Python_Module_2.pdf

Python Programming 20CSI44
Dept. of ISE, NCET 2021-2022 Page 1
while condition:
statement_1
statement_2
…………….
statement_n
statements_after_while
ITERATION
MODULE – 2
Iteration is a processing repeating some task. In a real time programming, we require a set of
statements to be repeated certain number of times and/or till a condition is met. Every programming
language provides certain constructs to achieve the repetition of tasks. In this section, we will discuss
various such looping structures.
The while Statement
The while loop has the syntax as below –
Here, while is a keyword. The condition is evaluated first. Till its value remains true, the
statement_1 to statement_n will be executed. When the condition becomes false, the loop is
terminated and statements after the loop will be executed. Consider an example –
n=1
while n<=5:
print(n) #observe indentation n=n+1
print("over")
The output of above code segment would be –
1
2
3
4
5
over
In the above example, a variable n is initialized to 1. Then the condition n<=5 is being checked. As
the condition is true, the block of code containing print statement (print(n)) and increment statement
(n=n+1)are executed. After these two lines, condition is checked again. The procedure continues till
condition becomes false, that is when n becomes 6. Now, the while-loop is terminated and next
statement after the loop will be executed. Thus, in this example, the loop is iterated for 5 times.

Note that, a variable n is initialized before starting the loop and it is incremented inside the loop.
Such a variable that changes its value for every iteration and controls the total execution of the loop
is called as iteration variable or counter variable. If the count variable is not updated properly
within the loop, then the loop may not terminate and keeps executing infinitely.
Infinite Loops, break and continue
A loop may execute infinite number of times when the condition is never going to become false. For
example,
n=1
while True:
print(n)
n=n+1
Here, the condition specified for the loop is the constant True, which will never get terminated.
Sometimes, the condition is given such a way that it will never become false and hence by restricting
the program control to go out of the loop. This situation may happen either due to wrong condition
or due to not updating the counter variable.
In some situations, we deliberately want to come out of the loop even before the normal termination
of the loop. For this purpose break statement is used. The following example depicts the usage of
break. Here, the values are taken from keyboard until a negative number is entered. Once the input is
found to be negative, the loop terminates.
while True:
x=int(input("Enter a number:")) if x>= 0:
print("You have entered ",x) else:
print("You have entered a negative number!!")
break #terminates the loop
Sample output:
Enter a number:23 You
have entered 23 Enter a
number:12 You have
entered 12 Enter a
number:45 You have
entered 45 Enter a number:0
You have entered 0 Enter a
number:-2
You have entered a negative number!!
In the above example, we have used the constant True as condition for while-loop, which will never
become false. So, there was a possibility of infinite loop. This has been avoided

by using break statement with a condition. The condition is kept inside the loop such a way that, if
the user input is a negative number, the loop terminates. This indicates that, the loop may terminate
with just one iteration (if user gives negative number for the very first time) or it may take thousands
of iteration (if user keeps on giving only positive numbers as input). Hence, the number of iterations
here is unpredictable. But, we are making sure that it will not be an infinite-loop, instead, the user
has control on the loop.
Sometimes, programmer would like to move to next iteration by skipping few statements in the loop,
based on some condition. For this purpose continue statement is used. For example, we would like
to find the sum of 5 even numbers taken as input from the keyboard. The logic is –
 Read a number from the keyboard
 If that number is odd, without doing anything else, just move to next iteration for reading
another number
 If the number is even, add it to sum and increment the accumulator variable.
 When accumulator crosses 5, stop the programThe
program for the above task can be written as –
sum=0 count=0
while True:
x=int(input("Enter a number:")) if x%2 !=0:
continue
else:
sum+=x
count+=1
if count==5:
break
print("Sum= ", sum)
Sample Output:
Enter a number:13 Enter
a number:12 Enter a
number:4 Enter a
number:5 Enter a
number:-3 Enter a
number:8 Enter a
number:7 Enter a
number:16 Enter a
number:6 Sum= 46

