A lot of Python books often mention that “everything in Python is an object”, and “objects are first class citizens”, but they don’t always explain what that these things actually mean. Let’s try to fix that up now.
Everything in Python is an Object
Dive Into Python gives a great explanation:
Different programming languages define “object” in different ways. In some, it means that all objects must have attributes and methods; in others, it means that all objects are subclassable. In Python, the definition is looser; some objects have neither attributes nor methods (more on this in Chapter 3), and not all objects are subclassable (more on this in Chapter 5). But everything is an object in the sense that it can be assigned to a variable or passed as an argument to a function (more in this in Chapter 4).
This is so important that I’m going to repeat it in case you missed it the first few times: everything in Python is an object. Strings are objects. Lists are objects. Functions are objects. Even modules are objects.
For more information on objects, see this effbot post.
First Class Citizens
For this definition I couldn’t go past Guido van Rossum’s blog post. After all, he’s the one who wrote Python so I’m sure he knows what he’s talking about :)
One of my goals for Python was to make it so that all objects were “first class.” By this, I meant that I wanted all objects that could be named in the language (e.g., integers, strings, functions, classes, modules, methods, etc.) to have equal status. That is, they can be assigned to variables, placed in lists, stored in dictionaries, passed as arguments, and so forth.
See the Coding section of my Knowledge Base for more information.
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