File Input & Output in Python

This Post is about using files with Python .

There are basically 3 operations we can perform :

1. Read

2. Write

3. Append

Reading Files

Reading from text file is also a 3 step process ie Open , Read and Close .

Simplest code to read a file is :

## open is passed with file_name and mode ('r' meaning read mode)
file = open("/Users/jatinmadaan/blog/file_io/sample.txt",'r')

## Looping through file object to print content
for line in file : 
    print (line)

file.close()

O/P :

Policy_id,Name,Age,Country,Plan_Type 111111,ABC1,20,India,Health1 111112,ABC2,30,India,Health2 111113,ABC3,20,India,Health1 111114,ABC4,30,India,Term1

Also we check and filter out data before printing as well :

## open is passed with file_name and mode ('r' meaning read mode)
file = open("/Users/jatinmadaan/blog/file_io/sample.txt",'r')

## Looping through file object to print content
for line in file :
    ## We can check for data here itself 
    if "111112" in line:
        print(line)

file.close()

O/P :

111112,ABC2,30,India,Health2

Problem with using open () is that in case we forget to close the file then it leads to data corruption , also if in between there is any failure then file remains open (we can handle this with exceptional handling ).

To avoid above issue we can use with open function which will automatically take care of closing and avoids corruption of files .

eg :

with open("sample.txt",'r') as file :
    for line in file : 
        if "111112" in line :
            print (line)

O/P : 111112,ABC2,30,India,Health2

We can also use readline() to parse line by data

eg :

with open("sample.txt",'r') as file :
    line = file.readline()
    while line:
        print(line)
        line = file.readline()

O/P :

Policy_id,Name,Age,Country,Plan_Type 111111,ABC1,20,India,Health1 111112,ABC2,30,India,Health2 111113,ABC3,20,India,Health1 111114,ABC4,30,India,Term1

Here output contains extra spaces between lines , to remove those we can use end='' in print function .

eg :

with open(sample.txt",'r') as file 
    line = file.readline()
    while line:
        print(line,end='')
        line = file.readline()

O/P :

Policy_id,Name,Age,Country,Plan_Type 111111,ABC1,20,India,Health1 111112,ABC2,30,India,Health2 111113,ABC3,20,India,Health1 111114,ABC4,30,India,Term1

There are also other functions eg readlines() and read() but they read entire file and put in memory which is very inefficient for large files.

2 . Writing to files

Mode used for writing is 'w' , in this mode file is overwritten if exists else a new file is created and data loaded into it.

eg : Let's create a list of fruits and store that data into a file

fruits=["Apple","Mango","Orange"]

with open("fruits_data.txt",'w') as fruits_data :
    for line in fruits :
        print(line,file=fruits_data)

O/P :

$ cat fruits_data.txt

Apple

Mango

Orange

3. Appending Data

Appending mode 'a' writes data at end of file , in case file doest not exists new file is created and data inserted.

eg :

new_policy="222222,DEF1,50,US,Term50"
with open("sample.txt",'a') as new_data :
    print(new_policy,file=new_data)

Before running of code

$ tail -1 sample.txt

111121,ABC11,20,India,life3

After running of code

$ tail -1 sample.txt

222222,DEF1,50,US,Term50

Above are very basic and fundamental file operations to get starting with working on I/O in python using text files .