File handling in Python

MODULE 2 – PART 3
FILES
By,
Ravi Kumar B N
Assistant professor, Dept. of CSE
BMSIT & M

INTRODUCTION
✓ Fileisa namedlocationonthe systemstoragewhichrecordsdata forlateraccess.
It enablespersistentstorage ina non-volatilememoryi.e.Harddisk.
✓ It is requiredtoworkwithfilesforeitherwritingtoa fileorreaddatafromit.It is
essentialtostorethe filespermanentlyin secondarystorage.
✓ In python,fileprocessingtakesplaceinthe followingorder.
• Opena filethatreturnsa filehandle.
• Usethe handletoperformreadorwrite action.
• Close thefilehandle.

TYPES OF FILES & OPERATIONS ON FILES
Typesof Files
1. TextFiles- All docfiles/excelfilesetc
2. BinaryFiles- Audiofiles,VideoFiles,imagesetc
OperationsonFile:
▪ Open
▪ Close
▪ Read
▪ Write

✓ Toreadorwritetoa file,you needtoopenitfirst.Toopena fileinPython,use itsbuilt-in
open() function.Thisfunctionreturnsa fileobject,i.e.,a handle.Youcan use ittoread
ormodifythe file.
✓open() filemethod:
file_object= open(“file_name”,” access_mode”)
file_object–Filehandlerthatpointstotheparticularlocationasa referencetoan object
file_name-Name ofthe file
access_mode-Read/write/appendmode. Bydefault,it isset toread-only<r>.
Ex: file1= open("app.log","w")
OPEN A FILE IN PYTHON

✓ Python stores a file in the form of bytes on the disk, so you need to decode them in
strings before reading. And, similarly, encode them while writing texts to the file. This is
done automaticallybyPythonInterpreter.
✓ If the open is successful, the operating system returns us a file handle. The file handle is
not the actual data contained in the file, but instead it is a “handle” that we can use to
read the data. You are given a handle if the requested file exists and you have the
properpermissionstoreadthe file.

✓ If the ﬁle does not exist, open will fail with a traceback and you will not get a
handletoaccessthecontents
✓ Afileopening maycauseanerrorduetosomeofthereasonsaslistedbelow
▪ Filemaynotexistinthe specifiedpath(whenwe trytoreada file)
▪ Filemayexist,butwe may nothave a permissiontoread/writea file
▪ Filemight have got corruptedandmay notbe inanopeningstate

Modes Description
r Opens a file only for reading
rb Opens a file only for reading but in a binary format
w Opens a file only for writing; overwrites the file if the file exists
wb Opens a file only for writing but in a binary format
a Opens a file for appending. It does not overwrite the file, just adds the data in the
file, and if file is not created, then it creates a new file
ab Opens a file for appending in a binary format
r+ Opens a file only 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.
FILE MODES

TEXT FILES & LINES
A text file can be thought of as a sequence oflines, much like a Python stringcan be thought of
as a sequence of characters. For example, this is a sample of a text file which records mail
activityfromvariousindividualsin anopensourceprojectdevelopmentteam:
Tobreakthe fileintolines,thereisa specialcharacterthatrepresentsthe “endofthe line”called
the newlinecharacter.In the textfileeachlinecan beseparatedusingescapecharactern
>>> stuff = 'XnY’
>>> print(stuff)
X
Y

READING THE 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
characterinthe file.
Therearedifferentwaysinwhichwe canreadthe filesinpython.
➢ read() //readsallthecontent
➢ read(n) //readsonlythefirstn characters
➢ readline() //readssingleline
➢ readlines() //readsalllines
Where, n isthe numberof bytestobe read
Note: If the file is too large to fit in main memory, you should write your program to read
the fileinchunks usinga fororwhileloop.

SAMPLE PROGRAMS
1.txt
Coronavirus Can Be Stopped Only by Harsh
Steps
Stay at home
wear a facemask
Clean your hands often
Monitor your symptoms
readexample.py
f1=open("1.txt",”r”)
# to read first n bytes
print("---first four
characters---")
print(f1.read(4))
# to read a first line
print("---first line---")
print(f1.readline())
# to read entire file
print("---Entire File---")
print(f1.read())
Output
---first four characters---
Coro
---first line---
navirus Can Be Stopped Only by
Harsh Steps
---Entire File---
Stay at home
wear a facemask
Clean your hands often
Monitor your symptoms
countlines.py
f1 = open(“1.txt”,”r”)
count = 0
for line in f1:
count = count + 1
print('Line Count:', count)
Output
Line Count: 5
Note: When the file is read using a for loop in this manner,
Python takes care of splitting the data in the file into
separate lines using the newline character.
Readlineexample.py
f1=open("1.txt") #by default read mode
#to read line wise
Output
Coronavirus Can Be Stopped Only by Harsh Steps
Stay at home
Countchars.py
#finds the length of the file
f1 = open('1.txt')
ch = f1.read()
print(len(ch))
Output
120
Note:In the above code it counts the
number of characters along with
newline character(n)

