Variable Scope

Overview#

This lesson introduces local and global scope with a few examples.

Learning Objectives#

In this lesson, students will:

Define variable scope.
Use the global keyword to access global variables.
Explain the order of scope precedence that Python follows when resolving variable names.

Duration#

20 minutes

Agenda#

Duration	Activity
3 mins	Welcome
5 mins	Local Scope
10 mins	Global scope
2 mins	Summary

Lesson Objectives#

After this lesson, you will be able to…

Define variable scope.
Use the global keyword to access global variables.
Explain the order of scope precedence that Python follows when resolving variable names.

Discussion: Delivering a Letter#

What if someone wanted to send Brandi a letter?

If you just had "For Brandi," the mail carrier would give the letter to the first Brandi they see!

They'd look:

First in the class. Is there a "Brandi" here? They get the letter!
No? OK, look in the town. Is there a "Brandi" here? They get the letter!
No? OK, look in the state. Is there a "Brandi" here? They get the letter!

Discussion: Your Address#

That's why scope matters. We might have to get more specific. To correctly deliver the letter, if the mail carrier only looked in the scope of:

Your class:

You're probably the only Brandi.
"For Brandi" is fine.

Your town:

There might be multiple Brandis in the town.
"For Brandi, on Main Street" is a bit more specific.

In your state:

There are multiple Main Streets in New York!
"For Brandi, on Main Street in Brooklyn" is more specific.

Discussion: What Is `x`?#

Python has scope, too. We can have many variables with the same name, and Python will look for the most specific one.

In different scopes, you can reuse the same name. Each one is a completely different variable.

Functions and classes create individual local scopes. A local variable doesn't exist outside its local function or class scope.

def my_func1():    x = 1  # This is a LOCAL variable.    print(x)  # 1

def my_func2():    x = 5  # This is a DIFFERENT local variable.    print(x)  # 5

print(x)  # x is OUT OF SCOPE - no x exists here!

Talking Points:

Any variable declared or assigned inside of a function is local to that function.
This is the most specific level of scope and is, ideally, where most of your variables should be declared.
Only the function in which the variable was declared has access to this scope — i.e., the variable is out of scope for everything but that function.

Global Scope#

Variables that are in global scope can be accessed anywhere.

Python will check for a local variable before using a global one.

x = 2

def my_func1():    x = 1    print(x)  # 1 - Python checks local scopes first.

def my_func2():    x = 5    print(x)  # 5 - Python checks local scopes first.

my_func1()my_func2()
print(x)  # 2 - Python found no local scope; prints global variable.

If some variables are specifically local, what are the variables outside of a function or class called?
Any variable declared or assigned outside of any function or class is considered "global."
Global variables are accessible from anywhere in the script. This is not necessarily a good thing, however, because those variables can be accessed, changed, or reassigned by anything, and this can lead to troublesome bugs.
This is another case where Python has our backs. It's preventing us from making an accidental error that could easily occur in many other languages.
Python assumes local unless otherwise specified.
- Meaning, these xs are three different variables.
Python does this to prevent unexpected behavior and accidental bad practice.
- It's considered sloppy to have too many global variables.
- If you have a large code base, you may have forgotten that you used a variable name elsewhere.
- If you're working on a team, another person may have used a variable name without your knowledge.

Multiple Variables, One Name#

Use case: x and y are frequently used to represent numbers.

Scope is important so they don't interact!

def add(x, y):    return x + y

def subtract(x, y):    return x - y

def multiply(x, y):    return x * y

def divide(x, y):    return x / y

divide(8, 2)  # Returns 4multiply(3, 1)  # Returns 3

Why would you want to have different variables with the same name? Do you expect each x and y in this code to perform independently?

We Do: Accessing Scopes#

Let's start with global scope:

foo = 5print(foo)foo = 7print(foo)

Python makes it a little trickier than other languages to fiddle around with global variables if we're not already in that scope. First, start up a blank script. The following line will assign a global variable named foo the value of 5. We can easily reassign and access that variable with the following lines. That's the global scope: There's no restriction on accessing or mutating a variable.

