How to parse and insert a line in a file using python or gnu tools

Question

I want to programmatically add 'someapp' into django settings.py file, which looks like this:

before:

...
...
INSTALLED_APPS = [
    'django.contrib.admin',
    'django.contrib.auth',
]
...

after:

...
...
INSTALLED_APPS = [
    'django.contrib.admin',
    'django.contrib.auth',
    'someapp',
]
...

What comes to my mind is the following step:

1. locate ^INSTALLED_APPS = [ string in the file
2. locate ] string to indicate the end of string parsed.
3. fetch all the items within [] to avoid conflict
4. append someapp to the end of []

This looks like a simple question, but I still want to know how to do it with best practice.

Although this is about python scripting, Bash script with GNU tools, like sed,awk,grep, are also welcomed.