\d{4})/',
cache_page(60 * 15)('viewArticles'), name = 'articles'),)
And, of course, it's no longer needed in myapp/views.py.
Caching a Template Fragment
You can also cache parts of a template, this is done by using the cache tag. Let's take our hello.html template −
{% extends "main_template.html" %}
{% block title %}My Hello Page{% endblock %}
{% block content %}
Hello World!!!Today is {{today}}
We are
{% if today.day == 1 %}
the first day of month.
{% elif today == 30 %}
the last day of month.
{% else %}
I don't know.
{%endif%}
{% for day in days_of_week %}
{{day}}
{% endfor %}
{% endblock %}
And to cache the content block, our template will become −
{% load cache %}
{% extends "main_template.html" %}
{% block title %}My Hello Page{% endblock %}
{% cache 500 content %}
{% block content %}
Hello World!!!Today is {{today}}
We are
{% if today.day == 1 %}
the first day of month.
{% elif today == 30 %}
the last day of month.
{% else %}
I don't know.
{%endif%}
{% for day in days_of_week %}
{{day}}
{% endfor %}
{% endblock %}
{% endcache %}
As you can see above, the cache tag will take 2 parameters − the time you want the block to be cached (in seconds) and the name to be given to the cache fragment.Django - Comments
Before starting, note that the Django Comments framework is deprecated, since the 1.5 version. Now you can use external feature for doing so, but if you still want to use it, it's still included in version 1.6 and 1.7. Starting version 1.8 it's absent but you can still get the code on a different GitHub account.
The comments framework makes it easy to attach comments to any model in your app.
To start using the Django comments framework −
Edit the project settings.py file and add 'django.contrib.sites', and 'django.contrib.comments', to INSTALLED_APPS option −
INSTALLED_APPS += ('django.contrib.sites', 'django.contrib.comments',)
Get the site id −
>>> from django.contrib.sites.models import Site
>>> Site().save()
>>> Site.objects.all()[0].id
u'56194498e13823167dd43c64'
Set the id you get in the settings.py file −
SITE_ID = u'56194498e13823167dd43c64'
Sync db, to create all the comments table or collection −
python manage.py syncdb
Add the comment app’s URLs to your project’s urls.py −
from django.conf.urls import include
url(r'^comments/', include('django.contrib.comments.urls')),
Now that we have the framework installed, let's change our hello templates to tracks comments on our Dreamreal model. We will list, save comments for a specific Dreamreal entry whose name will be passed as parameter to the /myapp/hello URL.
Dreamreal Model
class Dreamreal(models.Model):
website = models.CharField(max_length = 50)
mail = models.CharField(max_length = 50)
name = models.CharField(max_length = 50)
phonenumber = models.IntegerField()
class Meta:
db_table = "dreamreal"
hello view
def hello(request, Name):
today = datetime.datetime.now().date()
daysOfWeek = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']
dreamreal = Dreamreal.objects.get(name = Name)
return render(request, 'hello.html', locals())
hello.html template
{% extends "main_template.html" %}
{% load comments %}
{% block title %}My Hello Page{% endblock %}
{% block content %}
Our Dreamreal Entry:
Name : {{dreamreal.name}}
Website : {{dreamreal.website}}
Phone : {{dreamreal.phonenumber}}
Number of comments :
{% get_comment_count for dreamreal as comment_count %} {{ comment_count }}
List of comments :
{% render_comment_list for dreamreal %}
{% render_comment_form for dreamreal %}
{% endblock %}
Finally the mapping URL to our hello view −
url(r'^hello/(?P\w+)/', 'hello', name = 'hello'),
Now,
In our template (hello.html), load the comments framework with − {% load comments %}
We get the number of comments for the Dreamreal object pass by the view − {% get_comment_count for dreamreal as comment_count %}
We get the list of comments for the objects − {% render_comment_list for dreamreal %}
We display the default comments form − {% render_comment_form for dreamreal %}
When accessing /myapp/hello/steve you will get the comments info for the Dreamreal entry whose name is Steve. Accessing that URL will get you −
On posting a comment, you will get redirected to the following page −
If you go to /myapp/hello/steve again, you will get to see the following page −
As you can see, the number of comments is 1 now and you have the comment under the list of comments line.Django - RSS
Django comes with a syndication feed generating framework. With it you can create RSS or Atom feeds just by subclassing django.contrib.syndication.views.Feed class.
Let's create a feed for the latest comments done on the app (Also see Django - Comments Framework chapter). For this, let's create a myapp/feeds.py and define our feed (You can put your feeds classes anywhere you want in your code structure).
from django.contrib.syndication.views import Feed
from django.contrib.comments import Comment
from django.core.urlresolvers import reverse
class DreamrealCommentsFeed(Feed):
title = "Dreamreal's comments"
link = "/drcomments/"
description = "Updates on new comments on Dreamreal entry."
def items(self):
return Comment.objects.all().order_by("-submit_date")[:5]
def item_title(self, item):
return item.user_name
def item_description(self, item):
return item.comment
def item_link(self, item):
return reverse('comment', kwargs = {'object_pk':item.pk})
In our feed class, title, link, and description attributes correspond to the standard RSS , <link> and <description> elements.
