More Events

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CS 110

Variable Scope

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Scope

1. Scope — refers to the part of the program where a variable is accessible
2. Lifetime — the duration for which the variable exists
3. Global variable — a variable which is defined in the main body of a file (best avoided)
4. Local variable — a variable which is defined inside a function. Not accessible from outside the function
1. The parameter names in the function definition behave like local variables, but they contain the values that we pass into the function when we call it.

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Source: https://python-textbok.readthedocs.io/en/1.0/Variables_and_Scope.html

Scope 1: Consider the following program...

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def demo_1(name):

greeting = 'Welcome, ' + name

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demo_1('Jimmy')

print(greeting)

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�What will print to the screen?

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Scope 1: Local variables cannot be accessed outside function

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def demo_1(name):

greeting = 'Welcome, ' + name

demo_1('Jimmy')

print(greeting)

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�TAKEAWAY: If you want access to a value after the function ends, you have to return it.

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Scope 2: Which name will print to the screen?

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name = 'Lindsay'

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def demo_2(name):

print(name)

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demo_2('Walter')

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Scope 2: Which name will print to the screen?

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name = 'Lindsay'

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def demo_2(name):

print(name)

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demo_2('Walter')

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TAKEAWAY: Local variables take precedence over global variables.

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Scope 3: Global variables can be accessed inside a function

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name_1 = 'Lindsay'

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def demo_3(name):

print(name)

print(name_1)

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demo_3('Walter')

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Scope 4: Global variables can updated with global keyword

name = 'Lindsay'

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def modify_name(new_name):

global name

name = new_name

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print(name)�modify_name('Walter')�print(name)

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Scope 5: Without global keyword, Python assumes you’re creating a new local variable

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�name = 'Lindsay'

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def modify_name(new_name):

name = new_name

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print(name)�modify_name('Walter')�print(name)

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Back to Events

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Events: Ways of Dealing with User Input

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Events allow programs to listen to particular events (clicks, drags, etc.) and execute parts of a program based on the input. Events have two parts:

1. An event listener, which can detect that an event has occurred and the nature of it. Listeners notify your program that something has happened.
2. An event handler (the function), which is the block of code that responds to an event when is triggered by executing a code block. Event handlers are sometimes called “callbacks.”

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listen_for("LEFT-CLICK", do_something)

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Setting an Event Listener on the Pop-up Window