Difference between revisions of "Python-Flask/C2/Request-Response-Cycle-in-a-Flask-Web-App/English"

• About the request-response cycle in Flask.
• Different attributes of the request object.
• Different types of request data.
• Several HTTP response codes.

• Ubuntu Linux 16.04 OS
• Python 3.5.2
• Atom text editor 1.22.1 and
• Firefox web browser

You can use any text editor and web browser of your choice.

• Linux commands
• Python programming and
• HTML syntax

If not, then please go through the corresponding tutorials on this website.

For processing, the app instance needs to have access to certain information.

One of these objects is the request object.

This object encapsulates the contents of the HTTP request sent by the client.

• Query arguments sent as part of the URL in the browser.
• Form data collected from web forms.

We will discuss them in detail in this tutorial.

Type cd project_flask & Press Enter

$ . flask_venv/bin/activate

[Enter]

Type the command

Dot space flask_venv slash bin slash activate

And press Enter.

Highlight (flask_venv)

Type

$ atom hello_flask.py

Type “, request”

We will import the request object from the flask library.

In the line from flask import Flask, we will add comma space request

Type in hello_world():

(As the first line)

print("Request method is " + request.method)

Let us access request inside the hello_world view function.

Next to the function definition, type the code as shown.

print("Request method is " + request.method)

Type

$ export FLASK_APP=hello_flask.py

Switch to the terminal.

Type export <space> FLASK_APP <equal to> hello underscore flask dot py

And press Enter.

Type $ python3 -m flask run

[Enter]

python3 <space> hyphen m <space> flask <space> run

And press Enter.

Type http://localhost:5000/

And press Enter.

In the address bar, type http://localhost:5000 and press Enter.

The request object is indeed accessible inside the view functions.

The GET HTTP method is used to retrieve data from the web server.

It specifies the parameters in the URL of the request itself.

There are a number of ways to do this.

The simplest one is via Query Arguments.

Type

@app.route('/query')

def query_args():

(Tab)framework = request.args['framework']

(Tab)return '''<h1>The framework value is: {}</h1>'''.format(framework)

Switch back to the editor.

Before the if condition, type the code as shown here.

We will call it as query.

<h1>The framework value is: {}</h1>

framework = request.args['framework']

This is a dictionary object.

It has the parsed contents of the query string in the URL.

We will use this attribute to retrieve the data sent as query arguments.

$ python3 -m flask run [Enter]

And press Enter.

And press Enter.

and press Enter.

"GET /query HTTP/1.1" 400 -

This is one of the common errors you will encounter while developing Flask applications.

400 code indicates that the server could not understand the request due to invalid syntax.

We will scan the code to spot the error.

This is called debugging.

Press Ctrl+C

Before that, we need to stop the current server by pressing Ctrl and C keys together.

Type

$ export FLASK_ENV=development

and press Enter.

Type $ python3 -m flask run

python3 <space> (hyphen)m <space> flask <space> run

And press Enter.

Highlight Debug mode: on

Click on the refresh button.

Since the debug mode is active, we have received a traceback of errors.

This means that we get the line numbers where the errors have occurred in different files.

This is because an error in one file causes errors in other files as well.

This is termed as the cascading effect.

We have to look for the error in a file that we have written.

framework = request.args['framework']

BadRequestKeyError

That means that the key “framework” for the dictionary request.args does not exist.

To rectify this, we have to modify that line.

Type

framework = request.args.get('framework')

Now change the line as shown.

Highlight

framework = request.args.get('framework')

That means, instead of causing a 400 error, it will return None.

The framework value is None.

Click on the address bar

Type http://127.0.0.1:5000/query?framework=my_flask

Question mark framework <equal to> my_flask

And press Enter.

The framework value is: my_flask

We got the result as expected.

It can be used to collect data from the user using a web form.

And then store the data as a dictionary in the form attribute.

Let us look at an example.

Type

@app.route('/form', methods=['GET', 'POST'])

Let us define a new route called form.

We allow both GET and POST as possible request methods.

Type the code as shown here.

Type

def my_form():

if request.method == 'POST':

firstname = request.form.get('firstname')

lastname = request.form.get('lastname')

if request.method == 'POST':

firstname = request.form.get('firstname')

lastname = request.form.get('lastname')

• the client will send data to the server using the form.
• This is to update the server with some information

• For example - you enter your username and password while logging in to a website.
• In contrast, GET request method is used to get or read some data from the server.

We will obtain the entered values from the request object using the form attribute.

html_string1 = '''<h1>The firstname value is: {}</h1>

<h1>The lastname value is: {}</h1>'''.format(firstname, lastname)

return html_string1

This code will display the firstname as well as the lastname.

Type

html_string2 = <form method="POST">

Firstname: <input type="text" name="firstname">

Lastname: <input type="text" name="lastname">

<input type="submit" value="Submit">

</form>

return html_string2

This will simply display a web form in the output, if the request method is GET.

Type http://127.0.0.1:5000/form

In the address bar type the URL as http://localhost:5000/form and press Enter.

Last name as Tutorial

Last name as Tutorial.

And then click on Submit button located at the bottom of the web form.

Highlight

The text in the browser

This is one of the most frequent errors you will encounter during flask app development.

a = 10/0

a <equal to> 10 slash zero

As we are dividing 10 by zero, we should get an error.

Save the file.

Type http://localhost:5000/user/Sid

In the address bar type the URL as http://localhost:5000/user/Sid and press Enter

Highlight "GET /user/error HTTP/1.1" 500

Which means we have encountered an internal server error.

The 500 error simply implies that, there’s something wrong in the web server.

Let us summarise.

• The request-response cycle in Flask.
• Different attributes of the request object.
• Different types of request data.
• Several HTTP response codes.

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