The items method, return the elements that should go in the feed as item element. In our case the last five comments.
The item_title method, will get what will go as title for our feed item. In our case the title, will be the user name.
The item_description method, will get what will go as description for our feed item. In our case the comment itself.
The item_link method will build the link to the full item. In our case it will get you to the comment.
Now that we have our feed, let's add a comment view in views.py to display our comment −
from django.contrib.comments import Comment
def comment(request, object_pk):
mycomment = Comment.objects.get(object_pk = object_pk)
text = '<strong>User :</strong> %s <p>'%mycomment.user_name</p>
text += '<strong>Comment :</strong> %s <p>'%mycomment.comment</p>
return HttpResponse(text)
We also need some URLs in our myapp urls.py for mapping −
from myapp.feeds import DreamrealCommentsFeed
from django.conf.urls import patterns, url
urlpatterns += patterns('',
url(r'^latest/comments/', DreamrealCommentsFeed()),
url(r'^comment/(?P\w+)/', 'comment', name = 'comment'),
)
When accessing /myapp/latest/comments/ you will get our feed −
Then clicking on one of the usernames will get you to: /myapp/comment/comment_id as defined in our comment view before and you will get −
Thus, defining a RSS feed is just a matter of sub-classing the Feed class and making sure the URLs (one for accessing the feed and one for accessing the feed elements) are defined. Just as comment, this can be attached to any model in your app.Django - Ajax
Ajax essentially is a combination of technologies that are integrated together to reduce the number of page loads. We generally use Ajax to ease end-user experience. Using Ajax in Django can be done by directly using an Ajax library like JQuery or others. Let's say you want to use JQuery, then you need to download and serve the library on your server through Apache or others. Then use it in your template, just like you might do while developing any Ajax-based application.
Another way of using Ajax in Django is to use the Django Ajax framework. The most commonly used is django-dajax which is a powerful tool to easily and super-quickly develop asynchronous presentation logic in web applications, using Python and almost no JavaScript source code. It supports four of the most popular Ajax frameworks: Prototype, jQuery, Dojo and MooTools.
Using Django-dajax
First thing to do is to install django-dajax. This can be done using easy_install or pip −
$ pip install django_dajax
$ easy_install django_dajax
This will automatically install django-dajaxice, required by django-dajax. We then need to configure both dajax and dajaxice.
Add dajax and dajaxice in your project settings.py in INSTALLED_APPS option −
INSTALLED_APPS += (
'dajaxice',
'dajax'
)
Make sure in the same settings.py file, you have the following −
TEMPLATE_LOADERS = (
'django.template.loaders.filesystem.Loader',
'django.template.loaders.app_directories.Loader',
'django.template.loaders.eggs.Loader',
)
TEMPLATE_CONTEXT_PROCESSORS = (
'django.contrib.auth.context_processors.auth',
'django.core.context_processors.debug',
'django.core.context_processors.i18n',
'django.core.context_processors.media',
'django.core.context_processors.static',
'django.core.context_processors.request',
'django.contrib.messages.context_processors.messages'
)
STATICFILES_FINDERS = (
'django.contrib.staticfiles.finders.FileSystemFinder',
'django.contrib.staticfiles.finders.AppDirectoriesFinder',
'dajaxice.finders.DajaxiceFinder',
)
DAJAXICE_MEDIA_PREFIX = 'dajaxice'
Now go to the myapp/url.py file and make sure you have the following to set dajax URLs and to load dajax statics js files −
from dajaxice.core import dajaxice_autodiscover, dajaxice_config
from django.contrib.staticfiles.urls import staticfiles_urlpatterns
from django.conf import settings
Then dajax urls:
urlpatterns += patterns('',
url(r'^%s/' % settings.DAJAXICE_MEDIA_PREFIX, include('dajaxice.urls')),)
urlpatterns += staticfiles_urlpatterns()
Let us create a simple form based on our Dreamreal model to store it, using Ajax (means no refresh).
