Variables and Parameters are Local¶

An assignment statement in a function creates a local variable for the variable on the left hand side of the assignment operator.
It is called local because this variable only exists inside the function and you cannot use it outside.

For example, consider again the square function:

(bad_local)

The variable y only exists while the function is being executed – we call this its lifetime.
When the execution of the function terminates (returns), the local variables are destroyed.
Formal parameters are also local and act like local variables.
For example, the lifetime of x begins when square is called, and its lifetime ends when the function completes its execution.
On the other hand, it is legal for a function to access a global variable.

def badsquare(x):
    y = x ** power
    return y

power = 2
result = badsquare(10)
print(result)

(badsquare_1)

Although the badsquare function works, it is silly and poorly written.

First, Python looks at the variables that are defined as local variables in the function, i.e. the local scope.
If the variable name is not found in the local scope, then Python looks at the global variables, or global scope.
Assignment statements in the local function cannot change variables defined outside the function. Consider the following codelens example:

(cl_powerof_bad)

A shadow means that the global variable cannot be accessed by Python because the local variable will be found first.

Analogously, look at the following.

(cl_change_parm)