File Handling In Python

1. Open the File

2. Process the file (Read/write/append)

3. Close the file

4. Opening a file:

Opening A File

To perform any kid of operation on a file, the programmer first need to open the file by using open() method.

Syntax:

<file_object_name>=open(<filename>,<mode>)

File Pointer: File pointer/file handler is reference to the file on the disk which is being opened.

File Path: The Location of the file on the disk

File Access Mode: This specifies  the nature of operation(read/write/append) to be performed on the file.

File Access Modes

Read Mode: when a file is opened in read mode then the file must exist, otherwise python will raise an I/O error.

Write Mode: When a file is opened in write mode and if the file does not exist then a new file will be created at the specified location on the disk with the same name. If the file exist on the disk, then the file will be truncated, the user can overwrite new data into the file.

Append Mode: When a file is opened in append mode and if the file does not exist then a new file will be created at the specified location on the disk with the same name. If the file exist on the disk, then the file’s exixting data will be retained  and  new data can be appended at the end of the file.

Closing a File

The close() method is used to close an opened file, the execution of close() method breaks the reference of the file object to file on disk, so after execution of close() method no operation on the file can be performed, without reopening the file.

Syntax: <fileHandle>.close()

File Handling and File Types

Python offers wide range of modules/methods to perform file handling operation on both Text File and Binary File

Text File:

Text files stores the information on the file as sequence of ASCII or Unicode characters. These files can be open in any text editor in a human readable form.

Binary File:

Binary files stores information as a sequence of bytes, Binary files stores the information in the same format as it is stored in the memory.

Text File Handling:

Reading a Text File:

We can use any of these 3 methods/functions In order to perform read operation on a text file:

read([n])

readline([n])

readlines()

read() method:

read() function is used to read contents of a text file and returns the result as a string. If invoked with argument n it returns n number of bytes(characters) from file.

Syntax:                  

 <file_object>.read([n])

#program to read a text file 
f=open(“C:\\myfolder\\cs.txt”, “r”)
data=f.read()
print(data)
f.close()

#program to read first n characters of a text file 
n=int(input(“Enter no of chracetr you want to read”))
f=open(“C:\\myfolder\\cs.txt”, “r”)
data=f.read(n)
print(data)
f.close()

readline() method:

This function is used to read a line of input text file and returns the result as a string. If argument n specified then reads at n bytes(characters) of the current line.

Syntax:

<file_object>.readline([n])

#program to read first 2 lines of a text file 
f=open(“C:\\myfolder\\cs.txt”, “r”)
line1=f.readline()
print(line1)
line2=f.readline()
print(line2)
f.close()

readlines() method:

This function is reads all the line of the input text file and returns each line as string encapsulated inside a  List.

Syntax:

<file_object>.readlines()

#program to read a text file using readlines()
f=open(“C:\\myfolder\\cs.txt”, “r”)
data=f.readlines()
print(data)
f.close()

Writing into a Text File:

Python provides these two functions for writing into a text file

#program to write a string into a text file
f=open(“C:\\myfolder\\new.txt”,”w”)
f.write(“Programming is fun”)
f.close()

#program to write multiple lines into a text file using writeline()
L=[“programming is fun”, ”lets start writing python programs”]
f=open(“C:\\myfolder\\new.txt”,”w”)
f.writeline(L)
f.close()

Appending into a Text File:

When a file is opened in append mode, then new data can be added into the file without erasing the existing old data in the file, the new data will be appended at the end of the existing data.  To open a file append mode, file mode need to be set as append “a” while executing open() method.

#program to append a string into a text file
f=open(“C:\\myfolder\\new.txt”,”a”)
f.write(“Lets start programming”)
f.close()

Binary File Handling

Binary files can store more structed python objects like list, tuple, dictionary by converting these objects into byte stream( called serialization/pickling), these objects can brought back to original form while reading the file (called unpickling).

Pickle module is very useful when it come to binary file handling.  We will use dump() and load() methods of pickle module to write and read data objects into binary files.

Writing into a binary file:

The dump() function of pickle module can be used to write an object into a binary file.

