File Handling
As the part of programming requirement, we have to store our data permanently for
future purpose. For this requirement we should go for files.
Files are very common permanent storage areas to store our data.
Types of Files:
There are 2 types of files
1. Text Files:
Usually we can use text files to store character data
eg: abc.txt
2. Binary Files:
Usually we can use binary files to store binary data like images,video files, audio
files etc...
Opening a File:
Before performing any operation (like read or write) on the file,first we have to
open that
file.For this we should use Python's inbuilt function open()
But at the time of open, we have to specify mode,which represents the purpose of
opening file.
f = open(filename, mode)
The allowed modes in Python are
1. r : open an existing file for read operation. The file pointer is positioned at
the
beginning of the file.If the specified file does not exist then we will get
FileNotFoundError.This is default mode
2. w : open an existing file for write operation. If the file already contains some
data
then it will be overridden. If the specified file is not already avaialble then
this mode will
create that file.
3. a : open an existing file for append operation. It won't override existing
data.If the
specified file is not already avaialble then this mode will create a new file.
4. r+ : To read and write data into the file. The previous data in the file will
not be
deleted.The file pointer is placed at the beginning of the file.
5. w+ : To write and read data. It will override existing data.
6. a+ : To append and read data from the file.It wont override existing data.
7. x : To open a file in exclusive creation mode for write operation. If the file
already
exists then we will get FileExistsError.
Note: All the above modes are applicable for text files. If the above modes
suffixed with
'b' then these represents for binary files.
Eg: rb,wb,ab,r+b,w+b,a+b,xb
f = open("abc.txt","w")
We are opening abc.txt file for writing data.
Closing a File:
After completing our operations on the file,it is highly recommended to close the
file.
For this we have to use close() function.
f.close()
Various properties of File Object:
Once we opend a file and we got file object,we can get various details related to
that file by using its properties.
name : Name of opened file
mode : Mode in which the file is opened
closed : Returns boolean value indicates that file is closed or not
readable(): Retruns boolean value indicates that whether file is readable or not
writable(): Returns boolean value indicates that whether file is writable or not.
Eg:
1) f=open("abc.txt",'w')
2) print("File Name: ",f.name)
3) print("File Mode: ",f.mode)
4) print("Is File Readable: ",f.readable())
5) print("Is File Writable: ",f.writable())
6) print("Is File Closed : ",f.closed)
7) f.close()
8) print("Is File Closed : ",f.closed)
9)
10)
11) Output
12)
Writing data to text files:
We can write character data to the text files by using the following 2 methods.
write(str)
writelines(list of lines)
Eg:
1) f=open("abcd.txt",'w')
2) f.write("amol\n")
3) f.write("Software\n")
4) f.write("Solutions\n")
5) print("Data written to the file successfully")
6) f.close()
abcd.txt:
Note: In the above program, data present in the file will be overridden everytime
if we
run the program. Instead of overriding if we want append operation then we should
open
the file as follows.
f = open("abcd.txt","a")
Eg 2:
1) f=open("abcd.txt",'w')
2) list=["sunny\n","bunny\n","vinny\n","chinny"]
3) f.writelines(list)
4) print("List of lines written to the file successfully")
5) f.close()
Note: while writing data by using write() methods, compulsory we have to provide
line
seperator(\n),otherwise total data should be written to a single line.
Reading Character Data from text files:
We can read character data from text file by using the following read methods.
read(): To read total data from the file
read(n) : To read 'n' characters from the file
readline(): To read only one line
readlines(): To read all lines into a list
Eg 1: To read total data from the file
1) f=open("abc.txt",'r')
2) data=f.read()
3) print(data)
4) f.close()
5)
6) Output
Eg 2: To read only first 10 characters:
1) f=open("abc.txt",'r')
2) data=f.read(10)
3) print(data)
4) f.close()
5)
Eg 3: To read data line by line:
1) f=open("abc.txt",'r')
2) line1=f.readline()
3) print(line1,end='')
4) line2=f.readline()
5) print(line2,end='')
6) line3=f.readline()
7) print(line3,end='')
8) f.close()
9)
10) Output
Eg 4: To read all lines into list:
1) f=open("abc.txt",'r')
2) lines=f.readlines()
3) for line in lines:
4) print(line,end='')
5) f.close()
6)
7) Output
Eg 5:
1) f=open("abc.txt","r")
2) print(f.read(3))
3) print(f.readline())
4) print(f.read(4))
5) print("Remaining data")
6) print(f.read())
7)
8) Output
The with statement:
The with statement can be used while opening a file.We can use this to group file
operation statements within a block.
The advantage of with statement is it will take care closing of file,after
completing all
operations automatically even in the case of exceptions also, and we are not
required to
close explicitly.