We Do: Accessing Local Scope#

What if we add a variable in a local function scope and try to access it from the global scope?

foo = 5
# Delete your other code.# Add this function and print calls instead.def coolFunc():    bar = 8

coolFunc()print(foo)print(bar)

It fails!

Talking Points:

If you run this code, you will get an error: NameError: name 'bar' is not defined..
The variable bar is only accessible from inside the coolFunc() function.
We called the coolFunc() function, but as soon as it finished running, the variable bar ceased to exist. Even while the function was running, it was only accessible to itself. But, foo in the global scope was still accessible.

Scope Can Be Tricky#

What do you think happened here?

foo = 5

def incrementFoo():    foo = 6    print(foo)  # prints 6

print(foo)  # prints 5incrementFoo()print(foo)  # prints 5

Talking Points:

Hey! The variable foo went back to its old value after the function finished! Actually, not quite. Here's what happened:
- The line in the function where foo is assigned the value of 6 causes the creation of a new local variable.
- We then set this variable's value to 6, the function prints the value, and the function finishes. However, the global variable foo was never touched by the function.

I Do: The Global Keyword#

You can call a global variable intentionally with global.

What do you think happens if you forget the global keyword?

foo = 5

def incrementFoo():    global foo    foo += 1

print(foo)  # prints 5incrementFoo()print(foo)  # prints 6!

Talking Points:

It is very clearly marked as global, so anyone using it is fully aware it's global.
If you forget the global keyword, it simply becomes an unrelated local variable.

We Do: Global vs. Local#

In the following code, there are three print statements. Before you run the code, guess what those print statements will print.

Repl.it Note: Code is here

# Global variable x:x = 1
# Only local x in this function:def my_func1():    x = 2  # This is a different, local x    print(x)  # Print the local x

# Using global x:def my_func2():    global x  # We declare we'd like to use the global x.    print(x)  # Print that global x.    x = 3  # Change that global x.

my_func1()my_func2()
# Print global variable x.print(x)  # Did x get permanently changed by my_func2()?

You Do: Just a Day in the Jungle#

Open a new local file, piranhas.py.

Declare a global variable piranhas_hungry and set it to True.
Write two functions, swing_vine_over_river and jump_in_river.
In swing_vine_over_river, print Ahhh! Piranhas got me!.
- Change piranhas_hungry to False.
In jump_in_river, if piranhas_hungry is True, print I'm not going in there! There are hungry piranhas!.
- Otherwise, print Piranhas are full! Swimming happily through the Amazon!

# Call functions in this order.jump_in_river()swing_vine_over_river()jump_in_river()

We Do: Check Your Answers#

Did you remember the global keyword?
What happens if that keyword is removed?
Comment out line 4. What happens? Why?

Summary and Q&A#

Python checks scope to find the right variable.

Functions and classes create individual local scopes.
- A local variable doesn't exist outside its local function or class scope.
Any variable declared or assigned outside of any function or class is considered "global."
- Variables that are in global scope can be accessed anywhere.

Python will check for a local variable before using a global one.

There can be more levels. Python always works from the inside out — keep that in mind as your programs get more advanced!

https://repl.it/@SuperTernary/python-programming-scope-piranha?lite=true

Overview#

Learning Objectives#

Duration#

Agenda#

Lesson Objectives#

Discussion: Delivering a Letter#

Discussion: Your Address#

Discussion: What Is x?#

Global Scope#

Multiple Variables, One Name#

We Do: Accessing Scopes#

We Do: Accessing Local Scope#

Scope Can Be Tricky#

I Do: The Global Keyword#

We Do: Global vs. Local#

You Do: Just a Day in the Jungle#

We Do: Check Your Answers#

Summary and Q&A#

Additional Resources#

Discussion: What Is `x`?#