[Python] Mutable vs Immutable, DB Key Types, Queryset vs Object

Mutable vs Immutable Objects

Whenever an object is instantiated, it is assigned a unique object id. The type of the object is defined at the runtime and it can’t be changed afterwards. However, it’s state can be changed if it is a mutable object.

• mutable:
  • list, dictionary
• immutable:
  • int, float, tuple, boolean, string

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DB Field Key Types

• Parent Key(PK)
  • Used to serve as a unique identifier for each row in a table
  • Cannot accept NULL values
  • Creates clustered index
  • A Primary key supports auto increment value
• Unique Key(UK)
  • Uniquely determines a row which isn’t primary key
  • Can accepts NULL values
  • Creates non-clustered index
  • A unique key does not supports auto increment value
• Foreign Key(FK)
  • Field that refers to the PK in another table
  • There can be multiple FKs in one table

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Django Filter() vs Get()

• Filter()
  • Returns a queryset object
  • If no item was found matching your criteria, filter() returns an empty queryset instead of an error
• Get()
  • You expect one and only one item that matches your criteria
  • Returns an error if the item does not exist or if multiple items exist that match criteria
  • Utilize try/except block or shortcut functions such as get_object_or_404 in order to handle exceptions