for var in list/sequence:
statement_1
statement_2
………………
statement_n
statements_after_for
Definite Loops using for
The while loop iterates till the condition is met and hence, the number of iterations are usually
unknown prior to the loop. Hence, it is sometimes called as indefinite loop. When we know total
number of times the set of statements to be executed, for loop will be used. This is called as a
definite loop. The for-loop iterates over a set of numbers, a set of words, lines in a file etc. The
syntax of for-loop would be–
Here, for and in are keywords
list/sequence is a set of elements on which the loop is iterated. That is, the loop
will be executed till there is an element in list/sequence
statements constitutes body of the loop
Ex: In the below given example, a list names containing three strings has been created. Then the
counter variable x in the for-loop iterates over this list. The variable x takes the elements in names
one by one and the body of the loop is executed.
names=["Ram", "Shyam", "Bheem"] for x in
names:
print(x)
The output would be –
Ram
Shyam
Bheem
NOTE: In Python, list is an important data type. It can take a sequence of elements of different
types. It can take values as a comma separated sequence enclosed within square brackets. Elements
in the list can be extracted using index (just similar to extracting array elements in C/C++ language).
Various operations like indexing, slicing, merging, addition and deletion of elements etc. can be
applied on lists. The details discussion on Lists will be done in Module 3.
The for loop can be used to print (or extract) all the characters in a string as shown below –
for i in "Hello": print(i, end=‟t‟)
Output:
H e l l o

When we have a fixed set of numbers to iterate in a for loop, we can use a function
range(). The function range() takes the following format –
range(start, end, steps)
The start and end indicates starting and ending values in the sequence, where end is excluded in the
sequence (That is, sequence is up to end-1). The default value of start is 0. The argument steps
indicates the increment/decrement in the values of sequence with the default value as 1. Hence, the
argument steps is optional. Let us consider few examples on usage of range() function.
Ex1. Printing the values from 0 to 4 –
for i in range(5): print(i, end= „t‟)
Output:
0 1 2 3 4
Here, 0 is the default starting value. The statement range(5)is same as range(0,5)
and range(0,5,1).
Ex2. Printing the values from 5 to 1 –
for i in range(5,0,-1): print(i, end=
„t‟)
Output:
5 4 3 2 1
The function range(5,0,-1)indicates that the sequence of values are 5 to 0(excluded) in steps of -1
(downwards).
Ex3. Printing only even numbers less than 10 –
for i in range(0,10,2): print(i,
end= „t‟)
Output:
0 2 4 6 8
Loop Patterns
The while-loop and for-loop are usually used to go through a list of items or the contents of a file and
to check maximum or minimum data value. These loops are generally constructed by the following
procedure –
 Initializing one or more variables before the loop starts
 Performing some computation on each item in the loop body, possibly changing the variables
in the body of the loop
 Looking at the resulting variables when the loop completes
The construction of these loop patterns are demonstrated in the following examples.

Counting and Summing Loops: One can use the for loop for counting number of items in the list as
shown –
count = 0
for i in [4, -2, 41, 34, 25]:
count = count + 1
print(“Count:”, count)
Here, the variable count is initialized before the loop. Though the counter variable iis not being used
inside the body of the loop, it controls the number of iterations. The variable count is incremented in
every iteration, and at the end of the loop the total number of elements in the list is stored in it.
One more loop similar to the above is finding the sum of elements in the list –
total = 0
for x in [4, -2, 41, 34, 25]:
total = total + x
print(“Total:”, total)
Here, the variable total is called as accumulator because in every iteration, it accumulates the sum of
elements. In each iteration, this variable contains running total of values so far.
NOTE: In practice, both of the counting and summing loops are not necessary, because there are
built-in functions len()and sum()for the same tasks respectively.
Maximum and Minimum Loops: To find maximum element in the list, the following code can be
used –
big = None
print('Before Loop:', big) for x in [12,
0, 21,-3]:
if big is None or x > big : big = x
print('Iteration Variable:', x, 'Big:', big) print('Biggest:', big)
Output:
Before Loop: None
Iteration Variable: 12 Big: 12
Iteration Variable: -3 Big: 21
Biggest: 21

Here, we initialize the variable big to None. It is a special constant indicating empty. Hence, we
cannot use relational operator == while comparing it with big. Instead, the is operator must be used.
In every iteration, the counter variable x is compared with previous value of big. If x > big, then xis
assigned to big.
Similarly, one can have a loop for finding smallest of elements in the list as given below –
small = None
print('Before Loop:', small) for x in [12,
0, 21,-3]:
if small is None or x < small : small = x
print('Iteration Variable:', x, 'Small:', small) print('Smallest:', small)
Output:
Before Loop: None
Iteration Variable: 12 Small: 12
Iteration Variable: -3 Small: -3
Smallest: -3
NOTE: In Python, there are built-in functions max() and min()to compute maximum and minimum
values among. Hence, the above two loops need not be written by the programmer explicitly. The
inbuilt function min()has the following code in Python –
def min(values):
smallest = None
for value in values:
if smallest is None or value < smallest: smallest = value
return smallest
STRINGS
A string is a sequence of characters, enclosed either within a pair of single quotes or double quotes.
Each character of a string corresponds to an index number, starting with zero as shown below –
S= “Hello World”
character H e l l o w o r l d
index 0 1 2 3 4 5 6 7 8 9 10
The characters of a string can be accessed using index enclosed within square brackets. For
example,

