6.4. 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
If you press the ‘last >>’ button you will see an error message.
When we try to use
y on line 6 (outside the function) Python looks for a global
y but does not find one. This results in the
Name Error: 'y' is not defined.
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. Codelens helps you visualize this
because the local variables disappear after the function returns. Go back and step through the
statements paying particular attention to the variables that are created when the function is called.
Note when they are subsequently destroyed as the function returns.
Formal parameters are also local and act like local variables.
For example, the lifetime of
x begins when
and its lifetime ends when the function completes its execution.
So it is not possible for a function to set some local variable to a value, complete its execution, and then when it is called again next time, recover the local variable. Each call of the function creates new local variables, and their lifetimes expire when the function returns to the caller.
On the other hand, 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. 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, without further (discouraged) special syntax. 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
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