HTML forms with POST method in Flask - Step-by-Step Execution

Concept Flow - HTML forms with POST method

User fills form fields

↓

User clicks Submit button

↓

Browser sends POST request

↓

Flask server receives POST data

↓

Server processes data

↓

Server sends response page

↓

Browser displays response

This flow shows how a user fills a form, submits it via POST, the server processes the data, and sends back a response.

Execution Sample

Flask

from flask import Flask, request, render_template_string
app = Flask(__name__)

@app.route('/', methods=['GET', 'POST'])
def form():
    if request.method == 'POST':
        name = request.form['name']
        return f'Hello, {name}!'
    return '''<form method='POST'><input name='name'><button type='submit'>Submit</button></form>'''

A simple Flask app that shows a form and greets the user by name after POST submission.

Execution Table

Step	Action	Request Method	Form Data	Server Response
1	User visits page	GET	None	Show form with input and submit button
2	User types 'Alice' in input	GET	None	Form still shown, no change
3	User clicks Submit	POST	name=Alice	Server reads 'Alice' and returns 'Hello, Alice!'
4	Browser displays greeting	POST	name=Alice	Page shows: Hello, Alice!
5	User refreshes page	GET	None	Form shown again, ready for new input

💡 Execution stops after server sends response and browser displays it.

Variable Tracker

Variable	Start	After Step 3	After Step 5
request.method	GET	POST	GET
request.form['name']	N/A	Alice	N/A
name	N/A	Alice	N/A

Key Moments - 3 Insights

Why does the server check if request.method is 'POST'?

Because the form page is shown on GET requests, but the server only processes submitted data on POST requests, as shown in execution_table step 3.

What happens if the user refreshes the page after submitting the form?

The browser sends a GET request again, so the form is shown empty, as in execution_table step 5.

Why is the form's method attribute set to 'POST'?

To tell the browser to send form data in the request body, not in the URL, which matches the server expecting POST requests (execution_table step 3).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of request.method at step 3?

A POST

B GET

C PUT

D DELETE

💡 Hint

Check the 'Request Method' column at step 3 in the execution_table.

At which step does the server read the form data 'name=Alice'?

A Step 1

B Step 2

C Step 3

D Step 5

💡 Hint

Look at the 'Form Data' and 'Action' columns in execution_table.

If the form's method was changed to 'GET', what would change in the execution table?

A Request Method would be POST at step 3 but no form data sent

B Request Method would be GET at step 3 and form data sent in URL

C Server would not receive any data

D Browser would not send any request

💡 Hint

Think about how GET sends data compared to POST, and check the 'Request Method' column.

Concept Snapshot

HTML forms with POST method:
- Use <form method='POST'> to send data securely in request body.
- Server checks if request.method == 'POST' to process data.
- Access submitted data via request.form['fieldname'] in Flask.
- After processing, server returns response (e.g., greeting).
- GET requests show the form; POST requests handle submission.

Full Transcript

This visual trace shows how an HTML form with method POST works in a Flask app. The user visits the page (GET), sees a form, types a name, and submits it. The browser sends a POST request with the form data. The Flask server checks if the request method is POST, reads the form data from request.form, and returns a greeting message. The browser then displays this greeting. Refreshing the page sends a GET request again, showing the form anew. Key points include the difference between GET and POST, how form data is sent, and how the server processes it.