12.10. Global Variables¶
Variable names that are at the top-level, not inside any function definition, are called global.
It is legal for a function to access a global variable. However, this is considered bad form by nearly all programmers and should be avoided. This subsection includes some examples that illustrate the potential interactions of global and local variables. These will help you understand exactly how python works. Hopefully, they will also convince you that things can get pretty confusing when you mix local and global variables, and that you really shouldn’t do it.
Look at the following, nonsensical variation of the square function.
badsquare function works, it is silly and poorly written. We have done it here to illustrate
an important rule about how variables are looked up in Python.
First, Python looks at the variables that are defined as local variables in
the function. We call this the local scope. If the variable name is not
found in the local scope, then Python looks at the global variables,
or global scope. This is exactly the case illustrated in the code above.
power is not found locally in
badsquare but it does exist globally.
The appropriate way to write this function would be to pass power as a parameter.
For practice, you should rewrite the badsquare example to have a second parameter called power.
There is another variation on this theme of local versus global variables. Assignment statements in the local function cannot change variables defined outside the function. Consider the following codelens example:
Now step through the code. What do you notice about the values of variable
in the local scope compared to the variable
power in the global scope?
The value of
power in the local scope was different than the global scope.
That is because in this example
power was used on the left hand side of the
power = p. When a variable name is used on the
left hand side of an assignment statement Python creates a local variable.
When a local variable has the same name as a global variable we say that the
local shadows the global. A shadow means that the global variable cannot
be accessed by Python because the local variable will be found first. This is
another good reason not to use global variables. As you can see,
it makes your code confusing and difficult to understand.
If you really want to change the value of a global variable inside a function, you can can do it by explicitly declaring the variable to be global, as in the example below. Again, you should not do this in your code. The example is here only to cement your understanding of how python works.
To cement all of these ideas even further lets look at one final example.
square function we are going to make an assignment to the
x There’s no good reason to do this other than to emphasize
the fact that the parameter
x is a local variable. If you step through
the example in codelens you will see that although
x is 0 in the local
x in the global scope remains 2. This is confusing
to many beginning programmers who think that an assignment to a
formal parameter will cause a change to the value of the variable that was
used as the actual parameter, especially when the two share the same name.
But this example demonstrates that that is clearly not how Python operates.
Check your understanding
You may be asking yourself at this point when you should make some object a local variable and when should you make it a global variable. Generally, we do not recommend making variables global. Imagine you are trying to write a program that keeps track of money while purchasing groceries. You may make a variable that represents how much money the person has, called
wallet. You also want to make a function called
purchase, which will take the name of the item and its price, and then add the item to a list of groceries, and deduct the price from the amount stored in
wallet. If you initialize wallet before the function as a variable within the global scope instead of passing it as a third parameter for
purchase, then an error would occur because wallet would not be found in the local scope. Though there are ways to get around this, as outlined in this page, if your program was supposed to handle groceries for multiple people, then you would need to declare each wallet as a global variable in the functions that want to use wallet, and that would become very confusing and tedious to deal with.