A practical introduction to Python

An introduction to coding with Python

An introductory coding course with

Python…
Interpretive vs compiled languages
• Python is an interpretive language.
• This means that your code is not directly run by the hardware. It is
instead passed to a virtual machine, which is just another programme
that reads and interprets your code. If your code used the ‘+’
operation, this would be recognised by the interpreter at run time,
which would then call its own internal function ‘add(a,b)’, which would
then execute the machine code ‘ADD’.
• This is in contrast to compiled languages, where your code is translated
into native machine instructions, which are then directly executed by
the hardware. Here, the ‘+’ in your code would be translated directly in
the ‘ADD’ machine code.
 Advantages of Python?

Because Python is an interpretive

language, it has a number of advantages:
• Automatic memory management.
• Expressivity and syntax that is ‘English’.
• Ease of programming.
• Minimises development time.
• Python also has a focus on importing
modules, a feature that makes it useful
for scientific computing.
 Disadvantages
• Interpreted languages are slower than
compiled languages.
• The modules that you import are developed in
a decentralised manner; this can cause issues
based upon individual assumptions.
• Multi-threading is hard in Python
 Which language is the best

• No one language is
better than all
others.
• The ‘best’ language
depends on the task
you are using it for
and your personal
preference.
 Versions of Python
• There are currently two versions of Python in use;
Python 2 and Python 3.
• Python 3 is not backward compatible with Python 2.
• A lot of the imported modules were only available in
Python 2 for quite some time, leading to a slow
adoption of Python 3. However, this not really an
issue anymore.
• Support for Python 2 will ended in 2020. But
updates are still given
 The Anaconda IDE…

• The Anaconda distribution is the most

popular Python distribution out there.
• Most importable packages are pre-
installed.
• Offers a nice GUI in the form of Spyder.
• Before we go any further, let’s open
Spyder:
 Run
with some input

Write/Edit
OK?
NO
YES

YES
NO More
Inputs?
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 User Program
Filename, preferred extension is py
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IN[1]: Python Shell Prompt

IN[2]:
User Commands
IN[3]: (Statements)

IN[4]: ( )

Outputs

Python Shell is Interactive

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 Interacting with Python Programs
• Python program communicates its results to
user using print
• Most useful programs require information
from users
– Name and age for a travel reservation system
• Python 3 uses input to read user input as a
string (str)

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 input
• Take as argument a string to print as a prompt
• Returns the user typed value as a string
– details of how to process user string later

IN[1]:

IN[2]: ( )

IN[3]:

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 Elements of Python
• A Python program is a sequence of definitions
and commands (statements)
• Commands manipulate objects
• Each object is associated with a Type
• Type:
– A set of values
– A set of operations on these values
• Expressions: An operation (combination of
objects and operators)
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 Types in Python
• int
– Bounded integers, e.g. 732 or -5
• float
– Real numbers, e.g. 3.14 or 2.0
• long
– Long integers with unlimited precision
• str
– Strings, e.g. ‘hello’ or ‘C’

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 Types in Python
• Scalar
– Indivisible objects that do not have internal
structure
– int (signed integers), float (floating point), bool
(Boolean), NoneType
• NoneType is a special type with a single value
• The value is called None
• Non-Scalar
– Objects having internal structure
– str (strings)
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 Example of Types

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 Type Conversion (Type Cast)

• Conversion of value of one type to other

• We are used to int float conversion in Math
– Integer 3 is treated as float 3.0 when a real
number is expected
– Float 3.6 is truncated as 3, or rounded off as 4 for
integer contexts
• Type names are used as type converter
functions

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 Type Conversion Examples
Note that float to int conversion
is truncation, not rounding off

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 Type Conversion and Input

ii

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 22
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 The print() function
• A function is a separate part of the computer code
able to:
• cause some effect (e.g., send text to the terminal,
create a file, draw an image, play a sound, etc.); this
is something completely unheard of in the world of
mathematics;
• evaluate a value (e.g., the square root of a value or
the length of a given text) and return it as the
function's result; this is what makes Python
functions the relatives of mathematical concepts.
23
Welcome Python Programming
 The print() function
• The function name (print in this case) along with the
parentheses and argument(s), forms the function
invocation.
• The print() function has two keyword arguments
that you can use for keyword arguments. The first is
called end.
– a keyword argument consists of three elements: a
keyword identifying the argument (end here); an equal
sign (=); and a value assigned to that argument;
– any keyword arguments have to be put after the last
positional argument (this is very important)
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Welcome Python Programming
 The print() function
print("My name is", "Python.", end=" ")
print("Monty Python.")

