Section 1.1: The __repr__ Method

The __repr__ method defines how an object is represented as a string. As demonstrated earlier, when a Python object is printed in an interactive console, it outputs its representation. Here’s how you can implement it:

It’s important to ensure that the __repr__ method returns a string. If a non-string is returned, a TypeError will occur. The ideal string returned should enable users to reconstruct the object, allowing them to create a new object with identical attributes.

With an updated __repr__ method, you can now inspect the object:

>>> student0 = Student("John Smith")

>>> student0

Student('John Smith')

You can also use the built-in repr() function to obtain the representation string. Users might expect that this string can be evaluated to create another object using eval():

>>> student1 = eval(repr(student0))

>>> student1

Student('John Smith')

Additionally, when using f-strings, you can include the object's attribute and specify !r to denote that the raw representation should be used. Without !r, the f-string will call the __str__ method instead.

>>> name = "John Smith"

>>> print(f"Student({name!r})")

Student('John Smith')

>>> print(f"Student({name})")

Student(John Smith)

In some instances, a meaningful string for reconstruction might not be feasible. In such cases, it’s common to enclose the class name and a brief summary in angle brackets. The following code illustrates this with built-in classes.

Section 1.2: The __str__ Method

Another crucial special method for formatting is the __str__ method. Below is an example of how to override it within a custom class:

With the modified class, observe when this method is triggered:

>>> print(student0)

__str__ is called

>>> str(student0)

'__str__ is called'

>>> f"{student0}"

'__str__ is called'

The __str__ method can be invoked in multiple scenarios: using the print() function, the str() function, or within an f-string.

While the usage of __str__ is clear, the best practices for defining its output may vary. Generally, the output should provide a descriptive summary of the instance. If __str__ is not defined, Python will revert to calling __repr__.

Section 1.3: The __format__ Method

The __format__ method is another vital special method that allows for customized string formatting. Below is its signature in a custom class:

As previously noted, both f-strings and the built-in format() function can invoke the __format__ method. Here's how it behaves:

>>> student0 = Student("John Smith")

>>> f"{student0}"

'__format__ is called'

>>> format(student0)

'__format__ is called'

>>> print(student0)

<__main__.Student object at 0x114a92c10>

>>> str(student0)

'<__main__.Student object at 0x114a92c10>'

Unlike __repr__ and __str__, the __format__ method includes an additional parameter, format_spec, which defines how the object should be formatted. Below is an example showcasing custom specifications:

We define two specifications: 'i' for initials and 'C' for capitalization. The __format__ method will return the student's initials or the uppercase version of their name based on the specified format.

# Custom specifications for formatting

def __format__(self, format_spec):

if format_spec == 'i':

return self.name[0] # Return initials

elif format_spec == 'C':

return self.name.upper() # Return capitalized name

return self.name # Default formatting