Slides from the training on the topic "Python Programming Essentials"

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Functions in Python
Pavan Verma
@YinYangPavan
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Founder, P3 InfoTech Solutions Pvt. Ltd.
Python Programming Essentials

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What is a function
A function is a block of organized, reusable code
that is used to perform a single, related action
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Advantages of using functions
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Reusability. Reducing
duplication of code
Decomposing
complex problems
into simpler pieces
Abstraction Cleaner code

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Types of functions
Types of
Functions
Built-in
Functions
User Defined
Functions
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Defining a function
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def functionName():
# do something
def printList():
num_list = range(3)
print num_list
1. def keyword is used to define a function
2. def keyword is followed by a function name
3. Function name is followed by parenthesis ()
4. After this a block of python statements

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Calling a Function
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def printList():
num_list = range(3)
print num_list
printList()
Output:
[0, 1, 2]
To call a function, we specify the function name
with the round brackets
Call to function

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return keyword
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def printList():
num_list = range(3)
return num_list
my_list = printList()
print my_list
Output:
[0, 1, 2]
1. The return keyword is
used to return values
from a function
2. A function implicitly
returns None by default
even if you don’t use
return keyword

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Parameterized functions
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def printList(upper_limit):
num_list = range(upper_limit)
print "list: %s" % num_list
printList(5)
printList(2)
Output:
list: [0, 1, 2, 3, 4]
list: [0, 1]
1. Names given in the
function definition
are called
parameters
2. The values you
supply in the
function call are
called
arguments

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Default arguments
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def printList(upper_limit=4):
print "upper limit: %d" % upper_limit
num_list = range(upper_limit)
print "list: %s" % num_list
printList(5)
printList()
Output:
upper limit: 5
list: [0, 1, 2, 3, 4]
upper limit: 4
list: [0, 1, 2, 3]
value 4
1. When function is called
without any arguments default
value is used
2. During a function call if
argument is passed then the
passed value is used

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Default arguments (2)
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A non-default argument may not follow default
arguments while defining a function
def printList(upper_limit=4, step):

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Keyword arguments
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def printList(upper_limit, step=1):
print "upper limit: %d" % upper_limit
num_list = range(0, upper_limit, step)
print "list: %s" % num_list
printList(upper_limit=5, step=2)
Output:
upper limit: 5
list: [0, 2, 4]
range() function has two variations:
1.range(stop)
2.range(start, stop[, step])

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Keyword arguments (2)
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printList(step=2, upper_limit=5)
printList(upper_limit=5, step=2)
Advantages of using keyword arguments
Using the function is easier since you don’t have to worry
about or remember the order of the arguments

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Keyword arguments (3)
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You can give values to only those parameters which you
want, provided that the other parameters have default
argument value
printList(upper_limit=5)

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Keyword arguments (4)
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A non-keyword argument cannot follow keyword
arguments while calling a function
printList(upper_limit=4, 10)

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global variables
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counter = 10
def printVar():
print counter
printVar()
print "counter: %d" % counter
Output:
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counter: 10
Variable defined outside a function or class is a
global variable
counter is a
global variable

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global variables (2)
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counter = 10
def printVar():
counter = 20
print "Counter in: %d" % counter
printVar()
print "Counter out: %d" % counter
Output:
Counter in: 20
Counter out: 10
counter in function definition is
local variable tied to function’s
namespace
Hence the value of global
counter does not gets modified

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global variables (3)
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counter = 10
def printVar():
global counter
counter = 20
print "Counter in: %d" % counter
printVar()
print "Counter out: %d" % counter
Output:
Counter in: 20
Counter out: 20
In order to use the global
counter variable inside the function
you have to explicitly tell python to use
global variable using global keyword

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docstrings
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def add(x, y):
""" Return the sum of inputs x, y
"""
return x + y
print add.__doc__
Python documentation strings (or docstrings) provide a
convenient way of associating documentation with Python
modules, functions, classes, and methods
docstring
Access a docstring using
object.__doc__

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Summary
 What is a function
 Advantages of using
functions
 Defining a function
 Calling a function
 return keyword
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 Parameterized functions
 Default arguments
 Keyword arguments
 global variables
 docstrings

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Resources
 Default arguments
http://www.ibiblio.org/g2swap/byteofpython/read/default-argument-
values.html
 Keyword arguments
http://www.ibiblio.org/g2swap/byteofpython/read/keyword-
arguments.html
 return keyword
http://www.ibiblio.org/g2swap/byteofpython/read/return.html
 local variables http://www.ibiblio.org/g2swap/byteofpython/read/local-
variables.html
 Docstrings
http://www.ibiblio.org/g2swap/byteofpython/read/docstrings.html
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