At first, we need our Dreamreal form in myapp/form.py.
class DreamrealForm(forms.Form):
website = forms.CharField(max_length = 100)
name = forms.CharField(max_length = 100)
phonenumber = forms.CharField(max_length = 50)
email = forms.CharField(max_length = 100)
Then we need an ajax.py file in our application: myapp/ajax.py. That's where is our logic, that's where we put the function that will be saving our form then return the popup −
from dajaxice.utils import deserialize_form
from myapp.form import DreamrealForm
from dajax.core import Dajax
from myapp.models import Dreamreal
@dajaxice_register
def send_form(request, form):
dajax = Dajax()
form = DreamrealForm(deserialize_form(form))
if form.is_valid():
dajax.remove_css_class('#my_form input', 'error')
dr = Dreamreal()
dr.website = form.cleaned_data.get('website')
dr.name = form.cleaned_data.get('name')
dr.phonenumber = form.cleaned_data.get('phonenumber')
dr.save()
dajax.alert("Dreamreal Entry %s was successfully saved." %
form.cleaned_data.get('name'))
else:
dajax.remove_css_class('#my_form input', 'error')
for error in form.errors:
dajax.add_css_class('#id_%s' % error, 'error')
return dajax.json()
Now let's create the dreamreal.html template, which has our form −
<html>
<head></head>
<body>
<form action = "" method = "post" id = "my_form" accept-charset = "utf-8">
{{ form.as_p }}
<p><input type = "button" value = "Send" onclick = "send_form();"></p>
</form>
</body>
</html>
Add the view that goes with the template in myapp/views.py −
def dreamreal(request):
form = DreamrealForm()
return render(request, 'dreamreal.html', locals())
Add the corresponding URL in myapp/urls.py −
url(r'^dreamreal/', 'dreamreal', name = 'dreamreal'),
Now let's add the necessary in our template to make the Ajax work −
At the top of the file add −
{% load static %}
{% load dajaxice_templatetags %}
And in the <head> section of our dreamreal.html template add −
We are using the JQuery library for this example, so add −
<script src = "{% static '/static/jquery-1.11.3.min.js' %}"
type = "text/javascript" charset = "utf-8"></script>
<script src = "{% static '/static/dajax/jquery.dajax.core.js' %}"></script>
The Ajax function that will be called on click −
<script>
function send_form(){
Dajaxice.myapp.send_form(Dajax.process,{'form':$('#my_form').serialize(true)});
}
</script>
Note that you need the “jquery-1.11.3.min.js” in your static files directory, and also the jquery.dajax.core.js. To make sure all dajax static files are served under your static directory, run −
$python manage.py collectstatic
Note − Sometimes the jquery.dajax.core.js can be missing, if that happens, just download the source and take that file and put it under your static folder.
You will get to see the following screen, upon accessing /myapp/dreamreal/ −
On submit, you will get the following screen −
Flask Tutorial
Flask is a web application framework written in Python. Armin Ronacher, who leads an international group of Python enthusiasts named Pocco, develops it. Flask is based on Werkzeug WSGI toolkit and Jinja2 template engine. Both are Pocco projects.
Flask – Overview
What is Web Framework?
Web Application Framework or simply Web Framework represents a collection of libraries and modules that enables a web application developer to write applications without having to bother about low-level details such as protocols, thread management etc.
What is Flask?
Flask is a web application framework written in Python. It is developed by Armin Ronacher, who leads an international group of Python enthusiasts named Pocco. Flask is based on the Werkzeug WSGI toolkit and Jinja2 template engine. Both are Pocco projects.
WSGI
Web Server Gateway Interface (WSGI) has been adopted as a standard for Python web application development. WSGI is a specification for a universal interface between the web server and the web applications.
Werkzeug
It is a WSGI toolkit, which implements requests, response objects, and other utility functions. This enables building a web framework on top of it. The Flask framework uses Werkzeug as one of its bases.
Jinja2
Jinja2 is a popular templating engine for Python. A web templating system combines a template with a certain data source to render dynamic web pages.
Flask is often referred to as a micro framework. It aims to keep the core of an application simple yet extensible. Flask does not have built-in abstraction layer for database handling, nor does it have form a validation support. Instead, Flask supports the extensions to add such functionality to the application. Some of the popular Flask extensions are discussed later in the tutorial.Flask – Environment
Prerequisite
Python 2.6 or higher is usually required for installation of Flask. Although Flask and its dependencies work well with Python 3 (Python 3.3 onwards), many Flask extensions do not support it properly. Hence, it is recommended that Flask should be installed on Python 2.7.
Install virtualenv for development environment
virtualenv is a virtual Python environment builder. It helps a user to create multiple Python environments side-by-side. Thereby, it can avoid compatibility issues between the different versions of the libraries.
The following command installs virtualenv
pip install virtualenv
This command needs administrator privileges. Add sudo before pip on Linux/Mac OS. If you are on Windows, log in as Administrator. On Ubuntu virtualenv may be installed using its package manager.
