Misconception: Return is a mystery to students because they may not understand that functions have an entry point and an exit point. This results in mistakes such as assuming Python will return the right thing.

It is important to check if students are returning specific things correctly. If they are just assuming that Python will return the right thing, this may signify a deep misunderstanding about what Python can do.

Help students develop a better mental model of the protocol for when a function gets called (for how the function runs from beginning to end).

• Use tools like Python Tutor (http://pythontutor.com/visualize.html#) and Java Visualizer (http://cscircles.cemc.uwaterloo.ca/java_visualize/) to help students build their mental model of function calls.

  • This allows students to trace through the steps from the beginning of a function call through to return.

Make sure to distinguish the differences between print statements and return statements to help students understand that these elements are not related.

• Print statements contribute to this misconception because the functionality of a print statement overlaps with their mental model of what return is supposed to do.

• Until print statements come into the equation and induce confusion, teachers might think students understand print statements.

• One strategy to avoid this confusion is to delay introducing print statements until after students have a solid understanding of return.