Hidden features of Python

What are the lesser-known but useful features of the Python programming language?

• Try to limit answers to Python core.
• One feature per answer.
• Give an example and short description of the feature, not just a link to documentation.
• Label the feature using a title as the first line.

Quick links to answers:

• Argument Unpacking
• Braces
• Chaining Comparison Operators
• Decorators
• Default Argument Gotchas / Dangers of Mutable Default arguments
• Descriptors
• Dictionary default .get value
• Docstring Tests
• Ellipsis Slicing Syntax
• Enumeration
• For/else
• Function as iter() argument
• Generator expressions
• import this
• In Place Value Swapping
• List stepping
• __missing__ items
• Multi-line Regex
• Named string formatting
• Nested list/generator comprehensions
• New types at runtime
• .pth files
• ROT13 Encoding
• Regex Debugging
• Sending to Generators
• Tab Completion in Interactive Interpreter
• Ternary Expression
• try/except/else
• Unpacking+print() function
• with statement