"return" statement. How does different position of "return" makes it differ

In some other programming languages, indentation is generally used to improve readability but by itself is not required. Semi-colons or curly braces are used to signal end of statements and for code blocks. Even if indentation is not used, the code is still valid as long as it adheres to the syntax of the language.

In Python, indentation is important and is not optional.

Consider the following variations in indentation and how they affect the program.

  • Variation A (Your code)
def same_values(lst1, lst2):
    new_list = []
    for num in range(len(lst1)):
        if lst1[num] == lst2[num]:
            new_list.append(num)
            print(num)
        return new_list

Here, the return statement has the same indentation level as the if statement. Both the if statement and the return statement are part of the for loop’s body. The for loop will be executed and the first iteration will begin. If the condition is true, the statements in the body of the if statement will be executed. If not, the statements in the body of the if statement will be skipped. Then, the return statement will be executed. You won’t get to the second iteration of the for loop. Only one iteration of the for loop will be executed and as soon as the return statement is encountered, you will exit the function.

  • Variation B
def same_values(lst1, lst2):
    new_list = []
    for num in range(len(lst1)):
        if lst1[num] == lst2[num]:
            new_list.append(num)
            print(num)
            return new_list

Here the return statement has the same indentation as the other statements in the body of the if statement. So, this makes the return statement a part of the if statement’s body. The for loop will be executed and the first iteration will begin. If the condition is false, then nothing will happen and you will move on to the next iteration of the for loop. If the condition is true, the statements in the body of the if statement will be executed. After appending and printing num, you will again return immediately causing you to exit the function.

  • Variation C
def same_values(lst1, lst2):
    new_list = []
    for num in range(len(lst1)):
        if lst1[num] == lst2[num]:
            new_list.append(num)
            print(num)
    return new_list

Here, the return statement has the same indentation as the new_list = [] statement and the for loop. This means that the return statement is part of the same_values function’s body. Now, new_list will be initialized as the empty list, the for loop will run to completion. After the for loop has finished, new_list will be returned.

Consider the following example:

#
# Version A:
def func():
    for i in [3, 5, 4, 7, 8]:
        print(i)
        if (i % 2 == 0):
            print(f"{i} is Even!")
        return "Done"
    
print(func())

# Output:
3 
Done
# Version B
def func():
    for i in [3, 5, 4, 7, 8]:
        print(i)
        if (i % 2 == 0):
            print(f"{i} is Even!")
            return "Done"
    
print(func())

# Output:
3
5
4
4 is Even!
Done
# Version C
def func():
    for i in [3, 5, 4, 7, 8]:
        print(i)
        if (i % 2 == 0):
            print(f"{i} is Even!")
    return "Done"
    
print(func())

# Output:
3
5
4
4 is Even!
7
8
8 is Even!
Done

Play with a few examples of your own to better understand how indentation affects the results.

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