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Learn Python: Function Arguments
Default Return

What does a function return when it doesn’t return anything? This sounds like a riddle, but there is a correct answer. A Python function that does not have any explicit return statement will return None after completing. This means that all functions in Python return something, whether it’s explicit or not. For example:

def no_return(): print("You've hit the point of no return") # notice no return statement here_it_is = no_return() print(here_it_is) # Prints out "None"

Above we defined a function called no_return() and saved the result to a variable here_it_is. When we print() the value of here_it_is we get None, referring to Python’s None. It’s possible to make this syntax even more explicit — a return statement without any value returns None also.

def fifty_percent_off(item): # Check if item.cost exists if hasattr(item, 'cost'): return item.cost / 2 # If not, return None return sale_price = fifty_percent_off(product) if sale_price is None: print("This product is not for sale!")

Here we have implemented a function that returns the cost of a product with “50% Off” (or “half price”). We check if the item passed to our function has a cost attribute. If it exists, we return half the cost. If not, we simply return, which returns None.

When we plug a product into this function, we can expect two possibilities: the first is that the item has a cost and this function returns half of that. The other is that item does not have a listed cost and so the function returns None. We can put error handling in the rest of our code, if we get None back from this function that means whatever we’re looking at isn’t for sale!

Instructions

1.

Lots of everyday Python functions return None. What’s the return value of a call to print()? Since print() is a function it must return something.

Create a variable called prints_return and set it equal to a print statement. Make sure to give the print statement parentheses and give it an argument (like “hi!”).

2.

Now print out prints_return.

3.

Inside script.py is a list called sort_this_list. Create a new variable called list_sort_return and set it equal to sort_this_list.sort().

4.

What do you expect list_sort_return to contain?

Print out list_sort_return.

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