Lecture 17 Slides - Keyboard Events

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More Events

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CS 110

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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:

  • An event listener, which can detect that an event has occurred and the nature of it. Listeners notify your program that something has happened.
  • 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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setup_listener("LEFT-CLICK", do_something)

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We use this function to set the listener up.

A function name to run when the event happens (handler) that is defined to have exactly 1 input

Setting an Event Listener on the Pop-up Window

The event to listen for (a special string specified by Python)

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Variable Scope

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Scope

  • Scope — refers to the part of the program where a variable is accessible
  • Lifetime — the duration for which the variable exists
  • Global variable — a variable which is defined in the main body of a file (best avoided)
  • Local variable — a variable which is defined inside a function. Not accessible from outside the function
    • 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

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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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local variable

greeting variable was local to the demo_1 function. Throws error.

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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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Global variable

Parameters are local variables. �They take precedence over global variables

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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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Global variable

Parameters are local variables. �They 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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local variable

global variable

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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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