What does a pass do in Python?

In Python, pass is a null statement. It does nothing and it is often used as a placeholder or a stub in situations where a statement is required syntactically, but no action is needed or desired.

The pass statement is often used in situations such as:

1. Placeholder in function or class definitions: When defining a function or a class, you may not have the implementation yet, but you still need to provide a valid syntax for the function or class. In this case, you can use the pass statement as a placeholder until you have the actual implementation.

Example:

def my_function():
    pass # Placeholder for function implementation

2. Empty loops or conditional statements: Sometimes, you may want to create an empty loop or conditional statement for a certain condition, and you don't want any code to be executed in that case. In this situation, you can use the pass statement.

Example:

while True:
    pass # Empty loop

3. Ignoring exceptions: When handling exceptions, you may want to catch a specific exception but do nothing in the except block. In this case, you can use the pass statement to indicate that no action is needed.

Example:

try:
    some_function()
except ValueError:
    pass # Do nothing if a ValueError is raised

Also, 

When defining an abstract class in Python, you can use the pass statement to indicate that a method is abstract, i.e., it has no implementation in the abstract class itself. The concrete subclasses of the abstract class are then responsible for providing an implementation for the abstract methods.

Here's an example of using pass in an abstract class definition:

from abc import ABC, abstractmethod

class Shape(ABC):
    @abstractmethod
    def area(self):
        pass

@abstractmethod
    def perimeter(self):
        pass

In the above example, the Shape class is an abstract class that defines two abstract methods, area() and perimeter(). The pass statement is used in the method definitions to indicate that the methods are abstract and have no implementation in the abstract class itself. Any concrete subclass of Shape must provide an implementation for these methods.

So in this context, the pass statement is used to provide a placeholder for abstract methods that are meant to be implemented by concrete subclasses.