Output:
My name is Python. Monty Python.

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Welcome Python Programming
 The print() function
• print() function separates its outputted
arguments with spaces using keyword
argument : sep (as in separator).
print("My", "name", "is", "Monty", "Python.",
sep="-")
Output
My-name-is-Monty-Python.
• the sep argument's value may be an empty
string, too
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Welcome Python Programming
 The print() function
• Example: using the sep and end keywords, to
match the expected output
Output
Programming***Essentials***in...Python
• print("Programming","Essentials","in",
sep="***", end="...")
• print("Python")

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Welcome Python Programming
 Variables
• Variables in python can contain alphanumerical characters and
some special characters.
• By convention, it is common to have variable names that start
with lower case letters and have class names beginning with a
capital letter; but you can do whatever you want.
• Some keywords are reserved and cannot be used as variable
names due to them serving an in-built Python function; i.e. and,
continue, break. Your IDE will let you know if you try to use one
of these.
• Python is dynamically typed; the type of the variable is derived
from the value it is assigned.
 Variables
• A name associated with an m
object 64
• Assignment used for binding Acads
m = 64; c
3.1416
c = ‘Acads’;
f = 3.1416; f

• Variables can change their 2.7183

bindings
f = 2.7183;
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 Assignment Statement
• A simple assignment statement
Variable = Expression;
• Computes the value (object) of the expression
on the right hand side expression (RHS)
• Associates the name (variable) on the left
hand side (LHS) with the RHS value
• = is known as the assignment operator.

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 How to use a variable
• You're allowed to use as many variable
declarations as you need to achieve your goal,
like this:
var = 1
account_balance = 1000.0
client_name = “John Doe”
print(var, account_balance, client_name)
print(var)
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Welcome Python Programming
 Multiple Assignments
• Python allows multiple assignments
x, y = 10, 20 Binds x to 10 and y to 20

• Evaluation of multiple assignment statement:

– All the expressions on the RHS of the = are first
evaluated before any binding happens.
– Values of the expressions are bound to the
corresponding variable on the LHS.
x, y = 10, 20 x is bound to 21
x, y = y+1, x+1 and y to 11 at the
end of the program
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 Exercise (10 Minutes)
• Once upon a time in Appleland, John had three apples, Mary
had five apples, and Chanda had six apples. They were all very
happy and lived for a long time. End of story

Your task is to:

•create the variables: john, mary, and chanda;
•assign values to the variables. The values must be equal to
the numbers of fruit possessed by John, Mary, and Chanda
respectively;
•having stored the numbers in the variables, print the variables
on one line, and separate each of them with a comma;
•now create a new variable named total_apples equal to the
addition of the three previous variables.
•print the value stored in total_apples to the console;
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Welcome Python Programming
 Programming using Python

Operators and Expressions

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 Operators
• Arithmetic + - * // / % **

• Comparison == != > < >= <=

• Assignment = += -= *= //= /= %= **=

• Logical and or not
• Bitwise & | ^ ~ >> <<
• Membership in not in
• Identity is is not

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 Binary Operations
Op Meaning Example Remarks

+ Addition 9+2 is 11
9.1+2.0 is 11.1
- Subtraction 9-2 is 7
9.1-2.0 is 7.1
* Multiplication 9*2 is 18
9.1*2.0 is 18.2
/ Division 9/2 is 4.25 In Python3
9.1/2.0 is 4.55 Real div.
// Integer Division 9//2 is 4
% Remainder 9%2 is 1

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 The // operator
• Also referred to as “integer division”
• Result is a whole integer (floor of real
division)
– But the type need not be int
– the integral part of the real division
– rounded towards minus infinity
• Examples
9//4 is 2 (-1)//2 is -1 (-1)//(-2) is 0
1//2 is 0 1//(-2) is -1 9//4.5 is 2.0
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 The % operator
• The remainder operator % returns the
remainder of the result of dividing its
first operand by its second.
9%4 is 1 (-1)%2 is 1 (-1)//(-2) is 0
9%4.5 is 0.0 1%(-2) is 1 1%0.6 is 0.4

Ideally: x == (x//y)*y + x %y
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 Exponentiation and Multiplication

• An * (asterisk) sign is a multiplication operator.

