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Flask - The Basics

/ Published in: Python

Snippet showing a simple Flask application to illustrate the topics covered in the "Quickstart".

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from flask import Flask, url_for, request, render_template, flash, redirect, abort, session, escape
app = Flask(__name__)

# setting the secret_key generate with: >>> import os; os.urandom(24)
app.secret_key = '\x1eo\xc0\xd84\x8c\xfez\xbc\x0eQ\xc2\xb4we\xa8p\r\x13\\\x88\x97\x8e\xb9'

# route() decorator is used to bind a function to a URL
@app.route('/')
def index():
 return 'Index Page'

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

# route() accepts URLs with variable parts
@app.route('/hello/<user>')
def helloUser(user):
 return 'Hello %s!' % user

# rules may also be specified using a converter. Converter types:
# int accepts integers
# float like int but for floating point values
# path like the default but also accepts slashes
@app.route('/hello/<user>/<int:greetingsCount>')
def helloUserN(user, greetingsCount):
 greetingsCount = abs(greetingsCount)
 if greetingsCount > 3:
 app.logger.warning('Warning! The greetings count %d is above the limit (3).', greetingsCount)
 returnValue = ''
 while greetingsCount > 0:
 returnValue += 'Hello %s! ' % user
 greetingsCount -= 1;
 return returnValue

# building URLs using the url_for() function. Unknows variables are appended to the URL as query parameters
@app.route('/urls')
def printUrls():
 builtUrls = ''
 builtUrls += url_for('index') + ' '
 builtUrls += url_for('helloWorld') + ' '
 builtUrls += url_for('helloWorld', user='user') + ' '
 builtUrls += url_for('helloUser', user='user') + ' '
 builtUrls += url_for('helloUserN', user='user', greetingsCount='3') + ' '
 return builtUrls

# example on how to access the request object
# To access parameters submitted in the URL (?key=value) you can use the args attribute:
# searchword = request.args.get('key', '')
# by default, a route only answers to GET requests, but it can be changed, as below:
@app.route('/login', methods=['GET', 'POST'])
def login():
    if request.method == 'POST':
        if isCredentialsValid(request.form['username'], request.form['password']):
            # escape() used here does escaping for you. Can be used when you are not using the template engine
            session['username'] = escape(request.form['username'])
            return redirect(url_for('ghibliIndex'))
        else:
            app.logger.error('Invalid username and/or password provided.')
            flash('Invalid username and/or password.', 'error')
    
    # the code below this is executed if the request method
    # was GET or the credentials were invalid
    return render_template('login.html')
    
def isCredentialsValid(username, password):
    if username == 'user' and password == 'pass':
        return True
    return False

@app.route('/logout')
def logout():
    # remove the username from the session if it's there
    session.pop('username', None)
    return redirect(url_for('ghibliIndex'))

# example of how to deal with templates
@app.route('/ghibli')
@app.route('/ghibli/<name>')
def ghibliIndex(name=None):
 return render_template('ghibli.html', name=name, ghibliLogoUrl=url_for('static', filename='ghibli.gif'))

# example of how to deal with templates
@app.route('/upload', methods=['GET', 'POST'])
def upload():
    if request.method == 'POST':
        f = request.files['filename'];

# to save the file, the line below could be used
        # f.save('/var/www/uploads/uploaded_file.txt')
        
        # f.filename contains the name of the file on the client, before it was uploaded,
        # but it should not be trusted. If it is wanted to use it, pass it though the
        # secure_filename function
        # from werkzeug import secure_filename
        # f.save('/var/www/uploads/' + secure_filename(f.filename))

flash('Content of the uploaded file:', 'message')
        
        messages = []
        lines = f.readlines()
        for line in lines:
            flash(line, 'message')
            
        # example of how to use the redirect() function to guide use to an endpoint
        return redirect(url_for('upload'))
    return render_template('upload.html')

# useless route to illustrate the usage of abort() function
@app.route('/forbidden')
def forbidden():
    abort(401)
    
@app.errorhandler(404)
def page_not_found(error):
    app.logger.debug('Reached page not found handler.')
    return render_template('page_not_found.html', makkuroUrl=url_for('static', filename='makkuro.jpg')), 404

# this function could be written using the make_response() function
    # resp = make_response(render_template('page_not_found.html', makkuroUrl=url_for('static', filename='makkuro.jpg')), 404)
    # resp.headers['X-Something'] = 'A value'
    # return resp

if __name__ == '__main__':
    # app.run()
    
    # Enabling debug mode - NEVER USE IT IN PRODUCTION
    # Almost mandatory in development environments because, if enabled, the server will
    # reload itself on code changes
    app.run(debug=True)
    # or
    # app.debug = True
    # app.run()
    
    # Making the built-in server externally visible
    # It is disabled by default because in debugging mode a user of the application
    # can execute arbitrary Python code on the machine
    # Enable it if debug is disabled or users on the network can be trusted
    # app.run(host='0.0.0.0')

URL: http://flask.pocoo.org/docs/quickstart/

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