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Section 19.4 Common Function Issues

There are a few common mistakes you should watch out for while writing functions.

Subsection 19.4.1 Returning Ends a Function

One common mistake when writing functions is to try to do work after a return statement. When Python hits a return, it does just that: it returns to the place the function was called. The program below tries to use the addFive function to add 5 to the score; however, the code in the function that does the math comes after the return and thus never runs! Try running it in Codelens mode to see:

Warning 19.4.1.

A return ends a function immediately.

Subsection 19.4.2 Trying to Change a Parameter

The parameters for a function are variables that only exist inside of that function. You can make changes to their values, but doing so does not affect the rest of the program.
This version of the program does not work correctly either. score is passed as the argument from the main part of the program into addFive. That means addFive uses a copy of score’s value as its value for x. But x and score are not linked in any way. A change to x does nothing to change the value of score. Nothing is ever returned, so the rest of the program never learns about the changes that happened in addFive.
Again, try running in Codelens:

Warning 19.4.2.

The only way to get information (new values) out of a function is to return them.
Check Your Understanding

Checkpoint 19.4.3.

The following program is supposed to convert a Celsius temp into Fahrenheit.
Put the blocks in the right order and indentation. As usual, you will not use all the blocks.
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