Eg:
1) with open("abc.txt","w") as f:
2) f.write("amol\n")
3) f.write("Software\n")
4) f.write("Solutions\n")
5) print("Is File Closed: ",f.closed)
6) print("Is File Closed: ",f.closed)
7)
8) Output
The seek() and tell() methods:
tell():
==>We can use tell() method to return current position of the cursor(file pointer)
from
beginning of the file. [ can you plese telll current cursor position]
The position(index) of first character in files is zero just like string index.
Eg:
1) f=open("abc.txt","r")
2) print(f.tell())
3) print(f.read(2))
4) print(f.tell())
5) print(f.read(3))
6) print(f.tell())
seek():
We can use seek() method to move cursor(file pointer) to specified location.
[Can you please seek the cursor to a particular location]
f.seek(offset, fromwhere)
offset represents the number of positions
The allowed values for second attribute(from where) are
0---->From beginning of file(default value)
1---->From current position
2--->From end of the file
Note: Python 2 supports all 3 values but Python 3 supports only zero.
Eg:
1) data="All Students are Good"
2) f=open("amol.txt","w")
3) f.write(data)
4) with open("amol.txt","r+") as f:
5) text=f.read()
6) print(text)
7) print("The Current Cursor Position: ",f.tell())
8) f.seek(17)
9) print("The Current Cursor Position: ",f.tell())
10) f.write("GEMS!!!")
11) f.seek(0)
12) text=f.read()
13) print("Data After Modification:")
14) print(text)
How to check a particular file exists or not?
We can use os library to get information about files in our computer.
os module has path sub module,which contains isFile() function to check whether a
particular file exists or not?
os.path.isfile(fname)
Q. Write a program to check whether the given file exists or not. If it is
available then print its content?
1) import os,sys
2) fname=input("Enter File Name: ")
3) if os.path.isfile(fname):
4) print("File exists:",fname)
5) f=open(fname,"r")
6) else:
7) print("File does not exist:",fname)
8) sys.exit(0)
9) print("The content of file is:")
10) data=f.read()
11) print(data)
Note:
sys.exit(0) ===>To exit system without executing rest of the program.
argument represents status code . 0 means normal termination and it is the default
value.
Q. Program to print the number of lines,words and characters present in the
given file?
1) import os,sys
2) fname=input("Enter File Name: ")
3) if os.path.isfile(fname):
4) print("File exists:",fname)
5) f=open(fname,"r")
6) else:
7) print("File does not exist:",fname)
8) sys.exit(0)
9) lcount=wcount=ccount=0
10) for line in f:
11) lcount=lcount+1
12) ccount=ccount+len(line)
13) words=line.split()
14) wcount=wcount+len(words)
15) print("The number of Lines:",lcount)
16) print("The number of Words:",wcount)
17) print("The number of Characters:",ccount)
Handling csv files:
CSV==>Comma seperated values
As the part of programming,it is very common requirement to write and read data wrt
csv
files. Python provides csv module to handle csv files.
Writing data to csv file:
1) import csv
2) with open("emp.csv","w",newline='') as f:
3) w=csv.writer(f) # returns csv writer object
4) w.writerow(["ENO","ENAME","ESAL","EADDR"])
5) n=int(input("Enter Number of Employees:"))
6) for i in range(n):
7) eno=input("Enter Employee No:")
8) ename=input("Enter Employee Name:")
9) esal=input("Enter Employee Salary:")
10) eaddr=input("Enter Employee Address:")
11) w.writerow([eno,ename,esal,eaddr])
12) print("Total Employees data written to csv file successfully")
Note: Observe the difference with newline attribute and without
with open("emp.csv","w",newline='') as f:
with open("emp.csv","w") as f:
Note: If we are not using newline attribute then in the csv file blank lines will
be included
between data. To prevent these blank lines, newline attribute is required in
Python-3,but
in Python-2 just we can specify mode as 'wb' and we are not required to use newline
attribute.
Reading Data from csv file:
1) import csv
2) f=open("emp.csv",'r')
3) r=csv.reader(f) #returns csv reader object
4) data=list(r)
5) #print(data)
6) for line in data:
7) for word in line:
8) print(word,"\t",end='')
9) print()
Zipping and Unzipping Files:
It is very common requirement to zip and unzip files.
The main advantages are:
1. To improve memory utilization
2. We can reduce transport time
3. We can improve performance.
To perform zip and unzip operations, Python contains one in-bulit module zip file.
This module contains a class : ZipFile
To create Zip file:
We have to create ZipFile class object with name of the zip file,mode and constant
ZIP_DEFLATED. This constant represents we are creating zip file.
f = ZipFile("files.zip","w","ZIP_DEFLATED")
Once we create ZipFile object,we can add files by using write() method.
f.write(filename)
Eg:
1) from zipfile import *
2) f=ZipFile("files.zip",'w',ZIP_DEFLATED)
3) f.write("file1.txt")
4) f.write("file2.txt")
5) f.write("file3.txt")
6) f.close()
7) print("files.zip file created successfully")
To perform unzip operation:
We have to create ZipFile object as follows
f = ZipFile("files.zip","r",ZIP_STORED)
ZIP_STORED represents unzip operation. This is default value and hence we are not
required to specify.