>>> word1="Hello"
>>> word2='hi'
>>> x=word1[1] #2nd
character of word1 is extracted
>>> print(x) e
>>> y=word2[0] #1st
character of word1 is extracted
>>> print(y) h
Python supports negative indexing of string starting from the end of the string as shown below –
S= “Hello World”
character H e l l o w o r l D
Negative index -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1
The characters can be extracted using negative index also. For example,
>>> var=“Hello”
>>> print(var[-1]) o
>>> print(var[-4]) e
Whenever the string is too big to remember last positive index, one can use negative index to extract
characters at the end of string.
Getting Length of a String using len()
The len() function can be used to get length of a string.
>>> var="Hello"
>>> ln=len(var)
>>> print(ln)
5
The index for string varies from 0 to length-1. Trying to use the index value beyond this range
generates error.
>>> var="Hello"
>>> ln=len(var)
>>> ch=var[ln]
IndexError: string index out of range
Traversal through String with a Loop
Extracting every character of a string one at a time and then performing some action on that
character is known as traversal. A string can be traversed either using while loop or using for loop in
different ways. Few of such methods is shown here –

 Using for loop:
st="Hello" for i in
st:
print(i, end='t')
Output:
H e l l o
In the above example, the for loop is iterated from first to last character of the string st. That is, in
every iteration, the counter variable i takes the values as H, e, l, l and o. The loop terminates when
no character is left in st.
 Using while loop:
st="Hello" i=0
while i<len(st):
print(st[i], end=„t‟) i+=1
Output:
H e l l o
In this example, the variable i is initialized to 0 and it is iterated till the length of the string. In
every iteration, the value of i is incremented by 1 and the character in a string is extracted using i
as index.
String Slices
A segment or a portion of a string is called as slice. Only a required number of characters can be
extracted from a string using colon (:) symbol. The basic syntax for slicing a string would be –
st[i:j:k]
This will extract character from ith
character of st till (j-1)th
character in steps of k. If first index iis
not present, it means that slice should start from the beginning of the string. If the second index j is
not mentioned, it indicates the slice should be till the end of the string. The third parameter k, also
known as stride, is used to indicate number of steps to be incremented after extracting first character.
The default value of stride is 1.
Consider following examples along with their outputs to understand stringslicing.
st="Hello World" #refer this string for all examples
1. print("st[:] is", st[:]) #output Hello World
As both index values are not given, it assumed to be a full string.
2. print("st[0:5] is ", st[0:5]) #output is Hello
Starting from 0th
index to 4th
index (5 is exclusive), characters will be printed.

3. print("st[0:5:1] is", st[0:5:1]) #output is Hello
This code also prints characters from 0th to 4th index in the steps of 1. Comparing this
example with previous example, we can make out that when the stride value is 1, it is
optional to mention.
4. print("st[3:8] is ", st[3:8]) #output is lo Wo
Starting from 3rd
index to 7th
index (8 is exclusive), characters will be printed.
5. print("st[7:] is ", st[7:]) #output is orld
Starting from 7th
index to till the end of string, characters will be printed.
6. print(st[::2]) #outputs HloWrd
This example uses stride value as 2. So, starting from first character, every alternative
character (char+2) will be printed.
7. print("st[4:4] is ", st[4:4]) #gives empty string
Here, st[4:4] indicates, slicing should start from 4th
character and end with (4- 1)=3rd
character, which is not possible. Hence the output would be an empty string.
8. print(st[3:8:2]) #output is l o
Starting from 3rd character, till 7th character, every alternative index is considered.
9. print(st[1:8:3]) #output is eoo
Starting from index 1, till 7th
index, every 3rd
character is extracted here.
10. print(st[-4:-1]) #output is orl
Refer the diagram of negative indexing given earlier. Excluding the -1st character, all
characters at the indices -4, -3 and -2 will be displayed. Observe the role of stride with
default value 1 here. That is, it is computed as -4+1 =-3, -3+1=-2 etc.
11. print(st[-1:]) #output is d
Here, starting index is -1, ending index is not mentioned (means, it takes the index
10) and the stride is default value 1. So, we are trying to print characters from -1 (which is
the last character of negative indexing) till 10th
character (which is also the last character in
positive indexing) in incremental order of 1. Hence, we will get only last character as output.
12. print(st[:-1]) #output is Hello Worl
Here, starting index is default value 0 and ending is -1 (corresponds to last character in
negative indexing). But, in slicing, as last index is excluded always, -1st
character is omitted
and considered only up to -2nd
character.
13. print(st[::]) #outputs Hello World
Here, two colons have used as if stride will be present. But, as we haven‟t mentioned stride
its default value 1 is assumed. Hence this will be a full string.

