FAQ: Learn Python: Classes - Instance Variables

Frequently Asked Questions Python FAQ
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This community-built FAQ covers the “Instance Variables” exercise from the lesson “Learn Python: Classes”.

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This exercise can be found in the following Codecademy content:

Computer Science

FAQs on the exercise Instance Variables

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2 posts were split to a new topic: What is the difference between a class variable and instance variable?

3 
class Store:
  pass

alternative_rocks = Store()
isabelles_ices = Store()

alternative_rocks= "Alternative Rocks"
isabelles_ices = "Isabelle's Ices"

working_string = "{} {}".format(alternative_rocks, isabelles_ices)
print(working_string)

Without assigning(?) the instances variables, it just works fine. Why do we have to assign the instance variable for this lesson??

2 Likes
4

How do we know what is class variable and what is instance variable??
For the previous lesson, Instance Variables (https://www.codecademy.com/courses/learn-python-3/lessons/data-types/exercises/instance-variables), the instance variable, fake_key, just works fine without throwing an ‘AttributeError’ message in the terminal. (What I meant for “it works fine” is that when executing the last line, print(working_string) it didn’t throw an error.)

class FakeDict:
  pass

fake_dict1 = FakeDict()
fake_dict2 = FakeDict()

fake_dict1.fake_key = "This works!"
fake_dict2.fake_key = "This too!"

# Let's join the two strings together!
working_string = "{} {}".format(fake_dict1.fake_key, fake_dict2.fake_key)
print(working_string)
# prints "This works! This too!"

So what confuses me is this sentence from the lecture note of Attribute Function below.

“”“If we attempt to access an attribute that is neither a class variable nor an instance variable of the object Python will throw an AttributeError .”""

class NoCustomAttributes:
  pass

attributeless = NoCustomAttributes()

try:
  attributeless.fake_attribute
except AttributeError:
  print("This text gets printed!")

# prints "This text gets printed!"

How are these two codes, specifically the two instance variables (.fake_key, .fake_attribute) different? Those are instance variables, assumed that those are not defined in each class, and one is working fine without error but the latter one is not. This is very confusing…

1 Like
5

I am a little bit confused about instance variables.
if we don’t have some variables in our class definition then can we add them using a instance variable?
Or Do we use instance variable only to add in init method???
Someone please explain this…:blush:

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We do not need to define all of our class variables in the class definition? We can dynamically add attributes?

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Yes, but only to an instance. It does not become a common instance variable across all instances.

>>> class Foo:
	pass

>>> foo = Foo()
>>> foo.attr = 42
>>> foo.attr
42
>>> bar = Foo()
>>> bar.attr
Traceback (most recent call last):
  File "<pyshell#280>", line 1, in <module>
    bar.attr
AttributeError: 'Foo' object has no attribute 'attr'
>>> 
>>> Foo.attr
Traceback (most recent call last):
  File "<pyshell#278>", line 1, in <module>
    Foo.attr
AttributeError: type object 'Foo' has no attribute 'attr'
>>>
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how to check the available attributes of any class or object?

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What would be the purpose?

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if i am interacting with pre existing object and i am not sure what attributes does it have or not to work with or to add new attributes.
actually i am not friendly with this object thing but this thought just came to my mind, so i asked.

for example: if am working with list a and i don’t know what attributes does it have. Instead of guessing i would just check for those attributes.

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One way would be to look at the directory of attributes on the parent class…

>>> dir(list)

We can also read the docstring of it or any of the attributes that have one…

>>> help(list)

And the one place we should never skip is the documentation online…

1 Like
12

Hello, didn’t quite get why we are using init method. what’s the difference between class with Init and without it?

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A class without an __init__() method would have no unique object instance attributes.

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Can the instance attributes be named randomly?

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If you mean in any order, yes. The only requirement is that if one attribute refers to another, that attribute must follow the one it references.

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I think this whole instance variable thing is pretty sweet. I can’t wait to use it, but one thing confuses me. Why doesn’t it work on the str class?

class Store:

  pass

alternative_rocks = Store()

isabelles_ices = Store()

alternative_rocks.store_name = "Alternative Rocks"

isabelles_ices.store_name = "Isabelle's Ices"

print(isabelles_ices.store_name)

print(type(isabelles_ices))

someName = "thisname"

print(type(someName))

someName.whatever = "I want!"

print(someName.whatever)

Isabelle's Ices
<class '__main__.Store'>
<class 'str'>
Traceback (most recent call last):
  File "script.py", line 14, in <module>
    someName.whatever = "I want!"
AttributeError: 'str' object has no attribute 'whatever'
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And thus ends the conversation.

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I’m not sure if I understand.

class Store:
  pass

store1 = Store()
print(type(store1))
store1.whatever = "I want!"
print(store1.whatever)

Output:

<class '__main__.Store'>
I want!

…unless…

class yPythonJoke:
   pass
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No, not a joke. Reality. We can add attributes to a class instance publicly. They will however only belong to that instance.

>>> class Foo:
	pass

>>> foo = Foo()
>>> def add_bar():
	return 'bar'

>>> foo.add_bar = add_bar
>>> foo.add_bar()
'bar'
>>>
1 Like
20

I may be confused. :slight_smile:

In my original code, isn’t someName an instance of the str class?

If it is, why am I unable to add attributes to the instance?