http://effbot.org/zone/default-values.htm

…when you use a “mutable” object as a default value; that is, a value that can be modified in place, like a list or a dictionary.

An example:

>>> def function(data=[]):
...     data.append(1)
...     return data
...
>>> function()
[1]
>>> function()
[1, 1]
>>> function()
[1, 1, 1]

https://blogs.dropbox.com/tech/2019/09/our-journey-to-type-checking-4-million-lines-of-python/

Gradually migrating using mypy.

Once your project is tens of thousands of lines of code, and several engineers work on it, our experience tells us that understanding code becomes the key to maintaining developer productivity. 

Type comments were also handy for Python 2 support, which has no built-in notion of type annotations:

https://stackoverflow.com/questions/11520492/difference-between-del-remove-and-pop-on-lists

Object? Yes -> remove()

No -> Index range? Yes -> del

No -> return value? Yes -> pop()

No -> del