14. print(st[::-1]) #outputs dlroW olleH
This example shows the power of slicing in Python. Just with proper slicing, we could able
to reverse the string. Here, the meaning is a full string to be extracted in the order of -1.
Hence, the string is printed in the reverse order.
15. print(st[::-2]) #output is drWolH
Here, the string is printed in the reverse order in steps of -2. That is, every alternative
character in the reverse order is printed. Compare this with example (6) given above.
By the above set of examples, one can understand the power of string slicing and of Python script.
The slicing is a powerful tool of Python which makes many task simple pertaining to data types like
strings, Lists, Tuple, Dictionary etc. (Other types will be discussed in later Modules)
Strings are Immutable
The objects of string class are immutable. That is, once the strings are created (or initialized), they
cannot be modified. No character in the string can be edited/deleted/added. Instead, one can create a
new string using an existing string by imposing any modification required.
Try to attempt following assignment –
>>> st= “Hello World”
>>> st[3]='t'
TypeError: 'str' object does not support item assignment
Here, we are trying to change the 4th
character (index 3 means, 4th
character as the first index is 0) to
t. The error message clearly states that an assignment of new item (a string) is not possible on string
object. So, to achieve our requirement, we can create a new string using slices of existing string as
below –
>>> st= “Hello World”
>>> st1= st[:3]+ 't' + st[4:]
>>> print(st1)
Helto World #l is replaced by t in new string st1
Looping and Counting
Using loops on strings, we can count the frequency of occurrence of a character within another
string. The following program demonstrates such a pattern on computation called as a counter.
Initially, we accept one string and one character (single letter). Our aim to find the total number of
times the character has appeared in string. A variable count is initialized to zero, and incremented
each time a character is found. The program is given below –
def countChar(st,ch): count=0
for i in st:

if i==ch:
count+=1
return count
st=input("Enter a string:")
ch=input("Enter a character to be counted:") c=countChar(st,ch)
print("{0} appeared {1} times in {2}".format(ch,c,st))
Sample Output:
Enter a string: hello how are you?
Enter a character to be counted: h
h appeared 2 times in hello how are you?
The in Operator
The in operator of Python is a Boolean operator which takes two string operands. It returns True, if
the first operand appears in second operand, otherwise returns False. For example,
>>> 'el' in 'hello' #el is found in hello True
>>> 'x' in 'hello' #x is not found in hello False
String Comparison
Basic comparison operators like < (less than), > (greater than), == (equals) etc. can be applied on
string objects. Such comparison results in a Boolean value True or False. Internally, such
comparison happens using ASCII codes of respective characters. Consider following examples –
Ex1. st= “hello”
if st== „hello‟:
print(„same‟)
Output is same. As the value contained in st and hello both are same, the equality results in True.
Ex2. st= “hello”
if st<= „Hello‟:
print(„lesser‟)
else:
print(„greater‟)
Output is greater. The ASCII value of his greater than ASCII value of H. Hence, hello
is greater than Hello.

