How to define a class attribute with no default value in python

Question

I'm a C# developer learning python.

If I want to define a class with certain attributes, but not provide default values for the attributes, how is that done?

For example (this is my best guess of how to do it):

class Spam
    eggs = None
    cheese = None

Or, is this legal:

class Spam
    eggs
    cheese

Or something else?

Out of the 2 choices you provide, the first is legal, but not neccessary. Unless the attributes are used, there is no need to define them. Python dynamically creates attributes. — Dhara
– Dhara, Commented Feb 8, 2013 at 12:14
@David: depending on what you want to do with the variables, you might want to prepend a self. to them see here. — Francesco Montesano
– Francesco Montesano, Commented Feb 8, 2013 at 12:23
My God. Attributes are static by default. Thanks for pointing this out. — David
– David, Commented Feb 8, 2013 at 12:38

Martijn Pieters · Accepted Answer · 2013-02-08 12:13:05Z

11

You don't. You assign to the attributes later on, or set them to None.

An attribute without a value is not an attribute. Being dynamic, it's perfectly fine to assign the attribute later on without having to declare it in the class.

answered Feb 8, 2013 at 12:13

Martijn Pieters

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5 Comments

David Over a year ago

Um, so do you mean my first code suggestion is the correct one?

Martijn Pieters Over a year ago

@David: indeed. Or omit eggs and cheese altogether. It completely depends on what you are trying to achieve, what API your class is going to present.

David Over a year ago

`Being dynamic, it's perfectly fine to assign the attribute later on without having to declare it in the class.' - okay, I understand that, but presumably if I predefine the attributes I benefit from code completion in my IDE?

Martijn Pieters Over a year ago

@David: That depends on the IDE, but in theory, yes. I have mine set to tab-complete on any legal name defined in the current file.

Gareth Latty Over a year ago

@David That can help. Setting to None isn't an issue unless you want to use None as a value. In general, defining variables you know you are going to use on the class is a good idea as it makes the code easier to understand.

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