11 min readEverything about the new Flask 2.0

Progress Story Flask 2.0

Before jumping into Flask 2.0, let’s discuss what is Flask in general. If you are a python geek and know something about webservers, you have definitely heard about Flask or worked on it. Flask is classified as a microframework because it does not require particular tools or libraries and it has no database abstraction layer, form validation, or any other components where pre-existing third-party libraries provide common functions.

A few days back on 11th May 2021 pallets projects have released a major stable version of Flask that is Flask 2.0. In this blog, we will try to cover all the new changes that are introduced in the new version and also what all things are dropped from it.

Overview of Flask 2.0

To get the full list of changelog of Flask 2.0 you can follow the official link. In this blog, we are going to cover only the following changes

  • Drops the support of python 2 and 3.5.
  • Added route decorators for common HTTP methods.
    • For example, @app.post("/login") is a shortcut for @app.route("/login", methods=["POST"])
  • Built-in support for asynchronous routes, error handlers, before and after request functions, and teardown callbacks
  • Type hinting introduced
  • Nested blueprint introduced
  • Config.from_file introduced to import config from arbitrary file loaders and deprecated config.from_json
  • Improved CLI messages when the app failed to load when looking up commands.
  • When loading environment from .env or .flaskenv file, the current working directory will no longer be changed to the directory of .env file
  • flask shell will now have tab and history completion if readline is installed

1. Flask 2.0 Drops support of python 2 and 3.5

Don’t get too much excited with the news as the Flask 2.0 doesn’t support python 2 and python3.5. If your current application python version is below 3.5 then, the flask will not upgrade from 1.1.4 to 2.0.1. To crosscheck, first check your current python version using the command below

python --version

Flask 2.0 – Check python version

If the python version is lower then python 3.6 then upgrading the flask version will not work

pip install -U flask

Flask 2.0 – pip install -U flask or pip install flask==2.0.1

You can see the flask is upgraded to 1.1.4 instead of 2.0.1

pip freeze for Flask 2.0

Now check again with python version greater than 3.5

2. Flask 2.0 added route decorators for common HTTP methods.

This update is only syntactic sugar. As we all know to create a minimal server using Flask, we will create a route using Flask app context

Now, with Flask 2.0 we can replace route with the method, for example app.post() or app.get() or app.put() or app.delete(). Its just saving some extra words that’s it, but on a personal note this looks quite awesome.

The only concern will be, when there are multiple routes for the same functionality so rather than saving words it will increase the route lines. If you also have the same concern, please do write that in comment.

3. Async Await introduced in Flask 2.0

Starting with Flask 2.0, you can create asynchronous route handlers using async/await:

To run async await just install flask with extra async using below command

pip install flask[async]

Check out the below diagram for the working of corroutines

Flask 2.0 coroutines – https://testdriven.io/blog/flask-async/

As flask 2.0 is not an async server as it is running the coroutine function in the same process where the master flask server is running. Unfortunately, the flask has started the support for coroutines but not to the full extent. If you want to have async support in Flask to a full extent check out the library aioflask created by Miguel Grinberg

4. Type Hinting Introduced in Flask 2.0

This one is the best. Type hints were introduced in Python 3.5 and from that time it has been extensively used in the python world. But sadly type hints were not there in Flask. If you check the Zen of python

Flask 2.0 – zen of python

The second point is Explicit is better than ugly. Type hints will make sure that this point is getting followed. If you have Flask 1.0 and fire the below command


Flask 2.0 type hints


Flask 2.0 type hints

Now try with Flask 2.0.1 version. The output will have all the type hints in the implementation of Flask

Type hints in Flask 2.0

5. Nested Blueprints Introduced in Flask 2.0

Previously we have to create a separate blueprint even if the child endpoint is changing. That was leading to a code replication in URLs. Flask 2.0 has introduced a nested blueprint. Please check out the link for the official nested blueprint change

Nested blueprints in Flask 2.0

6. from_file method introduced to load configuration

Every project needs configuration. And flask has actually numerous ways to load config in the app context. For example

  • from_object => from class object type
  • from_envvar => from environment variables
  • from_pyfile => from python configuration file
  • from_json => from json file
Load config functions in Flask 2.0

Guess what! Now flask has introduced from_file that can load configuration from any type of file by simply passing the loader as a argument in it. So if you were longing to have TOML files holding your configuration information, rejoice! Now you can! 

Let’s create a toml file with below configuration in it and before going ahead just install the toml library in your environment.

pip install toml

# config.toml

DEBUG = true
SECRET_KEY = "development key"

Now all you have to do is use the new .from_file() method to read the variables from the configuration file. This is done by passing the file path and the file reader to the .from_file() method:

Also, if you were using the .from_json() to load your configs, that has been deprecated! You’ll need to update your code to use the .from_file() method. You probably had something like this before:

from flask import Flask

app = Flask('__name__')

Here’s the updated version using the .from_file() method:

import json

from flask import Flask

app = Flask('__name__')
app.config.from_file('config.json', json.load)

The most important thing to keep in mind when using .from_file() is that you have to have a function that parses that type of file, which means you might need to import new modules or libraries for reading and parsing that file. In the example above, I used the json module to read the configuration from a JSON file, whereas to read from a TOML file, I had to install the toml package.

7. Improved CLI messages when APP failed

In Flask 2, team has put a lot of effort in improving the CLI error messages that appears when the app failed in load up. As there is the support for lazy loading the app, when running flask CLI without providing the proper environment variables we must see a better warning instead of raw exception.

Error message in Flask 2

Flask 2.0 CLI error message

Error Message Flask 1.0

Flask 1.1.4 CLI error message

8. Current working directory changed for .env and .flaskenv

By the current implementation, if .env or .flaskenv was found in the top-level directory, Flask will change the current work directory to the directory that contains the .env or .flaskenv file.

When the user accidentally put a .env or .flaskenv in the top-level directory, then executing flask run in the current directory (contains app.py) will throw out NoAppException.

Checkout below example

.flaskenv for Flask 1.1.4 vs Flask 2.0.1

If we run the above project with the Flask 1.1.4 or below from inside the project directory, it will throw the below error.

Flask 1.1.4 current working directory changed due to .flaskenv

Now run the same project with Flask 2.0 it will not raise any error as it will not change the current working directory.

9. Flask shell will have tab and history completion

Flask shell is being used while debugging the project as by default it loads the flask app context in it. But it is generally hard to do the debugging in it as it doesn’t support any tab and history completion in it. In the new Flask update, the flask shell will have the tab and history completion by default in it. Just run the flask shell using the command below

flask shell

It will now work like just any interactive python shell with the tab completion and history completion feature.


This is the gist of mostly all of the updates that are in Flask 2.0. It’s not a giant leap for the mankind but yeah it’s small step and if you think it will make a difference in your production application then, you must have to go for it. My personal opinion, we can do much more in the async coroutines section as other frameworks like Quart/FastAPI are way to ahead of Flask right now, in terms of coroutines.

I hope, it has helped you or it will help you. If you want to discuss anything or anything related to tech, you can contact me here or on the Contact Page. If you are interested in becoming a part of the Progress story please reach out to me or check out the Create Blog page.

See you next time! Peace Out ✌️

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