When is Parentheses Omission Allowed in Python Function and Method Calls?

Understanding Parentheses Omission in Function and Method Calls

In Python, functions and methods are considered first-class objects, meaning they can be treated like any other data type. However, one unique aspect of first-class objects is the ability to use them without parentheses.

Code Example Explanation

The provided code snippet initializes two instances of the objectTest class, a and b, with the same value. The output shows that:

• a == b evaluates to False because they are different objects.
• a.get_value() == b.get_value evaluates to True because they return the same value.
• a.get_value() == b.get_value() also evaluates to True for the same reason.

Omission of Parentheses

The surprising result is the last expression, a.get_value == b.get_value. This evaluates to False even though get_value is the same method for both objects. This occurs because without parentheses, we are not calling the method but rather accessing the function object itself.

Why Omission is Allowed

Python allows the omission of parentheses because this provides a concise way to pass a reference to the callable (function or method) itself. This is particularly useful in several situations:

Using Callbacks:

Callbacks are functions or methods that are passed to another function to be executed at a later time. Omitting parentheses allows to pass the callback reference, as seen in the multiprocessing example:

t = Process(target=my_long_running_function)

Mapping Functions:

When using the map function, you can pass a callable without parentheses to fill in its arguments:

map(do_something, ['hey', 'what up', 'yo'])

Dynamic Invocation:

Parentheses omission enables dynamic invocation, where you can fetch and execute the desired callable from a collection:

str_ops = {'<': lt, '>': gt, '==': eq}
op = str_ops.get(my_operator)
if op:
    result = op(lhs, rhs)

In summary, omitting parentheses when calling functions or methods allows for the efficient passing of references to the callable itself, enabling various functionalities such as callbacks, dynamic invocation, and mapping.

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