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cgi Module

⚠️ REMOVED IN PYTHON 3.13: The cgi module was deprecated in Python 3.11 and removed in Python 3.13.

The cgi module provides utilities for parsing CGI (Common Gateway Interface) form data and query strings.

Complexity Reference

Operation Time Space Notes
parse_qs() O(n) O(n) n = query string length
parse_qsl() O(n) O(n) n = query string length
FieldStorage O(n) O(n) n = form data size
URL decoding O(n) O(n) n = string length

Parsing Query Strings

Query String Parsing

from urllib.parse import parse_qs, parse_qsl

# Modern approach (use urllib.parse instead)
query_string = "name=John&age=30&hobbies=reading&hobbies=gaming"

# Parse as dict (values are lists) - O(n)
params = parse_qs(query_string)
print(params)  # {'name': ['John'], 'age': ['30'], 'hobbies': ['reading', 'gaming']}

# Parse as list of tuples - O(n)
params_list = parse_qsl(query_string)
print(params_list)  # [('name', 'John'), ('age', '30'), ('hobbies', 'reading'), ('hobbies', 'gaming')]

Form Data Handling

FieldStorage (Deprecated)

import cgi

# Parse form data - O(n)
# In production, use Python's built-in wsgiref or modern frameworks
form = cgi.FieldStorage()

# Access form fields
if 'username' in form:
    username = form.getvalue('username')

# Access file uploads
if 'file' in form:
    fileitem = form['file']
    if fileitem.filename:
        filename = fileitem.filename
        content = fileitem.file.read()

URL Encoding/Decoding

Escape Functions

import cgi
from html import escape
from urllib.parse import quote, unquote

# Escape HTML - O(n)
user_input = '<script>alert("xss")</script>'
safe = escape(user_input)
# Result: &lt;script&gt;alert(&quot;xss&quot;)&lt;/script&gt;

# URL encoding - O(n)
text = "hello world & special chars!"
encoded = quote(text)
decoded = unquote(encoded)

Common Patterns (Legacy)

CGI Script Template

#!/usr/bin/env python3

import cgi
import sys

def main():
    # Print HTTP header
    print("Content-Type: text/html\n")

    # Parse form data - O(n)
    form = cgi.FieldStorage()

    # Get parameters
    name = form.getvalue('name', 'Guest')

    # Generate response
    from html import escape
    print(f"<h1>Hello, {escape(name)}!</h1>")

if __name__ == "__main__":
    main()

Error Handling

import cgi
import traceback

def handle_request():
    try:
        form = cgi.FieldStorage()
        # Process form
        print("Content-Type: text/html\n")
        print("<h1>Success</h1>")
    except Exception as e:
        # Print error page
        print("Content-Type: text/html\n")
        print(f"<h1>Error</h1><pre>")
        traceback.print_exc()
        print("</pre>")

handle_request()

Modern Alternatives

Using urllib.parse

from urllib.parse import parse_qs, parse_qsl, urlencode, quote, unquote

# Parse query string - O(n)
query = "name=John&age=30"
params = parse_qs(query)

# Encode parameters - O(n)
data = {'name': 'John', 'age': '30'}
encoded = urlencode(data)

# URL encoding - O(n)
text = "special chars: @ # $"
quoted = quote(text)
unquoted = unquote(quoted)

Using WSGI

from urllib.parse import parse_qs

def application(environ, start_response):
    # Get query string - O(n)
    query_string = environ.get('QUERY_STRING', '')
    params = parse_qs(query_string)

    # Process request
    response = b"Hello World"

    status = '200 OK'
    headers = [('Content-Type', 'text/plain')]
    start_response(status, headers)

    return [response]

Using Modern Framework

# Using Flask (recommended for CGI-like functionality)
from flask import Flask, request

app = Flask(__name__)

@app.route('/submit', methods=['GET', 'POST'])
def submit():
    name = request.args.get('name', 'Guest')
    age = request.form.get('age', 'Unknown')

    return f"<h1>Hello {name}, age {age}</h1>"

if __name__ == '__main__':
    app.run()

Removal Notice

# ❌ DON'T: Use cgi module (removed in 3.13)
import cgi
form = cgi.FieldStorage()

# ✅ DO: Use modern alternatives
# - urllib.parse for query strings
# - wsgiref for WSGI applications
# - Flask/Django for web applications
# - FastAPI for modern async applications