Python programming : Files

Introduction

A file is an object on a computer that stores data, information, settings, or commands used
with a computer program

Advantages of files

- Data is stored permanently

- Updation becomes easy

- Data can be shared among various programs

- Huge amount of data can be stored

Files
Types
Text Binary
Stores the data in the form of strings Stores data in the form of bytes
Example:
“Ram” is stored as 3 characters
890.45 is stored as 6 characters
Example:
“Ram” is stored as 3 bytes
89000.45 is stored as 8 bytes
Examples:
.txt, .c, .cpp
Examples:
.jpg, .gif or .png

Files
Opening a file
Name open()
Syntax file_handler = open("file_name", "open_mode", "buffering")
filename : Name of the file to be opened
open_mode: Purpose of opening the file
buffering: Used to stored the data temporarily
Opening Modes
w - To write the data
- If file already exist, the data will be lost
r - To read the data
- The file pointer is positioned at the begining of the file
a - To append data to the file
- The file pointer is placed at the end of the file
w+ - To write and read data
- The previous data will be deleted
r+ - To read and write
- The previous data will not be deleted
- The file pointer is placed at the begining of the file
a+ - To append and read data
- The file pointer will be at the end of the file
x - To open the file in exclusive creation mode
- The file creation fails, if already file exist
Example
f = open("myfile.txt", "w")
Here, buffer is optional, if omitted 4096 / 8192 bytes will be considered.

Files
Closing a file
Name close()
Syntax f.close()
Example #Open the file
f = open("myfile.txt", "w")
#Read the string
str = input("Enter the string: ")
#Write the string into the file
f.write(str)
#Close the file
f.close()

Files
Working with text files containing strings
To read the content from files,
f.read() : Reads all lines, displays line by line
f.readlines() : Displays all strings as elements in a list
f.read().splitlines(): To suppress the "n" in the list
Program
#To create a text file to store strings
#Open the file
f = open("myfile.txt", "r")
#Read the data from a file
str = f.read() #Reads all data
#Display the data
print(str)
#Close the file
f.close()
Note:
"""
f.read(n): Will read 'n' bytes from the file
"""

Files
Working with text files containing strings
f.seek(offset, fromwhere)
- offset : No. of bytes to move
- fromwhere : Begining, Current, End
- Example : f.seek(10, 0), move file handler from Beg forward 10 bytes.
# Appending and then reading strings, Open the file for reading data
f = open('myfile.txt', 'a+')
print('Enter text to append(@ at end): ')
while str != '@':
str = input() # accept string into str
# Write the string into file
if (str != '@'):
f.write(str+"n")
# Put the file pointer to the beginning of the file
f.seek(0,0)
# Read strings from the file
print('The file cotents are: ')
str = f.read()
print(str)
# Closing the file
f.close()

Files
Knowing If file exists or not
Sample:
if os.path.isfile(fname):
f = open(fname, "r")
else:
print(fname + "Does not exist")
sys.exit() #Terminate the program
# Checking if file exists and then reading data
import os, sys
# open the file for reading data
fname = input('Enter filename : ')
if os.path.isfile(fname):
f = open(fname, 'r')
else:
print(fname+' does not exist')
sys.exit()
# Read strings from the file
print('The file contents are: ')
str = f.read()
print(str)
# Closing the file
f.close()

Files
Exercise
Problem- 1
To count number of lines, words and characters in a text file
Problem- 2
To copy an image from one file to another

Files
The with statement
1. Can be used while opening the file
2. It will take care of closing the file, without using close() explicitly
3. Syntax: with open("file_name", "openmode") as fileObj:
Program -1
# With statement to open a file
with open('sample.txt', 'w') as f:
f.write('I am a learnern')
f.write('Python is attactiven')
Program -2
# Using with statement to open a file
with open('sample.txt', 'r') as f:
for line in f:
print(line)

Files
The pickle + Unpickle
1. To store the data of different types, we need to create the class for it.
2. Pickle/Serialization:
- Storing Object into a binary file in the form of bytes.
- Done by a method dump() of pickle module
- pickle.dump(object, file)
3. Unpickle/Deserialization
- Process where byte stream is converted back into the object.
- Object = pickle.load(file)

