Sequence and Mapping Methods

Describing Mapping Methods

• The following class methods can be used for mappings:

  • __len__()
  • __getitem__()
  • __setitem__()
  • __delitem__()
  • __contains__()

• __len__() has the following properties:

  • Called by the built-in len() function
  • Determines truth values unless the __bool__() method has been defined

• __getitem__() has the following properties:

  • Returns an item by key value
  • The key can be any object
  • The key is typically an integer for sequences

• __setitem__() has the following properties:

  • Assigns a value to an element

• __delitem__() has the following properties:

  • Deletes an element from the object

• __contains__() has the following properties:

  • Determines if a given element is in the object
  • Used to implement the in operator

>>> a = [1, 2, 3, 4, 5, 6]
>>> len(a)  # a.__len__()
6
>>> a[2]  # a.__getitem__(2)
3
>>> a[1] = 7  # a.__setitem__(1,7)
>>> print(a)
[1, 7, 3, 4, 5, 6]
>>> del a[2]  # a.__delitem__(2)
>>> print(a)
[1, 7, 4, 5, 6]  # a.__contains__(5)
>>> 5 in a
True

Example of Mapping Methods using Slicing

>>> a = [1,2,3,4,5,6] 
>>> a[1:5]
[2, 3, 4, 5]
>>> a[1:3] = [10,11,12]
>>> print(a)
[1, 10, 11, 12, 4, 5, 6]
>>> del a[1:4]
[1, 4, 5, 6]

>>> x = [1,2,3,4,5,6]
>>> x.__getitem__(slice(1,5,None))
[2, 3, 4, 5]
>>> x.__setitem__(slice(1,3,None), [10,11,12])
>>> print(x)
[1, 10, 11, 12, 4, 5, 6]
>>> x.__delitem__(slice(1,4,None))
[1, 4, 5, 6]

Summarizing Special Methods

References

• Python Essential Reference