What’s the difference between the square bracket and dot notations in Python?

by

The dot operator is used for accessing attributes of any object. For example, a complex number

>>> c = 3+4j

has (among others) the two attributes real and imag:

>>> c.real
3.0
>>> c.imag
4.0

As well as those, it has a method, conjugate(), which is also an attribute:

>>> c.conjugate
<built-in method conjugate of complex object at 0x7f4422d73050>
>>> c.conjugate()
(3-4j)

Square bracket notation is used for accessing members of a collection, whether that’s by key in the case of a dictionary or other mapping:

>>> d = {'a': 1, 'b': 2}
>>> d['a']
1

… or by index in the case of a sequence like a list or string:

>>> s = ['x', 'y', 'z']
>>> s[2]
'z'
>>> t="Kapow!"
>>> t[3]
'o'

These collections also, separately, have attributes:

>>> d.pop
<built-in method pop of dict object at 0x7f44204068c8>
>>> s.reverse
<built-in method reverse of list object at 0x7f4420454d08>
>>> t.lower
<built-in method lower of str object at 0x7f4422ce2688>

… and again, in the above cases, these attributes happen to be methods.

While all objects have some attributes, not all objects have members. For example, if we try to use square bracket notation to access a member of our complex number c:

>>> c[0]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'complex' object is not subscriptable

… we get an error (which makes sense, since there’s no obvious way for a complex number to have members).

It’s possible to define how [] and . access work in a user-defined class, using the special methods __getitem__() and __getattr__() respectively. Explaining how to do so is beyond the scope of this question, but you can read more about it in the Python Tutorial.

More Related Contents:

  1. How to access object attribute given string corresponding to name of that attribute
  2. How do you programmatically set an attribute?
  3. Python dictionary from an object’s fields
  4. How to check if an object has an attribute in Python?
  5. How can I create an object and add attributes to it?
  6. How to access (get or set) object attribute given string corresponding to name of that attribute
  7. How to check if an object has an attribute?
  8. How do I check if an object has an attribute?
  9. Python object attributes – methodology for access
  10. How to build a basic iterator?
  11. Difference between text and innerHTML using Selenium
  12. Spawning multiple instances of the same object concurrently in python
  13. Saving and loading multiple objects in pickle file?
  14. Python function attributes – uses and abuses [closed]
  15. Is there a short contains function for lists?
  16. super() fails with error: TypeError “argument 1 must be type, not classobj” when parent does not inherit from object
  17. Accessing Object Memory Address
  18. Elegant way to check if a nested key exists in a dict?
  19. python assign values to list elements in loop
  20. Difference between ‘cls’ and ‘self’ in Python classes?
  21. Evaluation of boolean expressions in Python
  22. What is the best way to do automatic attribute assignment in Python, and is it a good idea?
  23. Getting container/parent object from within python
  24. Retrieving JSON objects from a text file (using Python)
  25. “_set” in a queryset object in Django
  26. hasattr() vs try-except block to deal with non-existent attributes
  27. Should I use instance or class attributes if there will only be one instance? [closed]
  28. Python check instances of classes
  29. Python matplotlib decrease size of colorbar labels
  30. Openpyxl check for empty cell

Leave a Comment