• Two ** (asterisks) signs are used for
exponentiation operator.
• print(2 * 3) # multiplication
• print(2 ** 3) # exponentiation
Output
• 6
• 8
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Welcome Python Programming
 Solving simple mathematical problems

• The square of the hypotenuse is equal to the

sum of the squares of the other two sides.
• The following code evaluates the length of the
hypotenuse (i.e., the longest side of a right-
angled triangle, the one opposite of the right
angle) using the Pythagorean theorem:
make use of the ** operator to
evaluate the square root as:

√ (x)  = x(½)
Welcome Python Programming
c = √ a2 + b2  40
 Exercise
• Write a program that reads a float value, puts
it into a variable named x, and prints the value
of a variable named y:
y = 3x3 - 2x2 + 3x - 1

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Welcome Python Programming
 Conditional Statements
• In daily routine
– If it is very hot, I will skip
exercise.
– If there is a quiz tomorrow, I will
first study and then sleep.
Otherwise I will sleep now.
– If I have to buy coffee, I will
go left. Else I will go
straight. 42
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 if-else statement
• Compare two integers and print the min.

if x < y: 1. Check if x is less

print (x) than y.
2. If so, print x
else: 3. Otherwise, print y.
print (y)
print (‘is the minimum’)

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 Indentation
• Indentation is important in Python
– grouping of statement (block of statements)
– no explicit brackets, e.g. { }, to group statements
x,y = 6,10 Run
x the program
y

if x < y: 6 10
print (x)
else:
print (y) ed Output
print (‘is i pp
sk the min’) 6 44
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 if statement (no else!)
• General form of the if statement
e
if boolean-expr : tru

fals
e
S1
S1
S2
• Execution of if statement S2
– First the expression is evaluated.
– If it evaluates to a true value, then S1 is
executed and then control moves to the S2.
– If expression evaluates to false, then control
moves to the S2 directly.
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 if-else statement
• General form of the if-else statement
if boolean-expr : tru
e

fa
S1

ls
e
else: S1 S2
S2
S3 S3
• Execution of if-else statement
– First the expression is evaluated.
– If it evaluates to a true value, then S1 is executed and
then control moves to S3.
– If expression evaluates to false, then S2 is executed
and then control moves to S3.
– S1/S2 can be blocks of statements! 46
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 Nested if, if-else
if a <= b:
if a <= c:
…
else:
…
else:
if b <= c) :
…
else:
…

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 Elif
• A special kind of nesting is the chain of if-
else-if-else-… statements
• Can be written elegantly using if-elif-..-else
if cond1: if cond1:
s1 s1
else: elif cond2:
if cond2: s2
s2 elif cond3:
else: s3
if cond3: elif …
s3 else
else: last-block-of-stmt
…
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 Summary of if, if-else
• if-else, nested if's, elif.
• Multiple ways to solve a problem
–issues of readability,
maintainability
–and efficiency

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 Class Quiz
• What is the value of expression:
(5<2) and (3/0 > 1)

a) Run time crash/error

b) I don’t know / I don’t care

c) False The correct answer is

False

d) True
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 Short-circuit Evaluation
• Do not evaluate the second operand of binary
short-circuit logical operator if the result can be
deduced from the first operand
– Also applies to nested logical operators

true false false true

not( (2>5) and (3/0 > 1) ) or (4/0 < 2)

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Evaluates to true
Programming
51
 3 Factors for Expr Evaluation
• Precedence
– Applied to two different class of operators
– + and *, - and *, and and or, …
• Associativity
– Applied to operators of same class
– * and *, + and -, * and /, …
• Order
– Precedence and associativity identify the operands for
each operator
– Not which operand is evaluated first
– Python evaluates expressions from left to right
– While evaluating an assignment, the right-hand side is
52
evaluated before the left-hand side.
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Class Quiz
• What is the output of the following program:
y = 0.1*3
if y != 0.3:
print ('Launch a Missile')
else:
print ("Let's have peace")
Launch a Missile
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 Caution about Using Floats
• Representation of real numbers in a computer
can not be exact
– Computers have limited memory to store data
– Between any two distinct real numbers, there are
infinitely many real numbers.
• On a typical machine running Python, there are
53 bits of precision available for a Python float

