Hidden features of Python

Question

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

Answer 1

Chaining comparison operators:

>>> x = 5
>>> 1 < x < 10
True
>>> 10 < x < 20 
False
>>> x < 10 < x*10 < 100
True
>>> 10 > x <= 9
True
>>> 5 == x > 4
True

In case you're thinking it's doing 1 < x, which comes out as True, and then comparing True < 10, which is also True, then no, that's really not what happens (see the last example.) It's really translating into 1 < x and x < 10, and x < 10 and 10 < x * 10 and x*10 < 100, but with less typing and each term is only evaluated once.