4.5. Sets

Contents

• Sets
  • Quick example
  • Ordering
  • Operators

Sets are constructed from a sequence (or some other iterable object). Since sets cannot have duplicated, there are usually used to build sequence of unique items (e.g., set of identifiers).

4.5.1. Quick example

>>> a = set([1, 2, 3, 4])
>>> b = set([3, 4, 5, 6])
>>> a | b # Union
{1, 2, 3, 4, 5, 6}
>>> a & b # Intersection
{3, 4}
>>> a < b # Subset
False
>>> a - b # Difference
{1, 2}
>>> a ^ b # Symmetric Difference
{1, 2, 5, 6}

Note

the intersection, subset, difference and symmetric difference can be be called with method rather that symbols. See below for examples.

4.5.2. Ordering

Just as with dictionaries, the ordering of set elements is quite arbitrary, and shouldn’t be relied on.

4.5.3. Operators

As mentionned in the quick example section, each operator is associated to a symbol (e.g., &) and a method name (e.g. union).

>>> a = set([1, 2, 3])
>>> b = set([2, 3, 4])
>>> c = a.intersection(b) # equivalent to c = a & b
>>> a.intersection(b)
set([2, 3])

>>> c.issubset(a)
True
>>> c <= a
True

>>> c.issuperset(a)
False
>>> c >= a
False

>>> a.difference(b)
set([1])
>>> a - b
set([1])

>>> a.symmetric_difference(b)
set([1, 4])
>>> a ^ b
set([1, 4])

You can also copy a set using the copy method:

>>> a.copy()
set([1, 2, 3])