NOTE: A programmer must know ASCII values of some of the basic characters. Here are few –
A – Z : 65 – 90
a – z : 97 – 122
0 – 9 : 48 – 57
Space 32
Enter Key 13
String Methods
String is basically a class in Python. When we create a string in our program, an object of that class
will be created. A class is a collection of member variables and member methods (or functions).
When we create an object of a particular class, the object can use all the members (both variables
and methods) of that class. Python provides a rich set of built-in classes for various purposes. Each
class is enriched with a useful set of utility functions and variables that can be used by a
Programmer. A programmer can create a class based on his/her requirement, which are known as
user-defined classes.
The built-in set of members of any class can be accessed using the dot operator as shown–
objName.memberMethod(arguments)
The dot operator always binds the member name with the respective object name. This is very
essential because, there is a chance that more than one class has members with same name. To avoid
that conflict, almost all Object oriented languages have been designed with this common syntax of
using dot operator. (Detailed discussion on classes and objects will be done in later Modules.)
Python provides a function (or method) dir to list all the variables and methods of a particular class
object. Observe the following statements –
>>> s="hello" #string object is created with the name s
>>> type(s) #checking type of s
<class „str‟> #s is object of type class str
>>> dir(s) #display all methods and variables of object s
[' add ', ' class ', ' contains ', ' delattr ', ' dir ', ' doc ', ' eq ', ' format ', ' ge ', ' getattribute ',
' getitem ', ' getnewargs ', ' gt ', ' hash ', ' init ', ' init_subclass ', ' iter ', ' le ', ' len ', ' lt ',
' mod ', ' mul ', ' ne ', ' new ', ' reduce ', ' reduce_ex ', ' repr ', ' rmod ', ' rmul ', ' setattr
', ' sizeof ', ' str ', ' subclasshook ', 'capitalize', 'casefold', 'center', 'count', 'encode', 'endswith',
'expandtabs', 'find', 'format', 'format_map', 'index', 'isalnum', 'isalpha', 'isdecimal', 'isdigit',
'isidentifier', 'islower', 'isnumeric',

'isprintable', 'isspace', 'istitle', 'isupper', 'join', 'ljust',
'lower', 'lstrip', 'maketrans', 'partition', 'replace', 'rfind',
'rindex', 'rjust', 'rpartition', 'rsplit', 'rstrip', 'split', 'splitlines', 'startswith', 'strip', 'swapcase', 'title',
'translate', 'upper', 'zfill']
Note that, the above set of variables and methods are common for any object of string class that we
create. Each built-in method has a predefined set of arguments and return type. To know the usage,
working and behavior of any built-in method, one can use the command help. For example, if we
would like to know what is the purpose of islower() function (refer above list to check its
existence!!), how it behaves etc, we can use the statement –
>>> help(str.islower)
Help on method_descriptor:
islower(...)
S.islower() -> bool
Return True if all cased characters in S are lowercase and
there is at least one cased character in S, False otherwise.
This is built-in help-service provided by Python. Observe the className.memberName
format while using help.
The methods are usually called using the object name. This is known as method invocation. We say
that a method is invoked using an object.
Now, we will discuss some of the important methods of string class.
 capitalize(s) : This function takes one string argument s and returns a capitalized version of that
string. That is, the first character of s is converted to upper case, and all other characters to
lowercase. Observe the examples given below –
Ex1. >>> s="hello"
>>> s1=str.capitalize(s)
>>> print(s1)
Hello #1st
character is changed to uppercase
Ex2. >>> s="hello World"
>>> s1=str.capitalize(s)
>>> print(s1)
Hello world
Observe in Ex2 that the first character is converted to uppercase, and an in-between uppercase
letter W of the original string is converted to lowercase.

 s.upper(): This function returns a copy of a string s to uppercase. As strings are immutable, the
original string s will remain same.
>>> st= “hello”
>>> st1=st.upper()
>>> print(st1)
'HELLO'
>>> print( st) #no change in original string 'hello'
 s.lower(): This method is used to convert a string s to lowercase. It returns a copy of original
string after conversion, and original string is intact.
>>> st='HELLO'
>>> st1=st.lower()
>>> print(st1) hello
>>> print(st) #no change in original string HELLO
 s.find(s1) : The find() function is used to search for a substring s1 in the string s. If found, the
index position of first occurrence of s1 in s, is returned. If s1 is not found in s, then -1 is returned.
>>> st='hello'
>>> i=st.find('l')
>>> print(i) #output is 2
>>> i=st.find('lo')
>>> print(st.find(„x‟)) #output is -1
The find() function can take one more form with two additional arguments viz. start and end
positions for search.
>>> st="calender of Feb. cal of march"
>>> i= st.find(„cal‟)
Here, the substring „cal‟is found in the very first position of st, hence the result is 0.
>>> i=st.find('cal',10,20)
Here, the substring cal is searched in the string st between 10th
and 20th
position and hence the
result is 17.

