Python – Invoking Functions with and without Parentheses

Functions in Python are the defined blocks of code that perform a specific task. In this section, we will discuss the difference in invoking functions with and without Parentheses.

• When we call a function with parentheses, the function gets execute and returns the result to the callable.
• In another case, when we call a function without parentheses, a function reference is sent to the callable rather than executing the function itself.

Let’s discuss 3 important concepts:

• Invoking functions
• Return functions
• Passing functions

Invoking functions –

The below function performs a simple task, string concatenation. Here we will invoke the function `concatenate_string` with and without parentheses and see the difference.

Output

Function call with Parentheses: 
Hello, George
Function call without Parentheses:
<function concatenate_string at 0x7f0bb991df28>

For the first case, when we call concatenate_string('Hello, ', 'George'), the function executes and returns the concatenated string.
And, for the second case, when we call concatenate_string i.e. without parentheses, a reference is passed to the callable rather than executing the function itself. Here the callable can decide what to do with the reference.

Return Functions –

From the above discussion you can understand that, when the function is called with parentheses, the code is executed and returns the result. And, when it is called without parentheses, a function reference is returned to the callable.
So, what happens when a function is coded along with a return statement with parentheses and without parentheses. Let’s go through an example.

With Parentheses

Output

Hello, George

From the above example, it’s clear that the merge_string is a function within the function concatenate_string and the main function (concatenate_string) returns the sub-function (merge_string) to the caller.

 return merge_string(string1, string2) 

Here merge_string is invoked with parentheses, hence it executes and provides the result to the concatenate_string from where the result is passed to func.

Without Parentheses

Output:

<function concatenate_string..merge_string at 0x7f1e54ebc158>
Hello, George

Since the merge_string is used without parentheses, the concatenate_string passes the function reference to the callable func rather than executing the merge_string.

return merge_string

Hence, we can conclude that when we code sub-function with parentheses in a return statement, the main function executes the sub-function and passes the result to the callable. 
And, when we code subfunction without parentheses in a return statement, the main function passes the sub-function reference to the callable rather than executing it. Here the callable decides what to do with the reference.

Passing Functions –

You can pass a function as an argument by creating the reference, calling the function without parentheses, and provide it as an argument. Let’s look into the code.

Output:

Hello, George

In this case, the reference of concatenate_string is passed to the function_call as an argument.

function_call(concatenate_string)

Inside function_call, it executes the concatenate_string using the reference and returns the result to the callable.