Mappings

• A set of items indexed by arbitrary values.
• The built-in function len() returns the number of items in a mapping.

Dictionaries

{ key : value [, key : value]* }

• Comma-separated, brace-enclosed list of key : value pairs.
• Dictionaries are mutable (writeable).

• The indexes cannot contain lists or dictionaries or other mutable types that are compared by value rather than by object identity.
• Numeric indexes obey the normal rules for numeric comparison (i.e. 1 is equivalent to 1.0).

{}                 # 0 items
{1:'a'}            # 1 item
{1:'a', 'two':'b'}  # 2 items