Python Files

Files - Introduction 

The open() function helps us to read from or write to a file. 

The syntax for the open function is as below. 

FILEHANDLE = open(FILEPATH, ACCESSMODE, BUFFERINGOPTIONS) 

The access modes are read, write, append (add to existing file data). This is an optional parameter as the default open mode is read. 

r indicates read 

w indicates write 

a indicates append 

a+ indicates both read and append 

The below program opens a file called input.txt and prints the content in it. 

fh = open("input.txt","r") 

print (fh.read()) 

fh.close() 

The method read() reads the entire content in the file. 

The method close() releases the filehandle.

Files - Writing to a file 

The write() function helps us to write to a file. 

The below program opens a file called output.txt and writes a single like "This is the first line" to the file. 

f = open("output.txt","w") 

f.write("This is first line") 

f.close() 

The method close() flushes the buffer to the file and releases the filehandle.

Files - Appending content to a file 

A file can be opened in append mode and the write() function can be used to append content to the file. 

The below program opens a file called output.txt and appends the string "Rocks Stones". 

f = open("output.txt","a") 

f.write("Rocks Stones") 

f.close() 

If the file content initially was 

First Line 

Second Line 

then the revised content will be 

First Line 

Second LineRocks Stones 

To append the content in a new line (next line), the following code can be used with a \n 

f = open("output.txt","a") 

f.write("\nRocks Stones") 

f.close()

Another example program:

The content of output.txt is as below 

Actor1 - ABCD 

Actor2 - EFGH

f = open("output.txt","a")

f.write("\nActor3 - IJKL")

f.close()

#Print the file content as output

f = open("output.txt","r")

print (f.read())

f.close()

Now output.txt contains

Actor1 - ABCD 

Actor2 - EFGH 

Actor3 - IJKL

Files - with open 

A better way to open files is to use with open. 

The below program shows how to open a file using open and print the content in it. 

with open("output.txt","r") as f: 

line in f:

            print (line, end="")

The content of data.txt is as below 

FirstLine 

SecondLine 

ThirdLine 

FourthLine 

FifthLine 

SixthLine

with open("data.txt","r") as f:

    for count,line in enumerate(f, start=1): 

        if count % 2 == 1:

            print (line, end="")

That is the output of the program is as below. 

FirstLine 

ThirdLine 

FifthLine

File I/O - readline() 

The readline() method can be used to read a single line from a file from the current position. The below program prints the first four lines present in the file data.txt 

with open("data.txt","r") as f: 

for line in range(1,5):

            print (f.readline(), end="")

File I/O - random line access 

The linecache module can be used to randomly access a line in a file. 

The below program prints the 11th line in the file data.txt 

import linecache 

print (linecache.getline("data.txt",11), end="")

the below program prints the Nth line in the file data.txt

n = int(input())

import linecache

print (linecache.getline("data.txt",n))

File I/O - seek method 

The syntax of seek method is 

f.seek(offset, from_what) 

offset refers to the byte count from the position mentioned by from_what. 

- If from_what is 0, then it means from the beginning of the file. This is the default value. 

- If from_what is 1, then it means from the current position of cursor. 

- If from_what is 2, then it means from the end of the file. 

The below program prints the content in the file data.txt from the 6th byte (character). (As the index starts from zero, 5 denotes 6th byte) 

with open("data.txt","r") as f: 

f.seek(5) 

        print (f.read())

If the content of the file is abcdefghijklmnop, then the output of the program is fghijklmnop 

Renaming a file 

The os module helps in renaming a file.

The below program renames data.txt to data1.txt 

import os 

os.rename("data.txt","data1.txt")

fileinput module helps to perform inplace edit on files.

 Let us consider the file data.txt which contains the following content. 

banana 

apple 

berry 

melon 

Files - Inplace edit

We wish to append an asterisk in the end , if the line starts with the letter b. The below program produces the desired output given below which is now the content of data.txt 

apple 

banana* 

melon 

berry* 

import fileinput 

for line in fileinput.input('data.txt', inplace=1): 

    if line.startswith('b'): 

        print (line.rstrip()+"*") 

    else: 

        print (line, end="") 

The print will redirect the content to the file being edited inplace rather than the standard output. 

We do the rstrip() to remove the new line character in the end so that we can append the asterisk in the end. The new line is automatically added by the print statement as we have not overridden the default behaviour using the end parameter.