just get the information stored on that page and send it. This is probably the most common method.

HEAD

The browser tells the server to get the information, but it is only interested in the headers, not the content of the page. An application is supposed to handle that as if a GET request was received but to not deliver the actual content. In Flask you don’t have to deal with that at all, the underlying Werkzeug library handles that for you.

POST

The browser tells the server that it wants to post some new information to that URL and that the server must ensure the data is stored and only stored once. This is how HTML forms usually transmit data to the server.

PUT

Similar to POST but the server might trigger the store procedure multiple times by overwriting the old values more than once. Now you might be asking why this is useful, but there are some good reasons to do it this way. Consider that the connection is lost during transmission: in this situation a system between the browser and the server might receive the request safely a second time without breaking things. With POST that would not be possible because it must only be triggered once.

DELETE

Remove the information at the given location.

OPTIONS

Provides a quick way for a client to figure out which methods are supported by this URL. Starting with Flask 0.6, this is implemented for you automatically.

Rendering Templates

Generating HTML from within Python is not fun, and actually pretty cumbersome because you have to do the HTML escaping on your own to keep the application secure. Because of that Flask configures the Jinja2 template engine for you automatically. To render a template you can use the render_template() method. All you have to do is provide the name of the template and the variables you want to pass to the template engine as keyword arguments. Here’s a simple example of how to render a template:

from flask import render_template
@app.route('/hello/')
@app.route('/hello/<name>')
def hello(name=None):
    return render_template('hello.html', name=name).

File Uploads You can handle uploaded files with Flask easily. Just make sure not to forget to set theenctype="multipart/form-data" attribute on your HTML form, otherwise the browser will not transmit your files at all. Uploaded files are stored in memory or at a temporary location on the filesystem. You can access those files by looking at the files attribute on the request object. Each uploaded file is stored in that dictionary. It behaves just like a standard Python file object, but it also has a save() method that allows you to store that file on the file system of the server. Here is a simple example showing how that works:

from flask import request

@app.route('/upload', methods=['GET', 'POST'])
def upload_file():
    if request.method == 'POST':
        f = request.files['the_file']
        f.save('/var/www/uploads/uploaded_file.txt')

If you want to know how the file was named on the client before it was uploaded to your application, you can access the filename attribute. However please keep in mind that this value can be forged so never ever trust that value. If you want to use the filename of the client to store the file on the server, pass it through the secure_filename() function that Werkzeug provides for you:

from flask import request
from werkzeug import secure_filename

@app.route('/upload', methods=['GET', 'POST'])
def upload_file():
    if request.method == 'POST':
        f = request.files['the_file']
        f.save('/var/www/uploads/' + secure_filename(f.filename))
    ...

Redirects and Errors To redirect a user to another endpoint, use the redirect() function; to abort a request early with an error code, use the abort() function:

from flask import abort, redirect, url_for

@app.route('/')
def index():
    return redirect(url_for('login'))

@app.route('/login')
def login():
    abort(401)
    this_is_never_executed()

This is all about Flask Api. You can implement lot of python features in it like sessions,logging and a lot.