Files
The pickle: Program
# A python program to create an Emp class witg employee details as instance variables.
# Emp class - save this as Emp.py
class Emp:
def_init_(self, id, name, sal):
self.id = id
self.name = name
self.sal = sal
def display(self):
print("{:5d} {:20s} {:10.2f}".format(self.id, self.name,self.sal))
# pickle - store Emp class object into emp.dat file
import Emp, pickle
# Open emp.dat file as a binary file for writing
f = open('emp.dat', 'wb')
n = int(input('How many employees? '))
for i in range(n):
id = int(input('Enter id: '))
name = input('Enter name: ')
sal = float(input('Enter salary: '))
for i in range(n):
id = int(input('Enter id: '))
name = input('Enter name: ')
sal = float(input('Enter salary: '))
# Create Emp class object
e = Emp.Emp(id, name, sal)
# Store the object e into the file f
pickle.dump(e, f)
#close the file
f.close()

Files
The unpickle: Program
# A python program to create an Emp class witg employee details as instance variables.
# Emp class - save this as Emp.py
class Emp:
def_init_(self, id, name, sal):
self.id = id
self.name = name
self.sal = sal
def display(self):
print("{:5d} {:20s} {:10.2f}".format(self.id, self.name,self.sal))
# unpickle or object de-serialization
import Emp, pickle
# Open the file to read objects
f = open('emp.dat', 'rb')
print('Employees details: ')
while True:
try:
#Read object from file f
obj = pickle.load(f)
# Display the contents of employee obj
obj.display()
except EOFError:
print('End of file reached....')
break
#Close the file
f.close()

Random Binary File Access
using mmap
1. Using mmap, binary data can be viewed as strings
mm = mmap.mmap(f.fileno(), 0)
2. Reading the data using read() and readline()
print(mm.read())
print(mm.readline())
3. We can also retrieve the data using teh slicing operator
print(mm[5: ])
print(mm[5: 10])
4. To modify / replace the data
mm[5: 10] = str
5. To find the first occurrance of the string in the file
n = mm.find(name)
6. To convert name from string to binary string
name = name.encode()
7. To convert bytes into a string
ph = ph.decode()
Demonstrate the code

Zip & Unzip

Zip:

- The file contents are compressed and hence the size will be reduced

- The format of data will be changed making it unreadable
Original
File
Compressed
File
Zipping
Unzipping

Zip & Unzip
Programs
# Zipping the contents of files
from zipfile import *
# create zip file
f = zipfile('test.zip', 'w', 'ZIP_DEFLATED')
# add some files. these are zipped
f.write('file1.txt')
# close the zip file
print('test.zip file created....')
f.close()
# A Python program to unzip the contents of the files
# that are available in a zip file.
# To view contents of zipped files
from zipfile import*
# open the zip file
z = Zipfile('test.zip', 'r')
# Extract all the file names which are int he zip file
z.extractall()

Working With Directories
Program-1
# A Python program to know the currently working directory.
import os
# get current working directory
current = os.getcwd()
print('Current sirectory= ', current)

Program-2
# A Python program to create a sub directory and then sub-sun directory in the current
directory.
import os
# create a sub directory by the name mysub
os.mkdir('mysub')
# create a sub-sub directory by the same mysub2
os.mkdir('mysub/mysub2')

Program-3
# A Python program to use the makedirs() function to create sub and sub-sub directories.
import os
# create sub and sub-sub directories
os.mkdirs('newsub/newsub2')

Program-4
# A Python program to remove a sub directory that is inside another directory.
import os
# to remove newsub2 directory
os.rmdir('newsub/newsub2')

Program-5
# A Python program to remove a group of directories in the path
import os
# to remove mysub3, mysub2 and then mysub.
os.removedirs('mysub/mysub2/mysub3')

Program-6
# A Python program to rename a directory.
import os
# to rename enum as newenum
os.rename('enum', 'newenum')

Program-7
# A Python program to display all contents of the current directory.
import os
for dirpath, dirnames, filenames in os.walk('.'):
print('Current path: ', dirpath)
print('Directories: ', dirnames)
print('Files: ', filenames)
print()

Running other programs
Program-7
The OS module has the system() method that is useful to run an executableprogram from our
Python program
Example-1
os.system(‘dir’) Display contents of current working DIR
Example-2 os.system(‘python demo.py’) Runs the demo.py code

Python programming : Files

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