Unable to understand code -newbie
Question:
I am new to python ( started 1 week ago) and this is the first time i am doing coding so i am not able to understand fairly simple things as well.
can you explain this function to to me? i understand that a function is being defined with 2 input required self and my_object, but what is happening next? please explain like you would to a newbie.
class chain():
def __init__(self, my_object):
self.o = my_object
def __getattr__(self, attr):
x = getattr(self.o, attr)
if hasattr(x, '__call__'):
method = x
return lambda *args: self if method(*args) is None else method(*args)
else:
prop = x
return prop
Answers:
The class chain
has a constructor that takes an argument my_object
and assigns it to an instance variable self.o
.
The method __getattr__
is a special magic method that has been overridden to delegate calls to the initial my_object
variable we first received.
The result of the delegated call is checked for a method named __call__
. If present, it is called and the returned value is returned. If not, the value itself is returned as-is.
Firstly, chain
is not a Function, it’s a Class.
A class in simple words is a definition of an object. (say Car)
Now the __init__
function of the class simply defines what’s “in it” meaning what variables or properties does it has. Say for example a class Car
:
class Car:
def __init__(self,maxspeed,color):
self.speed = maxspeed #So what's defined under **__init__** is a property of a class.
self.color = color
So here Class car has speed and color as variables(or attributes or properties)
Now there are methods , of simply function that control the behaviour of the object and it’s functionalities.
class Car:
def __init__(self,maxspeed,color):
self.speed = maxspeed #So what's defined under **__init__** is a property of a class.
self.color = color
def accelarate(self): #Method to increase the speed of car object.
self.sepped = self.speed + 10
Now the method you have is a magical one , __getattr__
Say a scenario where you want to acess the brand of the car , now you haven’t define self.brand
in it’s __init__
function so you you’ll get an error when you call it like:
>>>red_car = Car(100,red) #Creating an object named red_car of class Car
>>>red_car.color
>>>'red'
>>>red_car.brand
>>> Attribute Error , Class car dosen't has attribute brand
Now remove this error when calling an undefined property for a object or put simple we tell tell the class what to do if an undefined variable is called we use the method __getattr__
.
class Dummy(object):
def __getattr__(self, attr):
return attr.upper()
d = Dummy()
d.does_not_exist # 'DOES_NOT_EXIST'
d.what_about_this_one # 'WHAT_ABOUT_THIS_ONE'
In the above code does_not_exist
property (attribute) is NOT define but still we are not getting error as the getattr catches it and does as instructed. In this case it catches attr
capitalises it and returns it rather than throwing an error in your face.
I am new to python ( started 1 week ago) and this is the first time i am doing coding so i am not able to understand fairly simple things as well.
can you explain this function to to me? i understand that a function is being defined with 2 input required self and my_object, but what is happening next? please explain like you would to a newbie.
class chain():
def __init__(self, my_object):
self.o = my_object
def __getattr__(self, attr):
x = getattr(self.o, attr)
if hasattr(x, '__call__'):
method = x
return lambda *args: self if method(*args) is None else method(*args)
else:
prop = x
return prop
The class chain
has a constructor that takes an argument my_object
and assigns it to an instance variable self.o
.
The method __getattr__
is a special magic method that has been overridden to delegate calls to the initial my_object
variable we first received.
The result of the delegated call is checked for a method named __call__
. If present, it is called and the returned value is returned. If not, the value itself is returned as-is.
Firstly, chain
is not a Function, it’s a Class.
A class in simple words is a definition of an object. (say Car)
Now the __init__
function of the class simply defines what’s “in it” meaning what variables or properties does it has. Say for example a class Car
:
class Car:
def __init__(self,maxspeed,color):
self.speed = maxspeed #So what's defined under **__init__** is a property of a class.
self.color = color
So here Class car has speed and color as variables(or attributes or properties)
Now there are methods , of simply function that control the behaviour of the object and it’s functionalities.
class Car:
def __init__(self,maxspeed,color):
self.speed = maxspeed #So what's defined under **__init__** is a property of a class.
self.color = color
def accelarate(self): #Method to increase the speed of car object.
self.sepped = self.speed + 10
Now the method you have is a magical one , __getattr__
Say a scenario where you want to acess the brand of the car , now you haven’t define self.brand
in it’s __init__
function so you you’ll get an error when you call it like:
>>>red_car = Car(100,red) #Creating an object named red_car of class Car
>>>red_car.color
>>>'red'
>>>red_car.brand
>>> Attribute Error , Class car dosen't has attribute brand
Now remove this error when calling an undefined property for a object or put simple we tell tell the class what to do if an undefined variable is called we use the method __getattr__
.
class Dummy(object):
def __getattr__(self, attr):
return attr.upper()
d = Dummy()
d.does_not_exist # 'DOES_NOT_EXIST'
d.what_about_this_one # 'WHAT_ABOUT_THIS_ONE'
In the above code does_not_exist
property (attribute) is NOT define but still we are not getting error as the getattr catches it and does as instructed. In this case it catches attr
capitalises it and returns it rather than throwing an error in your face.