Sudo apt-get install virtualenv
Once installed, new virtual environment is created in a folder.
mkdir newproj
cd newproj
virtualenv venv
To activate corresponding environment, on Linux/OS X, use the following −
venv/bin/activate
On Windows, following can be used
venv\scripts\activate
We are now ready to install Flask in this environment.
pip install Flask
The above command can be run directly, without virtual environment for system-wide installation.Flask – Application
In order to test Flask installation, type the following code in the editor as Hello.py
from flask import Flask
app = Flask(__name__)
@app.route('/')
def hello_world():
return 'Hello World’
if __name__ == '__main__':
app.run()
Importing flask module in the project is mandatory. An object of Flask class is our WSGI application.
Flask constructor takes the name of current module (__name__) as argument.
The route() function of the Flask class is a decorator, which tells the application which URL should call the associated function.
app.route(rule, options)
The rule parameter represents URL binding with the function.
The options is a list of parameters to be forwarded to the underlying Rule object.
In the above example, ‘/’ URL is bound with hello_world() function. Hence, when the home page of web server is opened in browser, the output of this function will be rendered.
Finally the run() method of Flask class runs the application on the local development server.
app.run(host, port, debug, options)
All parameters are optional
Sr.No.
Parameters & Description
1
host
Hostname to listen on. Defaults to 127.0.0.1 (localhost). Set to ‘0.0.0.0’ to have server available externally
2
port
Defaults to 5000
3
debug
Defaults to false. If set to true, provides a debug information
4
options
To be forwarded to underlying Werkzeug server.
The above given Python script is executed from Python shell.
Python Hello.py
A message in Python shell informs you that
* Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
Open the above URL (localhost:5000) in the browser. ‘Hello World’ message will be displayed on it.
Debug mode
A Flask application is started by calling the run() method. However, while the application is under development, it should be restarted manually for each change in the code. To avoid this inconvenience, enable debug support. The server will then reload itself if the code changes. It will also provide a useful debugger to track the errors if any, in the application.
The Debug mode is enabled by setting the debug property of the application object to True before running or passing the debug parameter to the run() method.
app.debug = True
app.run()
app.run(debug = True)Flask – Routing
Modern web frameworks use the routing technique to help a user remember application URLs. It is useful to access the desired page directly without having to navigate from the home page.
The route() decorator in Flask is used to bind URL to a function. For example −
@app.route(‘/hello’)
def hello_world():
return ‘hello world’
Here, URL ‘/hello’ rule is bound to the hello_world() function. As a result, if a user visits http://localhost:5000/hello URL, the output of the hello_world() function will be rendered in the browser.
The add_url_rule() function of an application object is also available to bind a URL with a function as in the above example, route() is used.
A decorator’s purpose is also served by the following representation −
def hello_world():
return ‘hello world’
app.add_url_rule(‘/’, ‘hello’, hello_world)Flask – Variable Rules
It is possible to build a URL dynamically, by adding variable parts to the rule parameter. This variable part is marked as <variable-name>. It is passed as a keyword argument to the function with which the rule is associated.
In the following example, the rule parameter of route() decorator contains <name> variable part attached to URL ‘/hello’. Hence, if the http://localhost:5000/hello/TutorialsPoint is entered as a URL in the browser, ‘TutorialPoint’ will be supplied to hello() function as argument.
from flask import Flask
app = Flask(__name__)
@app.route('/hello/<name>')
def hello_name(name):
return 'Hello %s!' % name
if __name__ == '__main__':
app.run(debug = True)
Save the above script as hello.py and run it from Python shell. Next, open the browser and enter URL http://localhost:5000/hello/TutorialsPoint.
The following output will be displayed in the browser.
Hello TutorialsPoint!
In addition to the default string variable part, rules can be constructed using the following converters −
Sr.No.
Converters & Description
1
int
accepts integer
2
float
For floating point value
3
path
accepts slashes used as directory separator character
In the following code, all these constructors are used.
from flask import Flask
app = Flask(__name__)
@app.route('/blog/<int:postID>')
def show_blog(postID):
return 'Blog Number %d' % postID
@app.route('/rev/<float:revNo>')
def revision(revNo):
return 'Revision Number %f' % revNo
if __name__ == '__main__':
app.run()
Run the above code from Python Shell. Visit the URL http://localhost:5000/blog/11 in the browser.
The given number is used as argument to the show_blog() function. The browser displays the following output −
Blog Number 11
Enter this URL in the browser − http://localhost:5000/rev/1.1
The revision() function takes up the floating point number as argument. The following result appears in the browser window −
Revision Number 1.100000
The URL rules of Flask are based on Werkzeug’s routing module. This ensures that the URLs formed are unique and based on precedents laid down by Apache.
Consider the rules defined in the following script −
from flask import Flask
app = Flask(__name__)
@app.route('/flask')
def hello_flask():
return 'Hello Flask'
@app.route('/python/')
def hello_python():
return 'Hello Python'
if __name__ == '__main__':
app.run()
Both the rules appear similar but in the second rule, trailing slash (/) is used. As a result, it becomes a canonical URL. Hence, using /python or /python/ returns the same output. However, in case of the first rule, /flask/ URL results in 404 Not Found page.Flask – URL Building
The url_for() function is very useful for dynamically building a URL for a specific function. The function accepts the name of a function as first argument, and one or more keyword arguments, each corresponding to the variable part of URL.