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 Caution about Using Floats
• The value stored internally for the decimal
number 0.1 is the binary fraction
0.00011001100110011001100110011001100110011001100110011010

• Equivalent to decimal value

0.1000000000000000055511151231257827021181583404541015625

• Approximation is similar to decimal

approximation 1/3 = 0.333333333...
• No matter how many digits you use, you have
an approximation
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 Comparing Floats
• Because of the approximations, comparison of
floats is not exact.
• Solution?
• Instead of
x == y
use
abs(x-y) <= epsilon
where epsilon is a suitably chosen small value

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 Exercise
• Spathiphyllum, more commonly known as a peace lily or white sail
plant, is one of the most popular indoor houseplants that filters out
harmful toxins from the air. Some of the toxins that it neutralizes
include benzene, formaldehyde, and ammonia.
• Imagine that your computer program loves these plants. Whenever
it receives an input in the form of the word Spathiphyllum, it
involuntarily shouts to the console the following string:
"Spathiphyllum is the best plant ever!“
Write a program that utilizes the concept of conditional execution,
takes a string as input, and:
• prints the sentence "Yes - Spathiphyllum is the bestplant ever!" to
the screen if the inputted string is "Spathiphyllum" (upper-case - s)
• prints "No, I want a big Spathiphyllum!" if the inputted string is
"spathiphyllum" (lower-case)
• prints "Spathiphyllum! Not [input]!" otherwise. Note: [input] is the
string taken as input. 57
Welcome Python Programming
 Programming using Python

Loops
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 Printing Multiplication Table
5 X 1 = 5
5 X 2 = 10
5 X 3 = 15
5 X 4 = 20
5 X 5 = 25
5 X 6 = 30
5 X 7 = 35
5 X 8 = 40
5 X 9 = 45
5 X 10 = 50

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 Program…
n = int(input('Enter a Too
number:
much '))
print (n, 'X', 1, '=', n*1)
repetition!
print (n, 'X', 2, '=', n*2)
Can I avoid
print (n, 'X', 3, '=', n*3)it?
print (n, 'X', 4, '=', n*4)
print (n, 'X', 5, '=', n*5)
print (n, 'X', 6, '=', n*6)
….

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 Printing Multiplication Table

Input n Loop Entry

i=1

Loop Exit
i <=10
TRUE FALSE

Print n X i = n*i Stop

i = i+1

Loop 61
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 Printing Multiplication Table
Input n
i=1

TRUE
i <=10
FALSE n = int(input('n=? '))
i=1
Print n x i = ni Stop
i = i+1

while (i <= 10) :

print (n ,'X', i, '=', n*i)
i=i+1
print ('done‘)

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 While Statement
while (expression):
S1 FALSE
expression
S2
TRUE

S1 S2
1. Evaluate expression
2. If TRUE then
a) execute statement1
b) goto step 1.
3. If FALSE then execute statement2.
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 For Loop
• Print the sum of the reciprocals of the
first 100 natural numbers.

rsum=0.0# the reciprocal sum

# the for loop

for i in range(1,101):
rsum = rsum + 1.0/i
print ('sum is', rsum)
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 For loop in Python
• General form

for variable in sequence:

stmt

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 range
• range(s, e, d)
– generates the list:
[s, s+d, s+2*d, …, s+k*d]
where s+k*d < e <= s+(k+1)*d
• range(s, e) is equivalent to range(s, e, 1)
• range(e) is equivalent to range(0, e)
Exercise: What if d is negative? Use python
interpreter to find out. 66
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 Quiz
• What will be the output of the following
program

# print all odd numbers < 10

i = 1
while i <= 10:
if i%2==0: # even
continue
print (i, end=‘ ‘)
i = i+1
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 Continue and Update Expr
• Make sure continue does not bypass update-
expression for while loops

