Adding Attributes to a Python Class

I don’t want to be a pedant but a Mouse is a Mammal.

ok what if you are working in a class with user input? how would you call the str method??

I’m not exactly sure what you mean by “working in a class with user input.” There are a few approaches you can take if you need user input. The first one is to instantiate the class with the input already stored in a variable like this:

class Person:
    def __init__(self, name):
        self.name = name

name = input('enter your name: ')
person = Person(name)

Or you could ask for user input in the __init__ method like this:

class Person:
    def __init__(self):
        self.name = input('enter your name: ')

person = Person()

str() is used to convert a value into a string. input() returns a string, so I’m not sure why you would need to call that. If you’re talking about getting a string representation of the object instantiated from your class, you can do something like this:

class Person:
    def __init__(self, name):
        self.name = name

    def __str__(self):
        return 'this object has name ' + self.name

name = input('enter your name: ')
person = Person(name)
print(person)

This is beyond the scope of this course, but str defines the string representation of an object (This is similar to overriding ToString() in other languages). When we pass the person object to the print() function, it calls str under-the-hood and prints the string representation of the object. You can learn more about that here Let me know if you have any other questions!

Why do you use {} and format instead of just printing the attributes?

@kwf777 This was my first course. I was under the impression I needed to follow the article exactly, which used .format() because it’s a bit older. I use fstrings in all my courses now :)

At 5:35 output is changing without any run code.

Please re-record this with the new f-strings. Might be silly to ask but as someone who is really trying to learn python, this small issue just keeps messing me up and takes me out of the flow of your courses, when other videos on here use f-strings. Please and thank you.

Regarding species not being printed second time around after species was changed to ‘mouse’ was useful to highlight, but took reviewing. Could have been made clearer by saying something like it’s not printed out on right because yada yada etc (rather than assuming the learner knew the lack of output as a parallel from your technical explanation).

Hi Austin,for clarification that class attributes is a permenant attributes compare instance attributes that can modify even outside of class code blocks.

This video was awesome!

A rose by any other name smells as sweet … I’m a Java person, but not hugely “formally” trained. Thus, if the high keepers of the Java flame make a distinction between class attributes and instance attributes, (properties), I don’t remember. I simply define class vars in Java and give them default values, or leave them null and give them values during construction – the practical results always seem the same. (I’m bypassing visibility, protection, etc.) Here in Python the class and instance var terminology functionally doesn’t seem to make a lot of difference either, though I do see the conceptual difference. What I’m trying to express is that as soon as you say

__init__(self,myinstanceattr)
    self.myinstanceattr

then “myinstanceattr” becomes an inextricable part of every self example of the class just as a class attribute would be – the only difference is that “classattr” would have a pre-assigned default value (which I find can even be None). However, the instance attributes are “required” to be assigned values during the creation of the class instance (haven’t seen an automatic default init yet, though can define an equivalent). Once you have an instance, both the class and instance attributes can be accessed and modified using the “.” operator. So, long short: I’ll be waiting to see in the next vids if there’s any deep down difference behind “class” vs. “instance” vars, like speed of access or storage locations or something.

Very good presentations. Clean, well thought, great delivery. Thespian? Experienced teacher? Little things make me suspect you’re a C++ or Java guy also, either now or in your beginnings.

@Zarata Not thespian, but I do have a lot of experience teaching. Particularly with kids who don’t respond well to boring teachers. You were close with the languages too, my background is primarily C# and .NET :)

So in essence everything in python we’ve been learning is oop? Is that why certain built in functions work with str.object and not with tuple.object?

How does Python enforce encapsulation?

I might be wrong, but I think there’s a mistake at 1.28 (when you are comparing the two classes). Doesn’t a == b return False because your class doesn’t have __eq__ method rather than because the memory addresses are different? If you wanted to test for the latter, wouldn’t be a is b? Of course, they are different objects, but == is the value equality operator.

@Joseph Flanagan, agreed, it would have been better to use a == b in this case.

However, this is the kind of mistake that somehow still works. When a class does not define .__eq__(), == is essentially equal to is. More precisely:

User-defined classes have __eq__() and __hash__() methods by default; with them, all objects compare unequal (except with themselves)

(Source: docs.python.org/3/reference/datamodel.html#object.__hash__)

Great course! What about attributes that are defined out of the __init__()? Are they still instantiated when it is called eg class Person:

def __init__(self, name):
    self.name = name
    self.age= age
    self.astrosign = astrosign

Will the last two attributes be called when the init is called?

@henrytirla Attributes declared outside of .__init__() or any other method are attached to the enclosing class. They’re defined even before any instance of that class exists and are shared across all existing instances.

For example, consider this Person class:

>>> class Person:
... 
...     counter = 42
... 
...     def __init__(self, name):
...         self.name = name
...
>>> Person.counter  # There are no instances of this class yet
42
>>> alice = Person("Alice")
>>> bob = Person("Bob")
>>> alice.counter
42
>>> bob.counter
42

However, when you wish to change the value of a class attribute, you should always refer to it through the class name instead of a particular instance. Otherwise, unexpected things start to happen. You’re creating an instance attribute with the same name that masks the class attribute:

>>> alice.counter = 15  # Defines an instance attribute in Alice
>>> alice.counter
15
>>> bob.counter  # Bob has no such attribute
42
>>> Person.counter  # ...so, it delegates to the class attribute
42 

Would it be better to set the species in the __init__ function? For example, self.species = 'mammal'. I guess I’ve just never seen it done the way shown in the tutorial.

@chananya7276 “Better” is a highly subjective term because it all depends on the circumstances 😉 This lesson introduces the concept of class attributes, and species was defined as one for the sake of example.

Hey Austin, not able to understand the phrase at 03:56, “… we can use the access modifier, which in python is a dot(.)”. Aren’t access modifiers public, protected and private in general?

I am new to Python. If class attributes is what I understand is default, would changing it as per your example make sense?

Can I get clarification that a class attribute when changed by one instance, is changed for all instances?

The class variable is shared among all instances of a class, rather than being unique to each instance. It is called static variable in Java.