DSC 20 Lecture 23

Source: https://docs.google.com/presentation/d/1vRgHqvw7YxlADO_tKuuNWONEkH1onKFwYxtCT9kFCcU/edit#slide=id.p1

String Representation and Special Methods

String Representations

• An object value should behave like the kind of data it is meant to represent
• For instance, by producing a string representation of itself
  In Python, all objects produce two string representations
• The str is easy for humans, to be readable
  • Informal representation
• The repr is easy for the Python interpreter
  • Official representation

The repr String for an Object

The result of calling repr on a value is what Python prints in an interactive session
Some objects do not have a simple Python-readable string
The repr function returns a Python expression (a string) that evaluates to an equal object (same object when passed to eval()): https://docs.python.org/3/library/functions.html#repr

• i.e. eval(repr(object)) == object

eval()

utility which lets Python program run Python code within itself, by evaluating expressions.

• It is used on sites like codepad.org to allow you to execute scripts in a test environment.
  eval(expression, globals=None, locals=None)
• expression: this string is parsed and evaluated as a Python expression
• globals (optional): a dictionary to specify the available global methods and variables
• locals (optional): another dictionary to specify the local methods and variables

Polymorphic Functions

Polymorphic function: a function that applies to many (poly) different forms (morph) of data
str and repr are both polymorphic; they apply to any object
repr asks an argument to display itself
invokes a zero-argument method __repr__ on its argument

User-defined classes

• Need to have methods __str__ and __repr__ in order to be able to print string representation of an object.
• If __str__ does not exist, then __repr__ will be used
• At the very least, you should implement __repr__