Python Introduction - Google's Python Class
developers.google.com/edu/python/introduction
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
Python is a dynamic, interpreted language. Source code does not declare the types of variables or parameters or methods. This makes the code short and flexible, and you lose the compile-time type checking in the source code. Python tracks the types of all values at runtime and flags code that does not make sense as it runs. (todo: link here to the companion video segment for this section)