# print all odd numbers < 10

i = 1 i is not incremented
while i <= 10: when even number
if i%2==0: # even encountered.
continue Infinite loop!!
print (i, end=‘ ‘)
i = i+1
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 Exercise
• A junior magician has picked a secret number. He has hidden it in a variable
named secret_number (e.g 787). He wants everyone who runs his program
to play the Guess the secret number game, and guess what number he has
picked for them. Those who don't guess the number will be stuck in an
endless loop forever! Unfortunately, he does not know how to complete the
code.
Your task is to help the magician come up with the code in such a way so that
the code:
• will ask the user to enter an integer number;
• will use a while loop;
• will check whether the number entered by the user is the same as the
number picked by the magician. If the number chosen by the user is
different than the magician's secret number, the user should see the
message "Ha ha! You're stuck in my loop!" and be prompted to enter a
number again. If the number entered by the user matches the number
picked by the magician, the number should be printed to the screen, and the
magician should say the following words: "Well done, muggle! You are free
now."
 Programming using Python

f(unctions)
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 Parts of a function

Input

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Programming, Functions
 def max (a, b):
‘’’return maximum among a and b’’’
keyword if (a > b):
return a 2 arguments
else: a and b
Function Name return b (formal args)

x = max(6, 4) Body of thefunction,

indented w.r.t the
def keyword

Call to the function. Documentation comment

Actual args are 6 and 4. (docstring), type
help <function-name>
on prompt to get help for the function
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def max (a, b):
‘‘‘return maximum among a and b’’’
if (a > b):
return a
else:
return b

In[3] : help(max)
Help on function max in module __main__:

max(a, b)
return maximum among a and b
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 Keyword Arguments
Note use of [0]
def printName(first, last, initials) : to get the first
if initials: character of a
string. More on
print (first[0] + '. ' + last[0] + '.') this later.
else:
print (first, last)
Call Output
printName('Acads', 'Institute', False) Acads Institute
printName('Acads', 'Institute', True) A. I.
printName(last='Institute', initials=False, first='Acads') Acads Institute
printName('Acads', initials=True, last='Institute') A. I.

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 Keyword Arguments
• Parameter passing where formal is bound to
actual using formal's name
• Can mix keyword and non-keyword arguments
– All non-keyword arguments precede keyword
arguments in the call
– Non-keyword arguments are matched by position
(order is important)
– Order of keyword arguments is not important

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 Default Values
def printName(first, last, initials=False) :
if initials:
print (first[0] + '. ' + last[0] + '.')
Note the use
else: of “default”
value
print (first, last)
Call Output
printName('Acads', 'Institute') Acads Institute
printName(first='Acads', last='Institute', initials=True) A. I.
printName(last='Institute', first='Acads') Acads Institute
printName('Acads', last='Institute') Acads Institute

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 Default Values
• Allows user to call a function with fewer
arguments
• Useful when some argument has a fixed value
for most of the calls
• All arguments with default values must be at
the end of argument list
– non-default argument can not follow default
argument

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 Globals
• Globals allow functions to communicate with
each other indirectly
– Without parameter passing/return value
• Convenient when two seemingly “far-apart”
functions want to share data
– No direct caller/callee relation
• If a function has to update a global, it must re-
declare the global variable with global keyword.

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 Globals
PI = 3.14 >>> print(area (100))
def perimeter(r): 31400.0
return 2 * PI * r >>> print(perimeter(10))
def area(r): 62.800000000000004
return PI * r * r >>> update_pi()
def update_pi(): >>> print(area(100))
global PI 31415.999999999996
PI = 3.14159 >>> print(perimeter(10))
62.832
defines PI to be of float type with value
3.14. PI can be used across functions. Any
change to PI in update_pi will be visible to 79
all due to the use of global.
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 Programming with Python

S T R I NGS
T UP L E S
L I S T S
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 Strings
• Strings in Python have type str
• They represent sequence of characters
– Python does not have a type corresponding to
character.
• Strings are enclosed in single quotes(') or double
quotes(“)
– Both are equivalent
• Backslash (\) is used to escape quotes and
special characters
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 Strings

• More readable when print is used

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 Length of a String
• len function gives the length of a string

\n is a single character:
the special character
representing newline 83
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 Concatenate and Repeat
• In Python, + and * operations have special
meaning when operating on strings
• + is used for concatenation of (two) strings
• * is used to repeat a string, an int number of
time
• Function/Operator Overloading

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 Concatenate and Repeat

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 Indexing
• Strings can be indexed
• First character has index 0

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 Indexing
• Negative indices start counting from the right
• Negatives indices start from -1
• -1 means last, -2 second last, ...