The following script demonstrates use of url_for() function.
from flask import Flask, redirect, url_for
app = Flask(__name__)
@app.route('/admin')
def hello_admin():
return 'Hello Admin'
@app.route('/guest/<guest>')
def hello_guest(guest):
return 'Hello %s as Guest' % guest
@app.route('/user/<name>')
def hello_user(name):
if name =='admin':
return redirect(url_for('hello_admin'))
else:
return redirect(url_for('hello_guest',guest = name))
if __name__ == '__main__':
app.run(debug = True)
The above script has a function user(name) which accepts a value to its argument from the URL.
The User() function checks if an argument received matches ‘admin’ or not. If it matches, the application is redirected to the hello_admin() function using url_for(), otherwise to the hello_guest() function passing the received argument as guest parameter to it.
Save the above code and run from Python shell.
Open the browser and enter URL as − http://localhost:5000/user/admin
The application response in browser is −
Hello Admin
Enter the following URL in the browser − http://localhost:5000/user/mvl
The application response now changes to −
Hello mvl as GuestFlask – HTTP methods
Http protocol is the foundation of data communication in world wide web. Different methods of data retrieval from specified URL are defined in this protocol.
The following table summarizes different http methods −
Sr.No.
Methods & Description
1
GET
Sends data in unencrypted form to the server. Most common method.
2
HEAD
Same as GET, but without response body
3
POST
Used to send HTML form data to server. Data received by POST method is not cached by server.
4
PUT
Replaces all current representations of the target resource with the uploaded content.
5
DELETE
Removes all current representations of the target resource given by a URL
By default, the Flask route responds to the GET requests. However, this preference can be altered by providing methods argument to route() decorator.
In order to demonstrate the use of POST method in URL routing, first let us create an HTML form and use the POST method to send form data to a URL.
Save the following script as login.html
<html>
<body>
<form action = "http://localhost:5000/login" method = "post">
<p>Enter Name:</p>
<p><input type = "text" name = "nm" /></p>
<p><input type = "submit" value = "submit" /></p>
</form>
</body>
</html>
Now enter the following script in Python shell.
from flask import Flask, redirect, url_for, request
app = Flask(__name__)
@app.route('/success/<name>')
def success(name):
return 'welcome %s' % name
@app.route('/login',methods = ['POST', 'GET'])
def login():
if request.method == 'POST':
user = request.form['nm']
return redirect(url_for('success',name = user))
else:
user = request.args.get('nm')
return redirect(url_for('success',name = user))
if __name__ == '__main__':
app.run(debug = True)
After the development server starts running, open login.html in the browser, enter name in the text field and click Submit.
Form data is POSTed to the URL in action clause of form tag.
http://localhost/login is mapped to the login() function. Since the server has received data by POST method, value of ‘nm’ parameter obtained from the form data is obtained by −
user = request.form['nm']
It is passed to ‘/success’ URL as variable part. The browser displays a welcome message in the window.
Change the method parameter to ‘GET’ in login.html and open it again in the browser. The data received on server is by the GET method. The value of ‘nm’ parameter is now obtained by −
User = request.args.get(‘nm’)
Here, args is dictionary object containing a list of pairs of form parameter and its corresponding value. The value corresponding to ‘nm’ parameter is passed on to ‘/success’ URL as before.Flask – Templates
It is possible to return the output of a function bound to a certain URL in the form of HTML. For instance, in the following script, hello() function will render ‘Hello World’ with <h1> tag attached to it.
from flask import Flask
app = Flask(__name__)
@app.route('/')
def index():
return '<html><body><h1>Hello World</h1></body></html>'
if __name__ == '__main__':
app.run(debug = True)
However, generating HTML content from Python code is cumbersome, especially when variable data and Python language elements like conditionals or loops need to be put. This would require frequent escaping from HTML.
This is where one can take advantage of Jinja2 template engine, on which Flask is based. Instead of returning hardcode HTML from the function, a HTML file can be rendered by the render_template() function.
from flask import Flask
app = Flask(__name__)
@app.route('/')
def index():
return render_template(‘hello.html’)
if __name__ == '__main__':
app.run(debug = True)
Flask will try to find the HTML file in the templates folder, in the same folder in which this script is present.
Application folder
Hello.py
templates
hello.html
The term ‘web templating system’ refers to designing an HTML script in which the variable data can be inserted dynamically. A web template system comprises of a template engine, some kind of data source and a template processor.
Flask uses jinja2 template engine. A web template contains HTML syntax interspersed placeholders for variables and expressions (in these case Python expressions) which are replaced values when the template is rendered.