>>> i=st.find('cal',10,15)
>>> print(i) #ouput is -1
In this example, the substring 'cal' has not appeared between 10th
and
15th
character of st. Hence, the result is -1.
 s.strip(): Returns a copy of string s by removing leading and trailing white spaces.
>>> st=" hello world "
>>> st1 = st.strip()
>>> print(st1)
hello world
The strip() function can be used with an argument chars, so that specified chars are removed from
beginning or ending of s as shown below –
>>> st="###Hello##"
>>> st1=st.strip('#')
>>> print(st1) #all hash symbols are removedHello
We can give more than one character for removal as shown below –
>>> st="Hello world"
>>> st.strip("Hld") ello wor
 S.startswith(prefix, start, end): This function has 3 arguments of which start and end are
option. This function returns True if S starts with the specified prefix, False otherwise.
>>> st="hello world"
>>> st.startswith("he") #returns True
When start argument is provided, the search begins from that position and returns True or False
based on search result.
>>> st="hello world"
>>> st.startswith("w",6) #True because w is at 6th position
When both start and end arguments are given, search begins at start and ends atend.
>>> st="xyz abc pqr ab mn gh“
>>> st.startswith("pqr ab mn",8,12) #returns False
>>> st.startswith("pqr ab mn",8,18) #returns True

The startswith() function requires case of the alphabet to match. So, when we are not sure about
the case of the argument, we can convert it to either upper case or lowercase and then use
startswith()function as below –
>>> st="Hello"
>>> st.startswith("he") #returns False
>>> st.lower().startswith("he") #returns True
 S.count(s1, start, end): The count() function takes three arguments – string, starting position
and ending position. This function returns the number of non-overlapping occurrences of
substring s1 in string S in the range of start and end.
>>> st="hello how are you? how about you?"
>>> st.count('h') #output is 3
>>> st.count(„how‟) #output is 2
>>> st.count(„how‟,3,10) #output is 1 because of range given
There are many more built-in methods for string class. Students are advised to explore more
for further study.
Format Operator
The format operator, % allows us to construct strings, replacing parts of the strings with the data stored in
variables. The first operand is the format string, which contains one or more format sequences that specify how
the second operand is formatted. The result is a string.
>>> sum=20
>>> '%d' %sum „20‟ #string „20‟, but not integer 20
Note that, when applied on both integer operands, the % symbol acts as a modulus operator. When the first
operand is a string, then it is a format operator. Consider few examples illustrating usage of format operator.
Ex1. >>> "The sum value %d is originally integer"%sum
Output : 'The sum value 20 is originally integer„
Ex2. >>> '%d %f %s'%(3,0.5,'hello')
Output : '3 0.500000 hello„
Ex3. >>> '%d %g %s'%(3,0.5,'hello')
Output : '3 0.5 hello„

Ex4. >>> '%d'% 'hello'
TypeError: %d format: a number is required, not str
Ex5. >>> '%d %d %d'%(2,5)
TypeError: not enough arguments for format string

FILE :
File handling is an important requirement of any programming language, as it allows us to store the
data permanently on the secondary storage and read the data from a permanent source. Here, we will
discuss how to perform various operations on files using the programming language Python.
Persistence
The programs that we have considered till now are based on console I/O. That is, the input was taken
from the keyboard and output was displayed onto the monitor. When the data to be read from the
keyboard is very large, console input becomes a laborious job. Also, the output or result of the
program has to be used for some other purpose later, it has to be stored permanently. Hence,
reading/writing from/to files are very essential requirement of programming.
We know that the programs stored in the hard disk are brought into main memory to execute them.
These programs generally communicate with CPU using conditional execution, iteration, functions
etc. But, the content of main memory will be erased when we turn-off our computer. We have
discussed these concepts in Module1 with the help of Figure 1.1. Here we will discuss about
working with secondary memory or files. The files stored on the secondary memory are permanent
and can be transferred to other machines using pen-drives/CD.
Opening Files
To perform any operation on a file, one must open a file. File opening involves communication with
operating system. In Python, a file can be opened using a built-in function open(). While opening a
file, we must specify the name of the file to be opened. Also, we must inform the OS about the
purpose of opening a file, which is termed as file opening mode. The syntax of open() function is as
below –
fhand= open(“filename”, “mode”)
Here, filename is name of the file to be opened. This string may be just a name of the file, or it may
include pathname also. Pathname of the file is optional when the file is stored
in current working directory
mode This string indicates the purpose of opening a file. It takes a predefined set of
values as given in Table 2.1
fhand It is a reference to an object of file class, which acts as a handler or tool for all
further operations on files.
When our Python program makes a request to open a specific file in a particular mode, then OS will
try to serve the request. When a file gets opened successfully, then a file object is returned. This is
known as file handle and is as shown in Figure 2.1. It will help to perform various operations on a
file through our program. If the file cannot be opened due to some reason, then error message
(traceback) will be displayed.

