Where names are visible.
WHAT SCOPE MEANS
Which parts of your code can see a given name.
THE BASIC RULE
A name assigned inside a function exists only inside it.
A name assigned at the top level is visible throughout the file.
WHY THAT MATTERS
It prevents functions interfering with each other.
WHAT SURPRISES PEOPLE
A function can read a name from the enclosing scope but cannot reassign it without declaring that intention.
WHAT TO DO
Pass values in as arguments and return results.
Rather than relying on names defined elsewhere.
WHY
Functions depending on external state are hard to test and hard to reason about.
GLOBAL VALUES
Possible, and usually a sign the design could be better.
WHAT TO USE INSTEAD
Arguments and return values Configuration passed explicitly
WHAT TO BE CAREFUL WITH
Reusing a name inside a function that also exists outside it Assuming a change inside a function affects the outside
WHAT DOES CARRY OUT
Changes to mutable objects passed in, such as lists and dictionaries.
