I have gone through most of the documentation of __getitem__
in the Python docs, but I am still unable to grasp the meaning of it.
So all I can understand is that __getitem__
is used to implement calls like self[key]
. But what is the use of it?
Lets say I have a python class defined in this way:
class Person:
def __init__(self,name,age):
self.name = name
self.age = age
def __getitem__(self,key):
print ("Inside `__getitem__` method!")
return getattr(self,key)
p = Person("Subhayan",32)
print (p["age"])
This returns the results as expected. But why use __getitem__
in the first place? I have also heard that Python calls __getitem__
internally. But why does it do it?
Can someone please explain this in more detail?
This question is related to
python
The magic method __getitem__
is basically used for accessing list items, dictionary entries, array elements etc. It is very useful for a quick lookup of instance attributes.
Here I am showing this with an example class Person that can be instantiated by 'name', 'age', and 'dob' (date of birth). The __getitem__
method is written in a way that one can access the indexed instance attributes, such as first or last name, day, month or year of the dob, etc.
import copy
# Constants that can be used to index date of birth's Date-Month-Year
D = 0; M = 1; Y = -1
class Person(object):
def __init__(self, name, age, dob):
self.name = name
self.age = age
self.dob = dob
def __getitem__(self, indx):
print ("Calling __getitem__")
p = copy.copy(self)
p.name = p.name.split(" ")[indx]
p.dob = p.dob[indx] # or, p.dob = p.dob.__getitem__(indx)
return p
Suppose one user input is as follows:
p = Person(name = 'Jonab Gutu', age = 20, dob=(1, 1, 1999))
With the help of __getitem__
method, the user can access the indexed attributes. e.g.,
print p[0].name # print first (or last) name
print p[Y].dob # print (Date or Month or ) Year of the 'date of birth'
The []
syntax for getting item by key or index is just syntax sugar.
When you evaluate a[i]
Python calls a.__getitem__(i)
(or type(a).__getitem__(a, i)
, but this distinction is about inheritance models and is not important here). Even if the class of a
may not explicitly define this method, it is usually inherited from an ancestor class.
All the (Python 2.7) special method names and their semantics are listed here: https://docs.python.org/2.7/reference/datamodel.html#special-method-names
__getitem__
can be used to implement "lazy" dict
subclasses. The aim is to avoid instantiating a dictionary at once that either already has an inordinately large number of key-value pairs in existing containers, or has an expensive hashing process between existing containers of key-value pairs, or if the dictionary represents a single group of resources that are distributed over the internet.
As a simple example, suppose you have two lists, keys
and values
, whereby {k:v for k,v in zip(keys, values)}
is the dictionary that you need, which must be made lazy for speed or efficiency purposes:
class LazyDict(dict):
def __init__(self, keys, values):
self.keys = keys
self.values = values
super().__init__()
def __getitem__(self, key):
if key not in self:
try:
i = self.keys.index(key)
self.__setitem__(self.keys.pop(i), self.values.pop(i))
except ValueError, IndexError:
raise KeyError("No such key-value pair!!")
return super().__getitem__(key)
Usage:
>>> a = [1,2,3,4]
>>> b = [1,2,2,3]
>>> c = LazyDict(a,b)
>>> c[1]
1
>>> c[4]
3
>>> c[2]
2
>>> c[3]
2
>>> d = LazyDict(a,b)
>>> d.items()
dict_items([])
Source: Stackoverflow.com