completes functions

learning_python3.md

@@ -653,12 +653,32 @@ print("outside the function", name)
 
 ## Functions that *return* something
 
-TODO basic math functions
+While the `global` keyword can be useful, it's rarely used.
+This is because function can not only *do* thing, they can **return** objects.
+The `input` function we've been using is a prime example of this as it always give you a `str` object back with the content of the user input.
+We can copy this behaviour as follows.
+
+```python3
+def square_surface(length, width):
+    surface = length * width
+    return surface
+
+
+square = square_surface(20, 40)
+print(square)
+```
+
+🏃 Try it
+---
+
+Functions that return an object are essential to any modern programming language.
+Think of some calculations such as the Celsius to Fahrenheit converter and create the corresponding functions.
 
 ## Some links to read up on
 
 * some [best practices](https://able.bio/rhett/python-functions-and-best-practices--78aclaa) for functions
 * more details on [variable scope](https://pythongeeks.org/python-variable-scope/)
+* in depth [article](https://realpython.com/python-return-statement/) on return values in python
 
 # Coding challenge - Pretty Print
 
@@ -675,6 +695,8 @@ Kind of like the two examples below.
 #################
 ```
 
+As an extra challenge you could return a [multi line string](https://www.askpython.com/python/string/python-multiline-strings) and print it outside of the function!
+
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