Master Flask: Quick Guide to Installation, Routing, and Session Management

Introduction

Flask is a lightweight Python web framework that enables rapid development. It is easy to get started with, though it has some limitations that are not discussed here.

1. Install Flask

pip install flask  # note: flake is a linting tool, not to be confused with Flask

The command installs four dependent packages for template rendering and networking.

2. Basic Flask Usage

2.1 Start the Application

from flask import Flask<br/>app = Flask(__name__)<br/><br/>@app.route('/')  # route decorator creates a URL rule<br/>def hello():<br/>    return 'hello'  # view function response<br/><br/>if __name__ == '__main__':<br/>    app.run(debug=True)  # run in debug mode<br/><br># optional: specify host and port<br/>app.run(host='192.168.1.2', port=3243)

After modifying the view function, refreshing the browser shows the new output without restarting the server.

2.2 Project Structure

Create a project folder containing at least two subfolders: static for media files and templates for HTML templates. Flask does not generate these automatically.

The url_map stores the mapping between URLs and endpoints. An endpoint is the internal name (usually the view function name) that Flask uses to locate the correct handler.

2.3 Flask Configuration Options

Key attributes include:

static_url_path : URL prefix for static files

static_folder : Filesystem path for static files (default static)

template_folder : Filesystem path for templates (default templates)

2.4 Adding Configuration

2.4.1 Directly in app.run()

Enable debug mode:

app.run(debug=True)

2.4.2 Using app.config

app.config = True

2.4.3 External Config File

Create a file (e.g., hw.cfg) with DEBUG=True and load it:

app.config.from_pyfile('hw.cfg')

3. View Function Patterns

3.1 Sub‑page

@app.route('/index')<br/>def index():<br/>    return 'index'

3.2 Converters

@app.route('/index/<int:num>')  # <int> converter named num<br/>def num(num):<br/>    if num > 10:<br/>        return 'dog'<br/>    else:<br/>        return 'pig'

Flask’s default converters:

DEFAULT_CONVERTERS = {<br/>    'default': UnicodeConverter,<br/>    'string': UnicodeConverter,<br/>    'any': AnyConverter,<br/>    'path': PathConverter,<br/>    'int': IntegerConverter,<br/>    'float': FloatConverter,<br/>    'uuid': UUIDConverter}<br/>

You can also define a custom converter:

class rc(BaseConverter):<br/>    # custom validation, e.g., QQ email regex<br/>    def __init__(self, url_map):<br/>        super(rc, self).__init__(url_map)<br/>        self.regex = '[0-9a-zA-Z_]{0,19}@qq.com'<br/>    def to_python(self, value):<br/>        return value<br/>    def to_url(self, value):<br/>        return value<br/><br/>app.url_map.converters['em'] = rc  # register custom converter<br/>@app.route('/emm/<em:email>')<br/>def email(email):<br/>    return 'email is:%s' % email

3.3 Multiple Routes to Same View

@app.route('/1')<br/>@app.route('/2')<br/>def fg():<br/>    return '1122'

Both /1 and /2 return the same response.

4. Redirection

4.1 Direct Redirect

from flask import redirect<br/>@app.route('/refer')<br/>def refer():<br/>    return redirect('/1')

4.2 Indirect Redirect with url_for

from flask import redirect, url_for<br/>@app.route('/ref')<br/>def ref():<br/>    return redirect(url_for('fg'))

5. Error Handling

5.1 Using abort

@app.route('/use/<id>')<br/>def get_use(id):<br/>    if int(id) < 10:<br/>        abort(404)<br/>    elif int(id) > 20:<br/>        return '---Error---'<br/>    return 'hello:%s' % str(id)

5.2 Custom Error Handler

@app.errorhandler(404)<br/>def error(err):<br/>    return 'hello,%s' % err

5.3 Using make_response

@app.route('/error3')<br/>def err3():<br/>    resp = make_response('search error')<br/>    resp.status = '400'<br/>    resp.headers['hrr'] = 'zjj'<br/>    resp.headers['hw'] = 'zj'<br/>    return resp

6. JSON Responses

@app.route('/json1')<br/>def json1():<br/>    data = {'name': 'HW', 'first': 'ZJ'}<br/>    return jsonify(data)<br/><br/>@app.route('/json2')<br/>def json2():<br/>    return jsonify(hour=12, second=21)

7. Cookie Operations

7.1 Set Cookie

@app.route('/set_cookie')<br/>def set_cookie():<br/>    response = make_response('cookie设置成功')<br/>    time = datetime.datetime.today() + datetime.timedelta(days=30)<br/>    response.set_cookie('user', 'admin', expires=time)<br/>    response.set_cookie('pass', '123456', expires=time)<br/>    response.headers['X-Something'] = 'mything'<br/>    response.headers['Server'] = 'Linux'<br/>    return response

7.2 Get Cookie

@app.route('/get_cookie')<br/>def get_cookie():<br/>    name = "用户名:" + request.cookies.get('user') + "密码:" + request.cookies.get('pass')<br/>    return name

7.3 Delete Cookie

@app.route('/del_cookie1')<br/>def del_cookie1():<br/>    response = make_response('delete cookie 1')<br/>    response.set_cookie('user', '', expires=0)<br/>    response.set_cookie('pass', '', expires=0)<br/>    return response<br/><br/>@app.route('/del_cookie2')<br/>def del_cookie2():<br/>    response = make_response('delete cookie 2')<br/>    response.delete_cookie('user')<br/>    response.delete_cookie('pass')<br/>    return response

8. Session Operations

8.1 Set Session

# configure secret key<br/>app.config['SECRET_KEY'] = os.urandom(30)<br/><br># set session values<br/>session['user'] = 'hw'<br/>session['pass'] = 'zj'<br/><br># make session permanent (default 31 days)<br/>session.permanent = True<br/><br># optional: set custom lifetime (e.g., 2 hours)<br/>app.config['PERMANENT_SESSION_LIFETIME'] = timedelta(hours=2)

8.2 Get Session

@app.route('/session2')<br/>def session2():<br/>    us = session.get('user')<br/>    pa = session.get('pass')<br/>    return 'hello %s %s' % (us, pa)

8.3 Delete Session

# delete specific keys<br/>@app.route('/session3')<br/>def session3():<br/>    session.pop('user', None)<br/>    session.pop('pass', None)<br/>    return 'delete successful!!!!'<br/><br># clear all session data<br/>@app.route('/session4')<br/>def session4():<br/>    session.clear()<br/>    return 'delete successful!!!!'

Conclusion

This article covered the fundamentals of Flask, a powerful yet lightweight web framework that enables fast development of web applications. While only a portion of Flask’s capabilities are demonstrated here, the concepts presented provide a solid foundation for further exploration.