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 Indexing
• Using an index that is too large or too small
results in “index out of range” error

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 Slicing
• To obtain a substring
• s[start:end] means substring of s starting at
index start and ending at index end-1
• s[0:len(s)] is same as s
• Both start and end are optional
– If start is omitted, it defaults to 0
– If end is omitted, it defaults to the length of string
• s[:] is same as s[0:len(s)], that is same as s

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 Slicing

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 More Slicing

Understanding Indices for slicing

A c a d s
0 1 2 3 4 5
-5 -4 -3 -2 -1
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 A c a d s

Out of Range Slicing 0

-5
1
-4
2
-3
3
-2
4
-1

• Out of range indices are ignored for slicing

• when start and end have the same sign, if start
>=end, empty slice is returned
Why?

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 Tuples
• A tuple consists of a number of values
separated by commas

• Empty and Singleton Tuples

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 Nested Tuples
• Tuples can be nested

• Note that course tuple is copied into student.

– Changing course does not affect student

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 Length of a Tuple
• len function gives the length of a tuple

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 More Operations on Tuples
• Tuples can be concatenated, repeated,
indexed and sliced

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 Unpacking Sequences
• Strings and Tuples are examples of sequences
– Indexing, slicing, concatenation, repetition
operations applicable on sequences
• Sequence Unpacking operation can be applied
to sequences to get the components
– Multiple assignment statement
– LHS and RHS must have equal length

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 Unpacking Sequences

( )

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 Lists
• Ordered sequence of values
• Written as a sequence of comma-separated
values between square brackets
• Values can be of different types
– usually the items all have the same type

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 Lists
• List is also a sequence type
– Sequence operations are applicable

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 Lists
• List is also a sequence type
– Sequence operations are applicable

Repetition

( )

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 More Operations on Lists
• L.append(x) • L.pop()
• L.extend(seq) • L.index(x)
• L.insert(i, x) • L.count(x)
• L.remove(x) • L.sort()
• L.pop(i) • L.reverse()
x is any value, seq is a sequence value (list, string, tuple, …),
i is an integer value
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 Mutable and Immutable Types
• Tuples and List types look very similar
• However, there is one major difference: Lists
are mutable
– Contents of a list can be modified
• Tuples and Strings are immutable
– Contents can not be modified

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 Summary of Sequences
Operation Meaning
seq[i] i-th element of the sequence
len(seq) Length of the sequence
seq1 + seq2 Concatenate the two sequences
num*seq
seq*num Repeat seq num times

seq[start:end] slice starting from start, and ending at end-1

e in seq True if e is present is seq, False otherwise
e not in seq True if e is not present is seq, False otherwise
for e in seq Iterate over all elements in seq (e is bound to one element per
iteration)

Sequence types include String, Tuple and List.

Lists are mutable, Tuple and Strings immutable. 104
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 Summary of Sequences
• For details and many useful functions, refer to:
https://
docs.python.org/3.x/tutorial/datastructures.htm
l

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 Programming with Python

Sets and Dictionaries

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 Sets
• An unordered collection with no duplicate
elements
• Supports
– membership testing
– eliminating duplicate entries
– Set operations: union, intersection, difference, and
symmetric difference.

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 Sets

{ }

Create a set from

a sequence

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 Set Operations

{ }

{ }

{ }

{ }

{ }

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 Dictionaries
• Unordered set of key:value pairs,
• Keys have to be unique and immutable
• Key:value pairs enclosed inside curly braces
{...}
• Empty dictionary is created by writing {}
• Dictionaries are mutable
– add new key:value pairs,
– change the pairing
– delete a key (and associated value)
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 Operations on Dictionaries
Operation Meaning
len(d) Number of key:value pairs in d
d.keys() List containing the keys in d
d.values() List containing the values in d
k in d True if key k is in d
d[k] Value associated with key k in d
d.get(k, v) If k is present in d, then d[k] else v
d[k] = v Map the value v to key k in d
(replace d[k] if present)
del d[k] Remove key k (and associated value) from d
for k in d Iterate over the keys in d

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 Operations on Dictionaries

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 Operations on Dictionaries

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 Operations on Dictionaries

# Remember: for ... in iterates over keys only

# Sort values in a list

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 Dictionary Construction
• The dict constructor: builds dictionaries
directly from sequences of key-value pairs

