Frequent errors.
INDENTATION PROBLEMS
Mixing tabs and spaces.
Configure the editor for spaces and the problem disappears.
OFF-BY-ONE ERRORS
Positions start at zero, and ranges exclude the end.
MODIFYING A COLLECTION WHILE ITERATING
Produces skipped items or errors.
Iterate over a copy, or build a new collection.
MUTABLE DEFAULT ARGUMENTS
The same object reused across calls.
Use a marker default and create the value inside.
COMPARING WITH THE WRONG OPERATOR
Equality and identity are different checks.
ASSUMING A KEY OR INDEX EXISTS
Check, or access with a default.
CONFUSING A VALUE WITH ITS TEXT REPRESENTATION
Particularly numbers read from files.
FORGETTING A RETURN
A function that computes and returns nothing.
SHADOWING A BUILT-IN NAME
Which then becomes unavailable.
IMPORT ORDERING PROBLEMS
Circular imports, or a file named after a library.
WHAT CATCHES MOST OF THESE
Running frequently, in small pieces.