Syntax:    pickle.dump( <object_to_be_witten>, <filehandle>)

#Program to write a tuple object into the binary file using dump() method
import pickle
f=open(‘csSubjects.dat’, ’wb’)
tup1=(“AI”,”Networking”,”DBMS”,”Python”)
pickle.dump(tup1,f)
f.close()

#program to read objects from a binary file using load() method
import pickle
f=open(‘cs.dat’,’wb’)
d={“name”:”amit”,”roll”:20,”mark”:320}
pickle.dump(d,f)

header=Pickle.load(f)
sdata=pickle.load(f)
print(”Name:”,sdata[name])
print(”Roll No:”,sdata[roll])
print(”Mark:”,sdata[mark])
f.close()

Writing into Binary File (Example Program )

#program to write details(name, rollno, mark) of n number of students into a binary file

import pickle
f=open(“C:\\myfolder\\students.dat”,”wb”)
n=int(input(“Enter how many students details you want to enter”))
for i in range(n):
	name=input(“Enter students name:”)
	roll=int(input(“Enter students Roll No:”))
	mark=int(input(“Enter students Mark:”))
	data={“Name”:name,”RollNo”:roll,”Mark”:mark}
	f.dump(data,f)
print(“Details of ”,n,” number of students saved successfully”)
f.close()

Reading from Binary File (Example Program )

#Program to read the details of all students in the binary file Students.dat created in the last example
import pickle
f=open(“C:\\myfolder\\students.dat”,”rb”)
print(“*******Students Details*********”)
while True:
	try:
		data=pickle.load(f)
		print(“Name:”,data[”Name”])	
		print(“Roll Number:”,data[”RollNo”])	
		print(“Mark:”,data[”Mark”])	
	except EOFError:
		print(“Reached end of the file”)
		break			
f.close()

Performing Search operation on a Binary File (Example Program )

#Program to search the details of students  based on students Roll Number
import pickle
f=open(“C:\\myfolder\\students.dat”,”rb”)
sroll=int(input(“Enter the Roll Number of the student”))
found=False
print(“*******Student Details*********”)
while True:
            try:
	    data=pickle.load(f)
	             if data[“RollNo”]==sroll:
		         found=True
		         print(“Name:”,data[“Name”])	
		         print(“Roll Number:”,data[”RollNo”])	
		         print(“Mark:”,data[“Mark”])	
	except EOFError:
		break	
If found==False:
	print(“Data not found”)		
f.close()

Append into a Binary File (Example Program )

#Program to append details of a new student into the binary file Student.dat

import pickle
f=open(“C:\\myfolder\\students.dat”,”ab”)
name=input(“Enter students name:”)
roll=int(input(“Enter students Roll No:”))
mark=int(input(“Enter students Mark:”))
data={“Name”:name,”RollNo”:roll,”Mark”:mark}
f.dump(data,f)		
f.close()

Update a Binary File (Example Program )

#Program to update mark of a  student in the  the binary file Student.dat
import pickle
f=open(`C:\\myfolder\\students.dat’,`rb’)
alldata=[]
sroll=int(input(`Enter the Roll Number of the student:’))
newmark=int(input(`Enter the Mark to be updated: ‘))
while True:
            try:
	    sdata=pickle.load(f)
	    alldata.append(sdata)			
            except EOFError:
	     break	
f.close()
for record in alldata:
            if record[`RollNo’]==sroll:
	   record[`Mark’]=newMark
f=open(`C:\\myfolder\\students.dat’,`rb’)
for record in alldata:
           pickle.dump(record)			
f.close()

Comma Separated File (CSV) File Handling

CSV stands for “Comma Separated Values.” It is used to store data in a tabular form in a text file. The fact that CSV files are actually text files, It becomes very useful to export high volume data to a database or to a server.

Reading/Writing operations on a CSV file can be performed using the csv module of python. to use the methods of csv module we have to import the csv  module into our program.

Writing into a csv File

As csv file is basically a text file, to write into a csv file mode “w” must be specified along with the file path in the open() method to open the file.

f=open(`C:\\myfolder\\new.csv’,`w’)

After the the file is being open and a file handle is created, we need to create a writer object using writer() method.

<writerObject>=csv.writer(<fileHandle>)

The writerow() method can be used to write a List object into the csv file.

<writerObject>.writerow(<[List]>)

#program to write four rows into csv file

import csv
f=open(`C:\\myfolder\\new.csv’,`w’)
writer1=csv.writer(f)
Writer1.writerow([`Name’,`Class’,`Mark’])
Writer1.writerow([`Amit’,12,450)
Writer1.writerow([`Sahil’,12,460])
Writer1.writerow([`Anjali’,11,410)
f.close()

Reading From a csv File

To read data from a csv file, the file need to opened in the same way as any text file

( Note: The file extension is .csv)

f=open(`C:\\myfolder\\new.csv’,`r’)

After the the file is being open and a file handle is created, we need to create a reader object using reader() method.

<readerObject>=csv.reader(<fileHandle>)

Once created, a reader object carries all data from the referenced csv file.

#program to read  a csv file

import csv
f=open(`C:\\myfolder\\new.csv’,`r’)
reader1=csv.reader(f)
for record in reader1:
	print(record)
f.close()