Shadowing and Parameters

A function can read a global variable. But when it assigns to a name that also exists globally, it creates a new local variable that shadows the global one. The global is untouched.

EXAMPLE

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YOUR TURN

Your turn → The global stayed 10. In the JavaScript tab, remove the `let` inside change() and run it: the function now changes the global. In Python, add `global count` as the first line of change() to do the same.

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Syntax has to be exact. Every quote, bracket and parenthesis needs its partner, and in Python the spaces at the start of a line count. One slip and the program won't run; the message will say where.

Parameters are local too. When you pass a value into a function, the parameter is a fresh local name holding a copy of that value. Reassigning the parameter inside the function does not touch the caller's variable.

EXAMPLE

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YOUR TURN

Your turn → Write a function add_tax(price) that returns the price with 7% tax added. Call it, store the result, and print both the original price and the result.

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Syntax has to be exact. Every quote, bracket and parenthesis needs its partner, and in Python the spaces at the start of a line count. One slip and the program won't run; the message will say where.

✦ The exception worth knowing

Lists and dictionaries behave differently. Passing one in does not copy it, it passes a reference to the same object, so changing its contents inside a function DOES affect the caller. Numbers and strings are copied, collections are shared. Module 18 works through this properly.

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