Classes And Dictionaries¶

Facts Up-Front ¶

class is a statement
Creates a class
Actually: a shorthand for creating a type
A class is callable
- Creates an object (⟶ self)
- Calls __init__ on it (see Constructor)

Using Raw Dictionaries As Objects ¶

Raw dictionary to hold attributes
person = {}

str type keys as attributes

person['firstname'] = 'Joerg'
person['lastname'] = 'Faschingbauer'

Attribute access is clumsy
person['firstname']
'Joerg'
The type of the object does not refect that it is a person
type(person)
dict
Much writing
Many opportunities for bugs/typos
⟶ BAD!

Enter Classes: An Empty Class, And Its Effects ¶

Better notation for the same thing: class

Create empty class (one without class attributes and methods)
class Person: pass
Classes are first-class object ⟶ have a type
type(Person)
type
⟶ A-ha! Created a new type, obviously

Objects are instantiated by calling their type

person = Person()
type(person)

__main__.Person

Attributes ¶

Setting attributes

person.firstname = 'Joerg'
person.lastname = 'Faschingbauer'

Getting attributes
person.firstname
'Joerg'

Unknown attribute access

person.svnr                   # <--- attribute 'svnr' does not exist

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Cell In[10], line 1
----> 1 person.svnr                   # <--- attribute 'svnr' does not exist

AttributeError: 'Person' object has no attribute 'svnr'

Summary: Classes Or Raw Dictionaries ¶

Definitely less typing
Objects have type other than dict
Classes are callable ⟶ creates instance of it (the object)

Dynamic Attribute Access ¶

Attention

Do not overuse! (Except to save your job maybe)

hasattr()
getattr()
setattr()
dir()

obj.__dict__

class Person:
    pass

person = Person()
person.firstname = 'Joerg'
person.lastname = 'Faschingbauer'

hasattr(person, 'firstname')

True

getattr(person, 'firstname')

'Joerg'

getattr(person, 'svnr')

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Cell In[14], line 1
----> 1 getattr(person, 'svnr')

AttributeError: 'Person' object has no attribute 'svnr'

setattr(person, 'svnr', '1037190666')

print('Boing, now having an ID', person.svnr)

Boing, now having an ID 1037190666

Classes And Dictionaries¶

Facts Up-Front¶

Using Raw Dictionaries As Objects¶

Enter Classes: An Empty Class, And Its Effects¶

Attributes¶

Summary: Classes Or Raw Dictionaries¶

Dynamic Attribute Access¶