Understanding getitem method

Question

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

User · Answer

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

User · Answer

Cong Ma does a good job of explaining what   getitem   is used for - but I want to give you an example which might be useful  Imagine a class which models a building  Within the data for the building it includes a number of attributes  including descriptions of the companies that occupy each floor    Without using   getitem   we would have a class like this    class Building object        def   init   self  floors            self  floors    None  floors      def occupy self  floor number  data             self  floors floor number    data      def get floor data self  floor number             return self  floors floor number   building1   Building 4    Construct a building with 4 floors building1 occupy 0   Reception   building1 occupy 1   ABC Corp   building1 occupy 2   DEF Inc   print  building1 get floor data 2      We could however use   getitem    and its counterpart   setitem    to make the usage of the Building class  nicer    class Building object        def   init   self  floors            self  floors    None  floors      def   setitem   self  floor number  data             self  floors floor number    data      def   getitem   self  floor number             return self  floors floor number   building1   Building 4    Construct a building with 4 floors building1 0     Reception  building1 1     ABC Corp  building1 2     DEF Inc  print  building1 2      Whether you use   setitem   like this really depends on how you plan to abstract your data - in this case we have decided to treat a building as a container of floors  and you could also implement an iterator for the Building  and maybe even the ability to slice - i e  get more than one floor s data at a time - it depends on what you need

User · Answer

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

User · Answer

getitem   can be used to implement  quot lazy quot  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  quot No such key-value pair   quot           return super     getitem   key    Usage   gt  gt  gt  a    1 2 3 4   gt  gt  gt  b    1 2 2 3   gt  gt  gt  c   LazyDict a b   gt  gt  gt  c 1  1  gt  gt  gt  c 4  3  gt  gt  gt  c 2  2  gt  gt  gt  c 3  2  gt  gt  gt  d   LazyDict a b   gt  gt  gt  d items   dict items

[python] Understanding getitem method

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[python] Understanding __getitem__ method

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[python] Understanding getitem method