Once we created ZipFile object for unzip operation,we can get all file names
present in
that zip file by using namelist() method.
names = f.namelist()
Eg:
1) from zipfile import *
2) f=ZipFile("files.zip",'r',ZIP_STORED)
3) names=f.namelist()
4) for name in names:
5) print( "File Name: ",name)
6) print("The Content of this file is:")
7) f1=open(name,'r')
8) print(f1.read())
9) print()
Working with Directories:
It is very common requirement to perform operations for directories like
1. To know current working directory
2. To create a new directory
3. To remove an existing directory
4. To rename a directory
5. To list contents of the directory
etc..
To perform these operations,Python provides inbuilt module os,which contains
several
functions to perform directory related operations.
Q1. To Know Current Working Directory:
import os
cwd=os.getcwd()
print("Current Working Directory:",cwd)
Q2. To create a sub directory in the current working directory:
import os
os.mkdir("mysub")
print("mysub directory created in cwd")
Q3. To create a sub directory in mysub directory:
cwd
|-mysub
|-mysub2
import os
os.mkdir("mysub/mysub2")
print("mysub2 created inside mysub")
Note: Assume mysub already present in cwd.
Q4. To create multiple directories like sub1 in that sub2 in that sub3:
import os
os.makedirs("sub1/sub2/sub3")
print("sub1 and in that sub2 and in that sub3 directories created")
Q5. To remove a directory:
import os
os.rmdir("mysub/mysub2")
print("mysub2 directory deleted")
Q6. To remove multiple directories in the path:
import os
os.removedirs("sub1/sub2/sub3")
print("All 3 directories sub1,sub2 and sub3 removed")
Q7. To rename a directory:
import os
os.rename("mysub","newdir")
print("mysub directory renamed to newdir")
Q8. To know contents of directory:
os module provides listdir() to list out the contents of the specified directory.
It won't
display the contents of sub directory.
Eg:
1) import os
2) print(os.listdir("."))
3)
4) Output
The above program display contents of current working directory but not contents of
sub
directories.
If we want the contents of a directory including sub directories then we should go
for
walk() function.
Q9. To know contents of directory including sub directories:
We have to use walk() function
[Can you please walk in the directory so that we can aware all contents of that
directory]
os.walk(path,topdown=True,onerror=None,followlinks=False)
It returns an Iterator object whose contents can be displayed by using for loop
path-->Directory path. cwd means .
topdown=True --->Travel from top to bottom
onerror=None --->on error detected which function has to execute.
followlinks=True -->To visit directories pointed by symbolic links
Eg: To display all contents of Current working directory including sub directories:
1) import os
2) for dirpath,dirnames,filenames in os.walk('.'):
3) print("Current Directory Path:",dirpath)
4) print("Directories:",dirnames)
5) print("Files:",filenames)
6) print()
7)
8)
9) Output
Note: To display contents of particular directory,we have to provide that directory
name
as argument to walk() function.
os.walk("directoryname")
Q. What is the difference between listdir() and walk() functions?
In the case of listdir(), we will get contents of specified directory but not sub
directory
contents. But in the case of walk() function we will get contents of specified
directory and
its sub directories also.
Running Other programs from Python program:
os module contains system() function to run programs and commands.
It is exactly same as system() function in C language.
os.system("commad string")
The argument is any command which is executing from DOS.
Eg:
import os
os.system("dir *.py")
os.system("py abc.py")
How to get information about a File:
We can get statistics of a file like size, last accessed time,last modified time
etc by using
stat() function of os module.
stats = os.stat("abc.txt")
The statistics of a file includes the following parameters:
st_mode==>Protection Bits
st_ino==>Inode number
st_dev===>device
st_nlink===>no of hard links
st_uid===>userid of owner
st_gid==>group id of owner
st_size===>size of file in bytes
st_atime==>Time of most recent access
st_mtime==>Time of Most recent modification
st_ctime==> Time of Most recent meta data change
Note:
st_atime, st_mtime and st_ctime returns the time as number of milli seconds since
Jan 1st
1970 ,12:00AM. By using datetime module fromtimestamp() function,we can get exact
date and time.
Q. To print all statistics of file abc.txt:
1) import os
2) stats=os.stat("abc.txt")
3) print(stats)
4)
5) Output
Q. To print specified properties:
1) import os
2) from datetime import *
3) stats=os.stat("abc.txt")
4) print("File Size in Bytes:",stats.st_size)
5) print("File Last Accessed Time:",datetime.fromtimestamp(stats.st_atime))
6) print("File Last Modified Time:",datetime.fromtimestamp(stats.st_mtime))