The following code is saved as hello.html in the templates folder.
<!doctype html>
<html>
<body>
<h1>Hello {{ name }}!</h1>
</body>
</html>
Next, run the following script from Python shell.
from flask import Flask, render_template
app = Flask(__name__)
@app.route('/hello/<user>')
def hello_name(user):
return render_template('hello.html', name = user)
if __name__ == '__main__':
app.run(debug = True)
As the development server starts running, open the browser and enter URL as − http://localhost:5000/hello/mvl
The variable part of URL is inserted at {{ name }} place holder.
The jinja2 template engine uses the following delimiters for escaping from HTML.
{% ... %} for Statements
{{ ... }} for Expressions to print to the template output
{# ... #} for Comments not included in the template output
# ... ## for Line Statements
In the following example, use of conditional statement in the template is demonstrated. The URL rule to the hello() function accepts the integer parameter. It is passed to the hello.html template. Inside it, the value of number received (marks) is compared (greater or less than 50) and accordingly HTML is conditionally rendered.
The Python Script is as follows −
from flask import Flask, render_template
app = Flask(__name__)
@app.route('/hello/<int:score>')
def hello_name(score):
return render_template('hello.html', marks = score)
if __name__ == '__main__':
app.run(debug = True)
HTML template script of hello.html is as follows −
<!doctype html>
<html>
<body>
{% if marks>50 %}
<h1> Your result is pass!</h1>
{% else %}
<h1>Your result is fail</h1>
{% endif %}
</body>
</html>
Note that the conditional statements if-else and endif are enclosed in delimiter {%..%}.
Run the Python script and visit URL http://localhost/hello/60 and then http://localhost/hello/30 to see the output of HTML changing conditionally.
The Python loop constructs can also be employed inside the template. In the following script, the result() function sends a dictionary object to template results.html when URL http://localhost:5000/result is opened in the browser.
The Template part of result.html employs a for loop to render key and value pairs of dictionary object result{} as cells of an HTML table.
Run the following code from Python shell.
from flask import Flask, render_template
app = Flask(__name__)
@app.route('/result')
def result():
dict = {'phy':50,'che':60,'maths':70}
return render_template('result.html', result = dict)
if __name__ == '__main__':
app.run(debug = True)
Save the following HTML script as result.html in the templates folder.
<!doctype html>
<html>
<body>
<table border = 1>
{% for key, value in result.items() %}
<tr>
<th> {{ key }} </th>
<td> {{ value }} </td>
</tr>
{% endfor %}
</table>
</body>
</html>
Here, again the Python statements corresponding to the For loop are enclosed in {%..%} whereas, the expressions key and value are put inside {{ }}.
After the development starts running, open http://localhost:5000/result in the browser to get the following output.
Flask – Static Files
A web application often requires a static file such as a javascript file or a CSS file supporting the display of a web page. Usually, the web server is configured to serve them for you, but during the development, these files are served from static folder in your package or next to your module and it will be available at /static on the application.
A special endpoint ‘static’ is used to generate URL for static files.
In the following example, a javascript function defined in hello.js is called on OnClick event of HTML button in index.html, which is rendered on ‘/’ URL of the Flask application.
from flask import Flask, render_template
app = Flask(__name__)
@app.route("/")
def index():
return render_template("index.html")
if __name__ == '__main__':
app.run(debug = True)
The HTML script of index.html is given below.
<html>
<head>
<script type = "text/javascript"
src = "{{ url_for('static', filename = 'hello.js') }}" ></script>
</head>
<body>
<input type = "button" onclick = "sayHello()" value = "Say Hello" />
</body>
</html>
hello.js contains sayHello() function.
function sayHello() {
alert("Hello World")
}Flask – Request Object
The data from a client’s web page is sent to the server as a global request object. In order to process the request data, it should be imported from the Flask module.
Important attributes of request object are listed below −
Form − It is a dictionary object containing key and value pairs of form parameters and their values.
args − parsed contents of query string which is part of URL after question mark (?).
Cookies − dictionary object holding Cookie names and values.
files − data pertaining to uploaded file.
method − current request method.
Flask – Sending Form Data to Template
We have already seen that the http method can be specified in URL rule. The Form data received by the triggered function can collect it in the form of a dictionary object and forward it to a template to render it on a corresponding web page.
In the following example, ‘/’ URL renders a web page (student.html) which has a form. The data filled in it is posted to the ‘/result’ URL which triggers the result() function.
The results() function collects form data present in request.form in a dictionary object and sends it for rendering to result.html.
The template dynamically renders an HTML table of form data.
Given below is the Python code of application −
from flask import Flask, render_template, request
app = Flask(__name__)
@app.route('/')
def student():
return render_template('student.html')
@app.route('/result',methods = ['POST', 'GET'])
def result():
if request.method == 'POST':
result = request.form
return render_template("result.html",result = result)
if __name__ == '__main__':
app.run(debug = True)
Given below is the HTML script of student.html.