Figure 2.1 A File Handle
A file opening may cause an error due to some of the reasons as listed below –
o File may not exist in the specified path (when we try to read a file)
o File may exist, but we may not have a permission to read/write a file
o File might have got corrupted and may not be in an opening state
Since, there is no guarantee about getting a file handle from OS when we try to open a file, it is always
better to write the code for file opening using try-except block. This will help us to manage error
situation.
Mode Meaning
r Opens a file for reading purpose. If the specified file does not exist in the
specified path, or if you don‟t have permission, error message will be displayed.
This is the default mode of open() function in Python.
w Opens a file for writing purpose. If the file does not exist, then a new file with the
given name will be created and opened for writing. If the file
already exists, then its content will be over-written.
a Opens a file for appending the data. If the file exists, the new content will
be appended at the end of existing content. If no such file exists, it will be created
and new content will be written into it.
r+ Opens a file for reading and writing.
w+ Opens a file for both writing and reading. Overwrites the existing file if the file
exists. If the file does not exist, creates a new file for reading and
writing.
a+ Opens a file for both appending and reading. The file pointer is at the end of the file
if the file exists. The file opens in the append mode. If the file does not exist, it
creates a new file for reading and
writing.
rb Opens a file for reading only in binary format
wb Opens a file for writing only in binary format
ab Opens a file for appending only in binary format

Text Files and Lines
A text file is a file containing a sequence of lines. It contains only the plain text without any images,
tables etc. Different lines of a text file are separated by a newline character n. In the text files, this
newline character may be invisible, but helps in identifying every line in the file. There will be one
more special entry at the end to indicate end of file (EOF).
NOTE: There is one more type of file called binary file, which contains the data in the form of bits.
These files are capable of storing text, image, video, audio etc. All these data will be stored in the form
of a group of bytes whose formatting will be known. The supporting program can interpret these files
properly, whereas when opened using normal text editor, they look like messy, unreadable set of
characters
Reading Files
When we successfully open a file to read the data from it, the open() function returns the file handle
(or an object reference to file object) which will be pointing to the first character in the file. A text
file containing lines can be iterated using a for-loop starting from the beginning with the help of this
file handle. Consider the following example of counting number of lines in a file.
NOTE: Before executing the below given program, create a text file (using Notepad or similar
editor) myfile.txt in the current working directory (The directory where you are going store your
Python program). Open this text file and add few random lines to it and then close. Now, open a
Python script file, say countLines.py and save it in the same directory as that of your text file
myfile.txt. Then, type the following code in Python script countLines.py and execute the program.
(You can store text file and Python script file in different directories. But, if you do so, you have to
mention complete path of text file in the open() function.)
Sample Text file myfile.txt:
hello how are you? I
am doing fine what
about you?
Python script file countLines.py
fhand=open('myfile.txt','r') count =0
for line in fhand: count+=1
print("Line Number ",count, ":", line)
print("Total lines=",count) fhand.close()
Output:
Line Number 1 : hello how are you? Line Number
2 : I am doing fine
Line Number 3 : what about you? Total lines=
3
In the above program, initially, we will try to open the file 'myfile.txt. As we have already created
that file, the file handler will be returned and the object reference to this file will be stored in fhand.

Then, in the for-loop, we are using fhand as if it is a sequence of lines. For each line in the file, we
are counting it and printing the line. In fact, a line is identified internally with the help of new-line
character present at the end of each line. Though we have not typed n anywhere in the file
myfile.txt, after each line, we would have pressed enter-key. This act will insert a n, which is
invisible when we view the file through notepad. Once all lines are over, fhandwill reach end-of-file
and hence terminates the loop. Note that, when end of file is reached (that is, no more characters are
present in the file), then an attempt to read will return Noneor empty character „‟(two quotes without
space in between).
Once the operations on a file is completed, it is a practice to close the file using a function close().
Closing of a file ensures that no unwanted operations are done on a file handler. Moreover, when a
file was opened for writing or appending, closure of a file ensures that the last bit of data has been
uploaded properly into a file and the end-of-file is maintained properly. If the file handler variable
(in the above example, fhand) is used to assign some other file object (using open() function), then
Python closes the previous file automatically.
If you run the above program and check the output, there will be a gap of two lines between each of
the output lines. This is because, the new-line character n is also a part of the variable line in the
loop, and the print() function has default behavior of adding a line at the end (due to default setting
of end parameter of print()). To avoid this double-line spacing, we can remove the new-line
character attached at the end of variable line by using built-in string function rstrip() as below –
print("Line Number ",count, ":", line.rstrip())
It is obvious from the logic of above program that from a file, each line is read one at a time,
processed and discarded. Hence, there will not be a shortage of main memory even though we are
reading a very large file. But, when we are sure that the size of our file is quite small, then we can
use read() function to read the file contents. This function will read entire file content as a single
string. Then, required operations can be done on this string using built-in string functions. Consider
the below given example –
fhand=open('myfile.txt') s=fhand.read()
print(“Total number of characters:”,len(s)) print(“String up to 20
characters:”, s[:20])
After executing above program using previously created file myfile.txt, then the output would be –
Total number of characters:50
String up to 20 characters: hello how are you? I
Writing Files
To write a data into a file, we need to use the mode w in open() function.
>>> fhand=open(“mynewfile.txt","w")
>>> print(fhand)
<_io.TextIOWrapper name='mynewfile.txt' mode='w' encoding='cp1252'>

