Py-slides-3 easyforbeginnerspythoncourse.pptx

Python Functions
• There are two kinds of functions in Python.
- Built-in functions that are provided as part of Python - print(),
input(), type(), float(), int() ...
- Functions that we define ourselves and then use
• We treat function names as “new” reserved words
(i.e., we avoid them as variable names)

Function Definition
• In Python a function is some reusable code that takes
arguments(s) as input, does some computation, and then returns
a result or results
• We define a function using the def reserved word
• We call/invoke the function by using the function name,
parentheses, and arguments in an expression

Building our Own Functions
• We create a new function using the def keyword followed by
optional parameters in parentheses
• We indent the body of the function
• This defines the function but does not execute the body of the
function
def print_lyrics():
print("I'm a lumberjack, and I'm okay.")
print('I sleep all night and I work all day.')

x = 5
print('Hello')
def print_lyrics():
print('Yo')
x = x + 2
print(x)
Hello
Yo
7
print_lyrics():

Definitions and Uses
• Once we have defined a function, we can call (or invoke) it
as many times as we like
• This is the store and reuse pattern

x = 5
print('Hello')
def print_lyrics():
print('Yo')
print_lyrics()
x = x + 2
print(x)
Hello
Yo
I'm a lumberjack, and I'm okay.
I sleep all night and I work all day.
7

Arguments
• An argument is a value we pass into the function as its input
when we call the function
• We use arguments so we can direct the function to do different
kinds of work when we call it at different times
• We put the arguments in parentheses after the name of the
function
big = max('Hello world')
Argument

Parameters
A parameter is a variable which
we use in the function definition.
It is a “handle” that allows the
code in the function to access
the arguments for a particular
function invocation.
>>> def greet(lang):
... if lang == 'es':
... print('Hola')
... elif lang == 'fr':
... print('Bonjour')
... else:
... print('Hello')
...
>>> greet('en')
Hello
>>> greet('es')
Hola
>>> greet('fr')
Bonjour
>>>
Parameter
Argument

Return Values
Often a function will take its arguments, do some computation, and
return a value to be used as the value of the function call in the
calling expression. The return keyword is used for this.
def greet():
return "Hello"
print(greet(), "Glenn")
print(greet(), "Sally")
Hello Glenn
Hello Sally

Return Value
• A “fruitful” function is one
that produces a result (or
return value)
• The return statement ends
the function execution and
“sends back” the result of
the function
>>> def greet(lang):
... if lang == 'es':
... return 'Hola'
... elif lang == 'fr':
... return 'Bonjour'
... else:
... return 'Hello'
...
>>> print(greet('en'),'Glenn')
Hello Glenn
>>> print(greet('es'),'Sally')
Hola Sally
>>> print(greet('fr'),'Michael')
Bonjour Michael
>>>

Arguments, Parameters, and
Results
>>> big = max('Hello world')
>>> print(big)
w
def max(inp):
blah
blah
for x in inp:
blah
blah
return 'w'
'Hello world' 'w'
Argument
Parameter
Result

Multiple Parameters / Arguments
• We can define more than one
parameter in the function
definition
• We simply add more arguments
when we call the function
• We match the number and order
of arguments and parameters
def addtwo(a, b):
added = a + b
return added
x = addtwo(3, 5)
print(x)
8

To function or not to function...
• Organize your code into “paragraphs” - capture a complete
thought and “name it”
• Don’t repeat yourself - make it work once and then reuse it
• If something gets too long or complex, break it up into logical
chunks and put those chunks in functions
• Make a library of common stuff that you do over and over -
perhaps share this with your friends...

Exercise
Rewrite your pay computation with time-and-a-
half for overtime and create a function called
computepay which takes two parameters ( hours
and rate).
Enter Hours: 45
Enter Rate: 10
Pay: 475.0
475 = 40 * 10 + 5 * 15

Py-slides-3 easyforbeginnerspythoncourse.pptx

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