6.3 Sets

  • A set is an unordered collection of unique values.
  • May contain only immutable objects, like strings, ints, floats and tuples that contain only immutable elements.
  • Sets do not support indexing and slicing.

Creating a Set with Curly Braces

  • Duplicates are ignored, making sets great for duplicate elimination.
In [1]:
colors = {'red', 'orange', 'yellow', 'green', 'red', 'blue'}
  • Though the output below is sorted, sets are unordered—do not write order-dependent code.
In [2]:
colors
Out[2]:
{'blue', 'green', 'orange', 'red', 'yellow'}

Determining a Set’s Length

In [3]:
len(colors)
Out[3]:
5

Checking Whether a Value Is in a Set

In [4]:
'red' in colors
Out[4]:
True
In [5]:
'purple' in colors
Out[5]:
False
In [6]:
'purple' not in colors
Out[6]:
True

Iterating Through a Set

  • There’s no significance to the iteration order.
In [7]:
for color in colors:
    print(color.upper(), end=' ')
    

YELLOW BLUE GREEN RED ORANGE

Creating a Set with the Built-In set Function

In [8]:
numbers = list(range(10)) + list(range(5))
In [9]:
numbers
Out[9]:
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 1, 2, 3, 4]
In [10]:
set(numbers)
Out[10]:
{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
  • To create an empty set, must use the set(), because {} represents an empty dictionary.
  • Python displays an empty set as set() to avoid confusion with an empty dictionary ({}).
In [11]:
set()
Out[11]:
set()

Frozenset: An Immutable Set Type

  • Sets are mutable.
  • Set elements must be immutable; therefore, a set cannot have other sets as elements.
  • A frozenset is an immutable set—it cannot be modified after you create it, so a set can contain frozensets as elements.
  • The built-in function frozenset creates a frozenset from any iterable.

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