Hidden features of Python -

what lesser-known useful features of python programming language?

• try limit answers python core.
• one feature per answer.
• give example , short description of feature, not link documentation.
• label feature using title first line.

quick links 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 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 @ runtime
• .pth files
• rot13 encoding
• regex debugging
• sending generators
• tab completion in interactive interpreter
• ternary expression
• try/except/else
• unpacking+print() function
• with statement

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, comes out true, , comparing true < 10, true, no, that's not happens (see last example.) it's translating 1 < x , x < 10, , x < 10 , 10 < x * 10 , x*10 < 100, less typing , each term evaluated once.