How to Check If a Function Returns a Specific Value in Python

In Python, functions are designed to perform tasks and often send data back to the main program using the return statement. Verifying that a function returns the expected value is a fundamental part of logic control, bug fixing, and automated testing.

This guide explains how to capture function outputs, compare them against expected values using conditional logic, and handle common pitfalls like implicit None returns.

Understanding Function Returns

When a function executes a return statement, it immediately exits and passes a value back to the line of code that called it. To check this value, you must capture it in a variable.

def calculate_sum(x, y):
    """Calculates sum and returns the result."""
    return x + y

result = calculate_sum(10, 5)

print(f"Returned value: {result}")

Output:

Returned value: 15

Method 1: Using Conditional Logic (if-else)

The most common way to check a return value during runtime is to compare the captured variable against an expected value using standard comparison operators (==, !=, >, etc.).

Checking Exact Matches

This is useful when a function has conditional logic that might return different results based on inputs.

def conditional_math(x, y):
    """
    Returns the sum. 
    However, if the sum is greater than 10, it returns double the sum.
    """
    total = x + y
    if total > 10:
        return total * 2
    return total

val_a = conditional_math(2, 3)

val_b = conditional_math(10, 5)

if val_b == 30:
    print(f"Success: Function returned 30 as expected.")
else:
    print(f"Failure: Expected 30, got {val_b}")

Output:

Success: Function returned 30 as expected.

Method 2: Using Assertions (Debugging)

For testing and debugging, you can use the assert keyword. This checks if a condition is True; if it is False, the program crashes with an AssertionError. This is the precursor to writing unit tests.

def get_status_code():
    return 200

assert get_status_code() == 200
print("First assertion passed.")

try:
    assert get_status_code() == 404
except AssertionError:
    print("Second assertion failed: Value was not 404.")

Output:

First assertion passed.
Second assertion failed: Value was not 404.

Common Pitfall: Printing vs. Returning

A frequent error involves functions that print() a value instead of return-ing it. In Python, a function without a return statement implicitly returns None.

The "None" Trap

def fake_calculator(a, b):
    print(a + b)

result = fake_calculator(5, 5)

print(f"Captured value: {result}")

if result == 10:
    print("This will NOT print.")
else:
    print("Check failed: result is not 10.")

Output:

10
Captured value: None
Check failed: result is not 10.

The Fix

Replace print with return to pass the data back to the program.

def real_calculator(a, b):
    return a + b

result = real_calculator(5, 5)

if result == 10:
    print("Check passed: Result is 10.")

Output:

Check passed: Result is 10.

Conclusion

To successfully check if a function returns a specific value:

1. Capture the output: Assign the function call to a variable (e.g., result = func()).
2. Verify the logic: Ensure the function uses return, not print.
3. Compare: Use if result == expected_value: for runtime logic or assert result == expected_value for testing.