Last modified on 21 Jun 2020.
Know to work with classes and objects in python.
Classes vs Objects vs Instances vs Methods
- Class is a blueprint/template of an object.
- Each class has its own attributes (its states) and methods (its behaviors).
- Object is a bundle of related attributes and methods.
- Instance is a single and unique unit of a class.
- Many instances may have the same class. They have attributes and methods defined in the class.
Syntactic sugar &
Syntactic sugar is syntax within a programming language that is designed to make things easier to read or to express. For example, we use
arr[i,j] but behind the scene, it’s
class MyClass() def method(arg): print(arg) my_object = MyClass() my_object.method('foo') # TypeError: method() takes exactly 1 positional argument (2 given)
MyClass.method(my_object, 'foo'). That’s why we need
self or a decorator,
class MyClass(): def method(self, arg): print(arg)
# DON'T NEED `self` class MyClass(): @staticmethod def method(self, arg): print(arg)
Get all attributes of a class
# CHECK THERE IS AN ATTRIBUTE getattr(MyClass, 'report', None) # if there is a class, it return this class' detail # if not, return None
def props(cls): return [i for i in cls.__dict__.keys() if i[:1] != '_'] # access these attributes properties = props(MyClass) for att in properties: print(getattr(MyClass, att))
# Get dictionaries of all attributes & their values MyClass.__dict__
Import local class
Suppose that we have a folders/files structure like below,
# ORIGINAL STRUCTURE popai/ processings/ a.py # contains class ABC test/ b.py lib/ c.py # contains class XYZ
# UPDATED STRUCTURE popai/ __init__.py processings/ __init__.py a.py # contains class ABC test/ __init__.py b.py lib/ c.py # contains class XYZ
We want import both classes
# b.py from popai.processings.a import ABC
# a.py from popai.lib.c import XYZ
__init__.py like in the right box above.
Some errors may occur,
ValueError: attempted relative import beyond top-level package
Father and Son
# FATHER class father_class(): def __init__(self): self.abc = 1
# SON class son_class(father_class): def __init__(self): # son_class has attribute `abc` super().__init__() self.xyz = 2
If you want son takes all parameters of father and use additional parameters,
class Shape: def __init__(self, shapename): self.shapename = shapename class ColoredShape(Shape): def __init__(self, color, **kwargs): super().__init__(**kwargs) self.color = color cs = ColoredShape(color='red', shapename='circle')
Abstract Base Classes (ABC)
from abc import ABC, abstractmethod
# FATHER CLASS class BaseModel(ABC): def __init__(self): pass # child class must have @abstractmethod def fit(self, X): pass # child class must have @abstractmethod def predit(self, X): pass # children class don't need to have # but they can call def fit_predict(self, X): pass
# CHILD CLASS class LinearModel(BaseModel) def __init__(self): pass # must-have def fit(self, X): pass # must-have def predict(self, X): pass # this call can use .fix_predict() # from its father!