Other Core Types

Beyond the core types we’ve seen so far, there are others that may or may not qualify for membership in the set, depending on how broadly it is defined. Sets, for example, are a recent addition to the language that are neither mappings nor sequences; rather, they are unordered collections of unique and immutable objects. Sets are created by calling the built-in set function or using new set literals and expressions in 3.0, and they support the usual mathematical set operations (the choice of new {...} syntax for set literals in 3.0 makes sense, since sets are much like the keys of a valueless dictionary):

>>> X = set('spam')                 # Make a set out of a sequence in 2.6 and 3.0
>>> Y = {'h', 'a', 'm'}             # Make a set with new 3.0 set literals
>>> X, Y
({'a', 'p', 's', 'm'}, {'a', 'h', 'm'})

>>> X & Y                           # Intersection
{'a', 'm'}

>>> X | Y                           # Union
{'a', 'p', 's', 'h', 'm'}

>>> X – Y                           # Difference
{'p', 's'}

>>> {x ** 2 for x in [1, 2, 3, 4]}  # Set comprehensions in 3.0
{16, 1, 4, 9}

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In addition, Python recently grew a few new numeric types: decimal numbers (fixed-precision floating-point numbers) and fraction numbers (rational numbers with both a numerator and a denominator). Both can be used to work around the limitations and inherent inaccuracies of floating-point math:

>>> 1 / 3                           # Floating-point (use .0 in Python 2.6)
0.33333333333333331
>>> (2/3) + (1/2)
1.1666666666666665

>>> import decimal                  # Decimals: fixed precision
>>> d = decimal.Decimal('3.141')
>>> d + 1
Decimal('4.141')

>>> decimal.getcontext().prec = 2
>>> decimal.Decimal('1.00') / decimal.Decimal('3.00')
Decimal('0.33')

>>> from fractions import Fraction  # Fractions: numerator+denominator
>>> f = Fraction(2, 3)
>>> f + 1
Fraction(5, 3)
>>> f + Fraction(1, 2)
Fraction(7, 6)

Python also comes with Booleans (with predefined True and False objects that are essentially just the integers 1 and 0 with custom display logic), and it has long supported a special placeholder object called None commonly used to initialize names and objects:

>>> 1 > 2, 1 < 2                    # Booleans
(False, True)
>>> bool('spam')
True

>>> X = None                        # None placeholder
>>> print(X)
None
>>> L = [None] * 100                # Initialize a list of 100 Nones
>>> L
[None, None, None, None, None, None, None, None, None, None, None, None,
None, None, None, None, None, None, None, None, ...a list of 100 Nones...]