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 Programming with Python

File I/O

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 File I/O
• Files are persistent storage
• Allow data to be stored beyond program
lifetime
• The basic operations on files are
– open, close, read, write
• Python treat files as sequence of lines
– sequence operations work for the data read from
files

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 File I/O: open and close
open(filename, mode)
• While opening a file, you need to supply
– The name of the file, including the path
– The mode in which you want to open a file
– Common modes are r (read), w (write), a (append)
• Mode is optional, defaults to r
• open(..) returns a file object
• close() on the file object closes the file
– finishes any buffered operations
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 File I/O: Example

• Do some writing
• How to do it?
• see the next few slides

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 File I/O: read, write and append
• Reading from an open file returns the
contents of the file
– as sequence of lines in the program
• Writing to a file
– IMPORTANT: If opened with mode 'w', clears the
existing contents of the file
– Use append mode ('a') to preserve the contents
– Writing happens at the end

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 File I/O: Examples

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 File I/O: Examples
( )

( )

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 File I/O: Examples
( )

Note empty line due to '\n'

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 File I/O: Examples
Note the use of for ... in
for sequence

( )

]
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 File I/O: Examples

( )

( )

( )
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 Programming using Python

Modules and Packages

126
Welcome Python Programming
 Modules
• As program gets longer, need to organize them for
easier access and easier maintenance.
• Reuse same functions across programs without
copying its definition into each program.
• Python allows putting definitions in a file
– use them in a script or in an interactive instance of the
interpreter
• Such a file is called a module
– definitions from a module can be imported into other
modules or into the main module
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 Modules
• A module is a file containing Python
definitions and statements.
• The file name is the module name with the
suffix .py appended.
• Within a module, the module’s name is
available in the global variable __name__.

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 Modules Example
fib.py - C:\

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 Modules Example

Within a module, the

module’s name is
available as the value of
the global variable
3/28/23 Programming __name__. 130
 Importing Specific Functions
• To import specific functions from a module

• This brings only the imported functions in the current symbol

table
– No need of modulename. (absence of fib. in the example)

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 Importing ALL Functions
• To import all functions from a module, in the current
symbol table

• This imports all names except those beginning with

an underscore (_).
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 __main__ in Modules
• When you run a module on the command line with
python fib.py <arguments>
the code in the module will be executed, just as if
you imported it, but with the __name__ set to
"__main__".
• By adding this code at the end of your module
if __name__ == "__main__":
... # Some code here
you can make the file usable as a script as well as an
importable module
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 __main__ in Modules
if __name__ == "__main__":
import sys
print (fib_iter(int(sys.argv[1])))
• This code parses the command line only if the
module is executed as the “main” file:
$ python fib.py 10
55
• If the module is imported, the code is not run:
>>> import fib
>>>
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 Package
• A Python package is a collection of Python modules.
• Another level of organization.
• Packages are a way of structuring Python’s module
namespace by using dotted module names.
– The module name A.B designates a submodule named B in
a package named A.
– The use of dotted module names saves the authors of
multi-module packages like NumPy or Pillow from having
to worry about each other’s module names.

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 A sound Package

https://docs.python.org/3/tutorial/modules.html
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 A sound Package

What are these files

with funny names?

https://docs.python.org/3/tutorial/modules.html
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 __init.py__
• The __init__.py files are required to make
Python treat directories containing the file as
packages.
• This prevents directories with a common name, such
as string, unintentionally hiding valid modules
that occur later on the module search path.
• __init__.py can just be an empty file
• It can also execute initialization code for the package

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 Importing Modules from Packages

https://docs.python.org/3/tutorial/
modules.html 139
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 Importing Modules from Packages
import sound.effects.echo

• Loads the submodule sound.effects.echo

• It must be referenced with its full name:

sound.effects.echo.echofilter(
input, output,
delay=0.7, atten=4
)
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 Importing Modules from Packages
from sound.effects import echo
• This also loads the submodule echo
• Makes it available without package prefix
• It can be used as:
echo.echofilter(
input, output,
delay=0.7, atten=4
)
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 Importing Modules from Packages
from sound.effects.echo import echofilter

• This loads the submodule echo, but this makes its

function echofilter() directly available.

echofilter(input, output,
delay=0.7, atten=4)

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 Popular Packages
• pandas, numpy, scipy, matplotlib, …
• Provide a lot of useful functions

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