<html>
<body>
<form action = "http://localhost:5000/result" method = "POST">
<p>Name <input type = "text" name = "Name" /></p>
<p>Physics <input type = "text" name = "Physics" /></p>
<p>Chemistry <input type = "text" name = "chemistry" /></p>
<p>Maths <input type ="text" name = "Mathematics" /></p>
<p><input type = "submit" value = "submit" /></p>
</form>
</body>
</html>
Code of template (result.html) is given below −
<!doctype html>
<html>
<body>
<table border = 1>
{% for key, value in result.items() %}
<tr>
<th> {{ key }} </th>
<td> {{ value }} </td>
</tr>
{% endfor %}
</table>
</body>
</html>
Run the Python script and enter the URL http://localhost:5000/ in the browser.
When the Submit button is clicked, form data is rendered on result.html in the form of HTML table.
Flask – Cookies
A cookie is stored on a client’s computer in the form of a text file. Its purpose is to remember and track data pertaining to a client’s usage for better visitor experience and site statistics.
A Request object contains a cookie’s attribute. It is a dictionary object of all the cookie variables and their corresponding values, a client has transmitted. In addition to it, a cookie also stores its expiry time, path and domain name of the site.
In Flask, cookies are set on response object. Use make_response() function to get response object from return value of a view function. After that, use the set_cookie() function of response object to store a cookie.
Reading back a cookie is easy. The get() method of request.cookies attribute is used to read a cookie.
In the following Flask application, a simple form opens up as you visit ‘/’ URL.
@app.route('/')
def index():
return render_template('index.html')
This HTML page contains one text input.
<html>
<body>
<form action = "/setcookie" method = "POST">
<p><h3>Enter userID</h3></p>
<p><input type = 'text' name = 'nm'/></p>
<p><input type = 'submit' value = 'Login'/></p>
</form>
</body>
</html>
The Form is posted to ‘/setcookie’ URL. The associated view function sets a Cookie name userID and renders another page.
@app.route('/setcookie', methods = ['POST', 'GET'])
def setcookie():
if request.method == 'POST':
user = request.form['nm']
resp = make_response(render_template('readcookie.html'))
resp.set_cookie('userID', user)
return resp
‘readcookie.html’ contains a hyperlink to another view function getcookie(), which reads back and displays the cookie value in browser.
@app.route('/getcookie')
def getcookie():
name = request.cookies.get('userID')
return '<h1>welcome '+name+'</h1>'
Run the application and visit http://localhost:5000/
The result of setting a cookie is displayed like this −
The output of read back cookie is shown below.
Flask – Sessions
Like Cookie, Session data is stored on client. Session is the time interval when a client logs into a server and logs out of it. The data, which is needed to be held across this session, is stored in the client browser.
A session with each client is assigned a Session ID. The Session data is stored on top of cookies and the server signs them cryptographically. For this encryption, a Flask application needs a defined SECRET_KEY.
Session object is also a dictionary object containing key-value pairs of session variables and associated values.
For example, to set a ‘username’ session variable use the statement −
Session[‘username’] = ’admin’
To release a session variable use pop() method.
session.pop('username', None)
The following code is a simple demonstration of session works in Flask. URL ‘/’ simply prompts user to log in, as session variable ‘username’ is not set.
@app.route('/')
def index():
if 'username' in session:
username = session['username']
return 'Logged in as ' + username + '<br>' + \
"<b><a href = '/logout'>click here to log out</a></b>"
return "You are not logged in <br><a href = '/login'></b>" + \
"click here to log in</b></a>"
As user browses to ‘/login’ the login() view function, because it is called through GET method, opens up a login form.
A Form is posted back to ‘/login’ and now session variable is set. Application is redirected to ‘/’. This time session variable ‘username’ is found.
@app.route('/login', methods = ['GET', 'POST'])
def login():
if request.method == 'POST':
session['username'] = request.form['username']
return redirect(url_for('index'))
return '''
<form action = "" method = "post">
<p><input type = text name = username/></p>
<p<<input type = submit value = Login/></p>
</form>
'''
The application also contains a logout() view function, which pops out ‘username’ session variable. Hence, ‘/’ URL again shows the opening page.
@app.route('/logout')
def logout():
# remove the username from the session if it is there
session.pop('username', None)
return redirect(url_for('index'))
Run the application and visit the homepage. (Ensure to set secret_key of the application)
from flask import Flask, session, redirect, url_for, escape, request
app = Flask(__name__)
app.secret_key = 'any random string’
The output will be displayed as shown below. Click the link “click here to log in”.
The link will be directed to another screen. Type ‘admin’.
The screen will show you the message, ‘Logged in as admin’.
