What is "class ClassName : pass" in Python?

Question

In this tutorial, an entire class is defined as class DontAppend: pass. According to this Q&A on Stack Overflow, pass defines the class.

The consensus from Google, however, is that pass is a statement. I've never seen statements define a class other than within definitions of class methods.

As a statement, pass also doesn't define a named class attribute/property.

Since pass doesn't serve to define a method or an attribute, how does it actually define the class? Is this just a special convention, solely for defining an empty class?

Answer 1

The pass statement in python can be used as a placeholder for future code (or it may always remain as pass).

A pass statement helps in avoiding the error where empty code is not allowed. All the following are valid python statements -

class XYX:
    pass

class SomeException(Exception):
    pass

if somecondition:
    pass
else:
    x = y + z

Empty code is not allowed in loops, function definitions, class definitions, or in if statements. pass helps in such cases. It is pretty common to create custom exceptions by inheriting base exception classes and leaving the class body as pass

Answer 2

You say that

According to this Q&A on Stack Overflow, pass defines the class.

but no, you've misread the linked page. pass doesn't define the class. class DontAppend: pass defines the class.

In this usage, pass is just the class body, not the entire class definition. The class body is a sequence of arbitrary statements that will be executed to populate the class's namespace. You don't want to do anything in the body of this class, so pass, a statement that doesn't do anything, is an appropriate body to use.

After the class body executes (and does nothing, because it's just pass), the class statement will take the namespace (mostly empty, except for a few default contents) and create a new type object based on that namespace.

Answer 3

Here is the specification for the syntax of the python class definition statement:

classdef    ::=  [decorators] "class" classname [inheritance] ":" suite
inheritance ::=  "(" [argument_list] ")"
classname   ::=  identifier

So, as you can see, everything after the colon : is part of a "suite" (generally, this suite is called "the class body", or the "body of the class definition statement").

Here is the specification for a suite:

suite         ::=  stmt_list NEWLINE | NEWLINE INDENT statement+ DEDENT
statement     ::=  stmt_list NEWLINE | compound_stmt
stmt_list     ::=  simple_stmt (";" simple_stmt)* [";"]

So basically, a "suite" is any series of statements. Here is a valid class definition:

>>> class Foo:
...     print("inside the class body")
...
inside the class body
>>>

(Note, the body is executed, a class definition is executable code). So any valid Python statement can go in a class body. So what is pass then? Here is the spec:

pass_stmt ::= "pass"

pass is a null operation — when it is executed, nothing happens. It is useful as a placeholder when a statement is required syntactically, but no code needs to be executed, for example:

def f(arg): pass # a function that does nothing (yet)

class C: pass # a class with no methods (yet)

So some statement is required in the class body, you cannot just leave it blank if you want the class to be empty. So in that case, you can simply use pass.

Note, if you want more nitty-gritty details about how a class definition statement is executed and how that namespace becomes the namespace of the class object, this link to the data model will be useful

Answer 4

In Python, classes are objects, and they can be used to represent values or concepts. In this case, DontAppend is an empty class with no methods or attributes defined. It is simply used as a unique object to serve as a default value for the new_elem argument in good_function.

By defaulting new_elem to DontAppend, the function can check if a specific value has been passed for new_elem by comparing it to DontAppend. If a different value is provided, it means that the caller explicitly passed a value for new_elem, and that value will be appended to the starter_list. If new_elem is DontAppend, it indicates that no specific value was provided, and the function skips appending to the list.

Essentially, DontAppend acts as a special sentinel value to distinguish between the cases where a specific value is provided and where no specific value is provided for new_elem. It allows for more flexibility in how the function is called and provides a way to handle optional arguments.