If the file specified already exists, then the old contents will be erased and it will be ready to write
new data into it. If the file does not exists, then a new file with the given name will be created.
The write() method is used to write data into a file. This method returns number of characters
successfully written into a file. For example,
>>> s="hello how are you?"
>>> fhand.write(s) 18
Now, the file object keeps track of its position in a file. Hence, if we write one more line into the file,
it will be added at the end of previous line. Here is a complete program to write few lines into a file –
fhand=open('f1.txt','w')foriin
range(5):
line=input("Enter a line: ")
fhand.write(line+"n")
fhand.close()
The above program will ask the user to enter 5 lines in a loop. After every line has been entered, it
will be written into a file. Note that, as write() method doesn‟t add a new-line character by its own,
we need to write it explicitly at the end of every line. Once the loop gets over, the program
terminates. Now, we need to check the file f1.txt on the disk (in the same directory where the above
Python code is stored) to find our input lines that have been written into it.
Searching through a File
Most of the times, we would like to read a file to search for some specific data within it. This can be
achieved by using some string methods while reading a file. For example, we may be interested in
printing only the line which starts with a character h. Then we can use startswith() method.
fhand=open('myfile.txt')forlinein
fhand:
if line.startswith('h'): print(line)
fhand.close()
Assume the input file myfile.txt is containing the following lines –
hello how are you?
I am doing fine
how about you?
Now, if we run the above program, we will get the lines which starts with h – hello
how are you?
how about you?

Letting the User Choose the File Name
In a real time programming, it is always better to ask the user to enter a name of the file which
he/she would like to open, instead of hard-coding the name of a file inside the program.
fname=input("Enter a file name:")
fhand=open(fname)
count =0
In this program, the user input filename is received through variable fname, and the same has been
used as an argument to open() method. Now, if the user input is myfile.txt (discussed before), then
the result would be
Total lines=3
Everything goes well, if the user gives a proper file name as input. But, what if the input filename
cannot be opened (Due to some reason like – file doesn‟t exists, file permission denied etc)?
Obviously, Python throws an error. The programmer need to handle such run- time errors as
discussed in the next section.
Using try, except to Open a File
It is always a good programming practice to write the commands related to file opening within a try
block. Because, when a filename is a user input, it is prone to errors. Hence, one should handle it
carefully. The following program illustrates this –
fname=input("Enter a file name:") try:
fhand=open(fname) except:
print("File cannot be opened") exit()
count =0
In the above program, the command to open a file is kept within try block. If the specified file cannot
be opened due to any reason, then an error message is displayed saying File cannot be opened, and
the program is terminated. If the file could able to open successfully, then we will proceed further to
perform required task using that file.
Debugging
While performing operations on files, we may need to extract required set of lines or words or

characters. For that purpose, we may use string functions with appropriate delimiters that may exist
between the words/lines of a file. But, usually, the invisible characters like white-space, tabs and
new-line characters are confusing and it is hard to identify them properly. For example,
>>> s="1 2t 3n 4"
>>> print(s) 1 2
3
4
Here, by looking at the output, it may be difficult to make out where there is a space, where is a tab
etc. Python provides a utility function called as repr() to solve this problem. This method takes any
object as an argument and returns a string representation of that object. For example, the print() in
the above code snippet can be modified as –
>>> print(repr(s)) '1 2t
3n 4'
Note that, some of the systems use n as new-line character, and few others may use r (carriage
return) as a new-line character. The repr() method helps in identifying that too.
Courtesy : Chethana Hegde.

Python_Module_2.pdf

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