Flask Essentials For Web Developer

Python's web framework is quite similar to each other, thanks to the WSGI standard, so the first time when you learn a new Python web development framework, there are a few essential things you should know, and those will help you to get started fast.

1, Read parameter from Query String:

from flask import request
searchword = request.args.get("key")

2, Read parameter from post body

from flask import request
username = request.form.get("username")

3, Read uploading file content from form body

from flask import request
f = request.files['the_file']
content = f.read()
print(f.content_type)
print(f.mimetype)
print(f.name)
print(f.filename)

File object reference: Data Structures - Werkzeug Documentation (0.14)

4, Return a response

Directly return rendered string:

def handler():
	page_content = render_template("home.html")
	return page_content

Return a pre-made response object

from flask import make_response

def handler():
	page_content = render_template("home.html")
	resp = make_response(page_content, 200)
	resp.headers["x-key"] = "value"
	return resp

Get Cookie:

from flask import request

# get cookie example
def handler():
	username = request.cookies.get('username')

Set Cookie:

from flask import make_response

def handler():
	resp = make_response(render_template("home.html"))
	resp.set_cookie('username', 'the username')
	return resp

6, Redirect

301: Move permanently

Use this one if one of url is permanently moved to another location

from flask import redirect
def handler():
	return redirect("/some-url", 301)

302: Found

from flask import redirect
def handler():
	return redirect("/some-url", 302)

303: See other (most common)

Use this in the scenario that after user posted some data, system wants to redirect this user to get another page url

from flask import redirect
def handler():
	return redirect("/some-url", 303)

7, Return response with status code

# use an extra returned value as status code
def handler():
	page_content = render_template('error.html')
	return page_content, 404

8, Return response with custom headers

# use an extra returned value (second or third) as header
def handler():
	page_content = render_template("home.html")
	headers = {
		"x-extra-key": "value"
	}
	return page_content, headers
	# or
	return page_content, 200, headers

9, Sessions

Flask's built-in session will save session info into user's client side (into cookie) after encrypting it, so the app need to specify a secret_key used for encryption.
DO NOT give this key to any one! It should only be saved in the server side.

from flask import Flask, session, redirect

app = Flask(__name__)

# Set the secret key to some random bytes. Keep this really secret!
# generate a good secret key:
# $ python -c 'import os; print(os.urandom(16))'
app.secret_key = b'some-secret-key-for-encrypting-session-data'

def handler():
	if 'username' not in session:
		return "sign-in required"
	session['username'] = "some username"
	return redirect("/home", 303)

10, Request context

from flask import request

def handler():
	print(dir(request))

# here are some attributes might be commonly used
request.base_url
request.blueprint
request.endpoint
request.environ
request.full_path
request.headers
request.host
request.host_url
request.is_secure
request.is_xhr
request.method
request.mimetype
request.path
request.query_string
request.referrer
request.remote_addr
request.scheme
request.url
request.url_root
request.user_agent

Additionally, the request.environ is a dictionary, looks like this:

{
	'wsgi.version': (1, 0), 
	'wsgi.url_scheme': 'http', 
	// ...
	'SERVER_SOFTWARE': 'Werkzeug/0.14.1', 
	'REQUEST_METHOD': 'GET', 
	'SCRIPT_NAME': '', 
	'PATH_INFO': '/', 
	'QUERY_STRING': '', 
	'REMOTE_ADDR': '127.0.0.1', 
	'REMOTE_PORT': 64636, 
	'SERVER_NAME': '127.0.0.1', 
	'SERVER_PORT': '3333', 
	'SERVER_PROTOCOL': 'HTTP/1.1', 
	'HTTP_HOST': '127.0.0.1:3333', 
	'HTTP_CONNECTION': 'keep-alive', 
	'HTTP_CACHE_CONTROL': 'max-age=0', 
	'HTTP_UPGRADE_INSECURE_REQUESTS': '1', 
	'HTTP_USER_AGENT': 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_0)',
	'HTTP_DNT': '1', 
	'HTTP_ACCEPT': 'text/html,image/apng,*/*;q=0.8', 
	'HTTP_ACCEPT_ENCODING': 'gzip, deflate, br', 
	'HTTP_ACCEPT_LANGUAGE': 'en-US,en;q=0.9,zh-CN;q=0.8,zh;', 
	'werkzeug.request': <Request '<http://127.0.0.1:3333/>' [GET]>
}

request.headers is also a dictionary, for example:

{
	"Host": "127.0.0.1:3333",
	"Connection": "keep-alive",
	"Cache-Control": "max-age=0",
	"Upgrade-Insecure-Requests": "1",
	"User-Agent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/69.0.3497.100 Safari/537.36",
	"Dnt": "1",
	"Accept": "text/html,application/xhtml+xml,application/xml;",
	"Accept-Encoding": "gzip, deflate, br",
	"Accept-Language: en-US,en;q=0.9,zh-CN;q=0.8,zh;q=0.7,zh-TW;q=0.6"
}

11, Logging

from flask import current_app

current_app.logger.debug('A value for debugging')
current_app.logger.warning('A warning occurred (%d apples)', 42)
current_app.logger.error('An error occurred')

12, Flask Internal Config Value

Configuration Handling - Flask 1.0.2 documentation

ENV
DEBUG
SECRET_KEY
SESSION_COOKIE_NAME
SESSION_COOKIE_DOMAIN
SESSION_COOKIE_PATH
SESSION_COOKIE_HTTPONLY
SESSION_COOKIE_SECURE
PERMANENT_SESSION_LIFETIME: set cookie expire time
TEMPLATES_AUTO_RELOAD

13, Jinja Autoescaping

By default, Flask's render_template will auto escape all variables in jinja templates, if you want to unescape a variable, here are 2 ways:

1, use |safe:

{{ myvariable|safe }}

2, use autoescape block:

{% autoescape false %}
    <p>autoescaping is disabled here
    <p>{{ will_not_be_escaped }}</p>
{% endautoescape %}