WRITING THE FILES:
✓ Towritea dataintoa file,we needtouse themode ‘w’inopen()function.
✓ The write()methodisusedtowritedata intoa file.
>>> fhand=open(“mynewfile.txt","w")
>>> print(fhand)
<_io.TextIOWrapper name='mynewfile.txt' mode='w' encoding='cp1252'>
✓ We have twomethodsforwritingdataintoa fileasshown below
1. write(string)
2. writelines(list)
If the file specified already exists, then the old contents will be erased and it will be ready
to write new data intoit. If the file does not exists, then a new file with the given name will
be created.

write( ) method: It returns number of characters successfully written into a file. The file object
alsokeepstrackofpositionin thefile.
For example,
writelines()method: Example:This code adds the listofcontentsintothe fileincludingn
Writexample.py
fhand=open("2.txt",'w')
s="hello how are you?"
print(fhand.write(s))
Output:
18
Writelist.py
food = ["Citrusn", "Garlicn", "Almondn", "Gingern"]
my_file = open("immunity.txt", "w")
my_file.writelines(food)
Output
It creates a file immunity.txt
Citrus
Garlic
Almond
Ginger

Example for read binary ‘rb’ and write binary ‘wb’
Imagecopy.py
f1=open("bird.jpg",'rb')
f2=open("birdcopy.jpg",'wb')
for i in f1:
print(f2.write(i))
bird.jpg #input file birdcopy.jpg #output file

SEARCHING THROUGH A FILE
✓ When you are searching through data in a file, it is a very common pattern to read
through a file, ignoring most of the lines and only processing lines which meet a
particularcondition.
✓ Mostofthe times,we wouldlike toreada filetosearchforsome specificdata withinit.
Thiscan beachievedbyusingsome stringmethodswhile readinga file.
✓ For example, we may be interested in printing only the line which starts with a
specificcharacter.

SEARCH EXAMPLE
Search1.py
fhand = open('mbox.txt')
for line in fhand:
line = line.rstrip() #strips whitespace from right side of a string
if line.startswith('From:'):
print(line)
Or
fhand = open('mbox-short.txt')
for line in fhand:
line = line.rstrip()
# Skip 'uninteresting lines'
if not line.startswith('From:'):
continue
# Process our 'interesting' line
print(line)
Output
From: stephen.marquard@uct.ac.za Sat Jan 5 09:14:16 2008
From: louis@media.berkeley.edu
From: zqian@umich.edu Fri Jan 4 16:10:39 2008
mbox.txt
Return-Path: <postmaster@collab.sakaiproject.org>
From: louis@media.berkeley.edu
Subject: [sakai] svn commit:
From: zqian@umich.edu Fri Jan 4 16:10:39 2008
Return-Path: <postmaster@collab.sakaiproject.org>
Search2.py
Note:find lines where the search string is anywhere in the
line. Find() method returns either position of a string or
-1
fhand = open('mbox.txt')
for line in fhand:
line = line.rstrip()
if line.find('@uct.ac.za') == -1: continue
print(line)
Output

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.
Fileuser.py
fname=input("Enter a file name:")
f1=open(fname)
count =0
for line in f1:
count+=1
print("Line Number ",count, ":", line)
print("Total lines=",count)
f1.close()
Output:
Enter a file name:1.txt
Line Number 1 : Coronavirus Can Be Stopped Only by Harsh Steps
Line Number 2 : Stay at home
Line Number 3 : wear a facemask
Line Number 4 : Clean your hands often
Line Number 5 : Monitor your symptoms
Total lines= 5
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 1.txt (discussed before), then the result would be
Total lines=5
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 AND OPEN
When you try opening the file which doesn’t exist or if a file name is not valid, then the interpreter throws
you an error. Assume that the open call might fail and add recovery code when the open fails as follows:
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.
Tryfile.py
fname = input('Enter the file name: ')
try:
fhand = open(fname)
except:
print('File cannot be opened:', fname)
exit()
count = 0
for line in fhand:
if line.startswith('From:'):
count = count + 1
print('count=', count)
Output1:
Enter the file name: mbox.txt
count= 3
Output2:
Enter the file name: newmbox.txt
File cannot be opened: newmbox.txt

CLOSE A FILE IN PYTHON
✓ It’s always the best practice to close a file when your work gets finished.
However, Python runs a garbage collector to clean up the unused objects. While
closing a file, the system frees up all resources allocated to it
✓ The most basic way is to call the Python close() method.
Filepointer.close()
Example:
f = open("app.log“)
# do file operations.
f.close()

PROBLEMS WITH WHITE SPACE- REPR()
✓ When we are reading and writing files, we might run into problems with white
space. These errors can be hard to debug because spaces, tabs and newlines are
normally invisible.
>>> s = ‘1 2 t 3 n 4’
>>> print(s)
1 2 3
4
✓ The built in function repr( ) can take any object as an argument and returns a
string representation of the object. For strings, it represents whitespace,
characters with blackslash sequences:
>>> print(s)
‘1 2 t 3 n 4’
✓ This helps for debugging

EXERCISE PROBLEMS:
1) WAP to copy all the lines from one file to another file ( file1.txt to file2.txt) where the line
beginswithvowelsandalso demonstratethecomputationalfaultsintheprogram
2) Write a program to count the number of occurrences of a given word(accept the input
fromuser) ina file.
(hint: Canusestrip()andcount()methodsforeachwordin a line)
3) Input decimal number and convert it to binary number and write it in another file until
userenters0

File handling in Python

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