Python: interactive shell and shell_context_processor() decorator.

The

$ flask shell

command starts a Python interactive command prompt where we can explore our application data. To quote from the link Command Line Interface:

To explore the data in your application, you can start an interactive Python shell with the shell command. An application context will be active, and the app instance will be imported.

…

Use shell_context_processor() to add other automatic imports.

In this short post, we’re focusing on the shell_context_processor() decorator and how it works with the “flash shell” command.

The code for this post is built upon the code created for this post Python: Flask-RESTX and the Swagger UI automatic documentation. This code can be cloned from GitHub using:

E:\>git clone -b v1.0.0 https://github.com/behai-nguyen/flask-restx-demo.git

To recap, the layout of the project from the above git clone is as below — ☆ marks the file we’re going to update for this post:

f:\flask_restx_demo
|
|-- .env
|-- app.py ☆
|-- setup.py
|
|-- src\
|   |
|   |-- flask_restx_demo\
|       |
|       |-- __init__.py 
|       |-- config.py
|       |
|       |-- api\
|       |   |       
|       |   |-- __init__.py
|       |   |
|       |   |-- trees\
|       |       |
|       |       |-- bro.py
|       |       |-- dto.py
|       |       |-- routes.py 
|       |       |-- __init__.py
|       |
|       |-- models\
|           |       
|           |-- tree.py
|
|-- venv\

Updated file f:\flask_restx_demo\app.py

"""Flask Application entry point."""

from flask_restx_demo import (
    create_app,
    db,
)	

app = create_app()

@app.shell_context_processor
def shell():
    return {
        "db": db,
    }

The complete code for this post can be downloaded from GitHub using:

E:\>git clone -b v1.0.1 https://github.com/behai-nguyen/flask-restx-demo.git

Basically, we import the db object, and making it available to the Python interactive shell via the @app.shell_context_processor decorator. We list all objects we want to make available to the shell one after another, separated by comma ( , ). Please note that the method does not have to be:

def shell():

We could name it whatever we like, for so long it makes sense. What’s important is the @app.shell_context_processor decorator.

❶ Now we can try opening a Python interactive shell with:

(venv) F:\flask_restx_demo>venv\Scripts\flask.exe shell

We’ll get the following:

Python 3.10.1 (tags/v3.10.1:2cd268a, Dec  6 2021, 19:10:37) [MSC v.1929 64 bit (AMD64)] on win32
App: flask-restx-demo [development]
Instance: F:\flask_restx_demo\instance
>>>

❷ Remember an earlier quote from Command Line Interface?

An application context will be active, and the app instance will be imported.

We’d expect the app object to be available:

>>> print( app )

We should get:

<Flask 'flask-restx-demo'>
>>>

❸ Let’s look at the db object:

>>> print( db )

This is the output on my configuration:

<SQLAlchemy engine=sqlite:///F:\flask_restx_demo\flask_restx_demo.db>
>>>

❹ Similar to other Python interactive shells, we can also use the db object in an interactive manner:

>>> conn = db.engine.connect()
>>> res = conn.execute( 'select * from tree' )
>>> for r in res:
...    print( r )
... press Enter key

It’ll print out the… entire tree table:

<pre>
(1, 'Acer palmatum', 'Japanese maple', 'https://en.wikipedia.org/wiki/Acer_palmatum')
(2, 'Liquidambar', 'Sweetgums', 'https://en.wikipedia.org/wiki/Liquidambar')
(3, 'Lagerstroemia', 'Crepe myrtle', 'https://en.wikipedia.org/wiki/Lagerstroemia')
(4, 'Pinus Thunbergii', 'Black Pine', 'https://en.wikipedia.org/wiki/Pinus_thunbergii')
(5, 'Pinus parviflora', 'Japanese White Pine', 'https://en.wikipedia.org/wiki/Pinus_parviflora')
>>>

❺ I did run the same application on my Synology DS218 box. It’s not different to the Windows 10 Pro environment:

(venv) behai@omphalos-nas-01:/var/services/web/flask_restx_demo$ sudo venv/bin/flask shell

Python 3.9.6 (default, Jan  5 2022, 15:50:31)
[GCC 8.5.0] on linux
App: flask-restx-demo [development]
Instance: /volume1/web/flask_restx_demo/instance

Then:

>>> conn = db.engine.connect()
>>> res = conn.execute( 'select * from tree' )
>>> for r in res:
...     print( r )
... press Enter key

I’ve only one record in the tree table on this environment:

(1, 'Acer Buergerianum', 'Trident Maple', 'https://en.wikipedia.org/wiki/Acer_buergerianum')
>>>

❻ To exit the interactive shell:

>>> exit()

We’ll be returned to the Python virtualenv prompt.

The complete code for this post can be downloaded from GitHub using:

E:\>git clone -b v1.0.1 https://github.com/behai-nguyen/flask-restx-demo.git

For such small addition, we’ve such rich functionalities… I find this feature very enticing. I hope you find this post helpful in some manner. And thank you for reading.