Flask – Redirect & Errors
Flask class has a redirect() function. When called, it returns a response object and redirects the user to another target location with specified status code.
Prototype of redirect() function is as below −
Flask.redirect(location, statuscode, response)
In the above function −
location parameter is the URL where response should be redirected.
statuscode sent to browser’s header, defaults to 302.
response parameter is used to instantiate response.
The following status codes are standardized −
HTTP_300_MULTIPLE_CHOICES
HTTP_301_MOVED_PERMANENTLY
HTTP_302_FOUND
HTTP_303_SEE_OTHER
HTTP_304_NOT_MODIFIED
HTTP_305_USE_PROXY
HTTP_306_RESERVED
HTTP_307_TEMPORARY_REDIRECT
The default status code is 302, which is for ‘found’.
In the following example, the redirect() function is used to display the login page again when a login attempt fails.
from flask import Flask, redirect, url_for, render_template, request
# Initialize the Flask application
app = Flask(__name__)
@app.route('/')
def index():
return render_template('log_in.html')
@app.route('/login',methods = ['POST', 'GET'])
def login():
if request.method == 'POST' and request.form['username'] == 'admin' :
return redirect(url_for('success'))
else:
return redirect(url_for('index'))
@app.route('/success')
def success():
return 'logged in successfully'
if __name__ == '__main__':
app.run(debug = True)
Flask class has abort() function with an error code.
Flask.abort(code)
The Code parameter takes one of following values −
400 − for Bad Request
401 − for Unauthenticated
403 − for Forbidden
404 − for Not Found
406 − for Not Acceptable
415 − for Unsupported Media Type
429 − Too Many Requests
Let us make a slight change in the login() function in the above code. Instead of re-displaying the login page, if ‘Unauthourized’ page is to be displayed, replace it with call to abort(401).
from flask import Flask, redirect, url_for, render_template, request, abort
app = Flask(__name__)
@app.route('/')
def index():
return render_template('log_in.html')
@app.route('/login',methods = ['POST', 'GET'])
def login():
if request.method == 'POST':
if request.form['username'] == 'admin' :
return redirect(url_for('success'))
else:
abort(401)
else:
return redirect(url_for('index'))
@app.route('/success')
def success():
return 'logged in successfully'
if __name__ == '__main__':
app.run(debug = True)Flask – Message Flashing
A good GUI based application provides feedback to a user about the interaction. For example, the desktop applications use dialog or message box and JavaScript uses alerts for similar purpose.
Generating such informative messages is easy in Flask web application. Flashing system of Flask framework makes it possible to create a message in one view and render it in a view function called next.
A Flask module contains flash() method. It passes a message to the next request, which generally is a template.
flash(message, category)
Here,
message parameter is the actual message to be flashed.
category parameter is optional. It can be either ‘error’, ‘info’ or ‘warning’.
In order to remove message from session, template calls get_flashed_messages().
get_flashed_messages(with_categories, category_filter)
Both parameters are optional. The first parameter is a tuple if received messages are having category. The second parameter is useful to display only specific messages.
The following flashes received messages in a template.
{% with messages = get_flashed_messages() %}
{% if messages %}
{% for message in messages %}
{{ message }}
{% endfor %}
{% endif %}
{% endwith %}
Let us now see a simple example, demonstrating the flashing mechanism in Flask. In the following code, a ‘/’ URL displays link to the login page, with no message to flash.
@app.route('/')
def index():
return render_template('index.html')
The link leads a user to ‘/login’ URL which displays a login form. When submitted, the login() view function verifies a username and password and accordingly flashes a ‘success’ message or creates ‘error’ variable.
@app.route('/login', methods = ['GET', 'POST'])
def login():
error = None
if request.method == 'POST':
if request.form['username'] != 'admin' or \
request.form['password'] != 'admin':
error = 'Invalid username or password. Please try again!'
else:
flash('You were successfully logged in')
return redirect(url_for('index'))
return render_template('login.html', error = error)
In case of error, the login template is redisplayed with error message.
Login.html
<!doctype html>
<html>
<body>
<h1>Login</h1>
{% if error %}
<p><strong>Error:</strong> {{ error }}
{% endif %}
<form action = "" method = post>
<dl>
<dt>Username:</dt>
<dd>
<input type = text name = username
value = "{{request.form.username }}">
</dd>
<dt>Password:</dt>
<dd><input type = password name = password></dd>
</dl>
<p><input type = submit value = Login></p>
</form>
</body>
</html>
On the other hand, if login is successful, a success message is flashed on the index template.
Index.html
<!doctype html>
<html>
<head>
<title>Flask Message flashing
{% with messages = get_flashed_messages() %}
{% if messages %}
{% for message in messages %}
{% endfor %}
{% endif %}
{% endwith %}
Flask Message Flashing Example
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