Parameters and Variables

Earlier, we said that a parameter “behaves like a variable within the function.” While this is true, the relationship between variables and parameters is a bit more complicated.

Function Scope

The scope of a variable refers to where that variable is useable within a program. Scope consists of all locations in a program where a variable can be used or modified.

A variable is not always usable throughout an entire program. Where it can be used depends on where it is defined. Parameters and variables defined inside a function are only visible within that function.

Variable Shadowing

What about variables defined OUTSIDE of a function?

This situation is more complicated. A variable defined outside a function may be visible within the function, but using it or trying to change its value creates problems.

Tip

Do NOT define variables inside a function that use the same names found outside of the function.

Example

What if we did something like this:

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def remove_hyphens(phone_number):
   without_hyphens = phone_number.replace('-', '')
   return without_hyphens

phone_number = "614-555-5555"
no_hyph_number = remove_hyphens('56-78')

We don’t recommend doing this! Are you having trouble interpreting this code? When the function runs, does phone_number on line 2 have the value "614-555-5555" or '56-78'? Feel free to run this code in a code editor above to find out.

An interesting thing happens when a function parameter has the same name as a variable defined outside of that function.

While the variable phone_number declared on line 5 is visible inside remove_hyphens, it is hidden by the function parameter with the same name. When remove_hyphens('56-78') is called, and phone_number.replace('-', '') runs, phone_number has the value '56-78', which is the argument passed into the function.

This situation is called shadowing. We can imagine that a function parameter casts a shadow over a variable of the same name and hides it from view.

Warning

There is NO good reason to allow variable shadowing in your programs!

Avoid giving variables and function parameters the same name.

Check Your Understanding

Question

What does the following code output?

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def is_even(num): 
   return num % 2 == 0

num = 42
print(is_even(43))

True
False