Python Read Line and by Line and Write With New Line

What is Python readline?

Python readline() is a file method that helps to read one complete line from the given file. It has a trailing newline ("\n") at the end of the string returned.

Y'all can likewise make use of the size parameter to get a specific length of the line. The size parameter is optional, and by default, the entire line will be returned.

The flow of readline() is well understood in the screenshot shown beneath:

You have a file demo.txt, and when readline() is used, it returns the very start line from demo.txt.

How readline works

In this tutorial, you volition learn:

• Python File readline
• Characteristic of Python readline()
• Syntax
• Example: To read the first line using readline()
• Example: Using size argument in readline()
• Basic File IO in Python
• Read a File Line-past-Line in Python
• How to read all lines in a file at once?
• How to read a File line-by-line using for loop?
• How to read a File line-by-line using a while loop?

Characteristic of Python readline()

Here, are of import characteristics of Python read line:

• Python readline() method reads only one consummate line from the file given.
• It appends a newline ("\n") at the cease of the line.
• If yous open the file in normal read mode, readline() will return y'all the string.
• If you open the file in binary mode, readline() will return you binary object.
• You lot tin give size equally an argument to readline(), and it will go you the line as per the size given inclusive of the newline. Past default, the size is 0, and it returns the entire line.

Syntax

file.readline(size)          

Parameters

size: (optional) Here, y'all can specify the number, an integer value to readline(). Information technology volition go the string of that size. By default, the value of size is -1, and hence the entire string is returned.

ReturnValue

The readline() method returns the line from the file given.

Example: To read the commencement line using readline()

Here volition empathize how to read the line from the file given using the readline() method. We are going to make utilize of demo.txt file hither to read the contents.

The file contents of demo.txt are as follows:

demo.txt

Testing - FirstLine Testing - SecondLine Testing - Third Line Testing - Fourth Line Testing - Fifth Line          

The following are the steps to read a line from the file demo.txt.

Step 1)

First, open the file using the file open() method, equally shown below:

myfile = open("demo.txt", "r")          

The open up() method takes the outset parameter as the name of the file, and the second parameter is the mode is while you lot desire to open. Right now, we take used "r", which ways the file will open in read mode.

Step 2)

Employ the readline() method to read the line from the file demo.txt as shown below:

myline = myfile.readline()          

Step iii)

The line read is stored inside myline. Let us now print the line to see the details:

print(myline)          

Step four)

One time the reading is done, close the file using close() method equally shown below:

myfile.close()          

The entire code is as follows:

myfile = open("demo.txt", "r") myline = myfile.readline() impress(myline) myfile.shut()          

Output:

Testing - FirstLine          

Example: Using size argument in readline()

We have seen how to read the entire line from the file given. Y'all tin besides make use of the size parameter to get simply the required length of the line.

The given example has the size parameter given equally 10. The get-go line will be fetched, and it will return the line with characters from 0 to x.

Nosotros are going to brand use of demo.txt file used earlier. Salvage the file demo.txt and utilise the location of the demo.txt within open() function.

myfile = open("demo.txt", "r") myline = myfile.readline(10) print(myline) myfile.close()          

Output:

Testing -          

Basic File IO in Python

The basic file IO in Python to open a file for reading or writing is the built-in open up() function. The ii important arguments that goes in open() function are the file path, which is a string, and the mode that specifies whether the file is meant for reading or writing. The mode statement is a string.

Syntax:

open up("file path", "mode")          

Following are the modes available that can exist used with open() method:

Mode	Description
R	This volition open() the file in read mode.
Westward	Using w, you can write to the file.
a	Using a with open() will open up the file in write mode, and the contents will be appended at the terminate.
rb	The rb mode will open the file for binary data reading.
wb	The wb mode will open the file for writing binary information.

Since we demand the file for reading, we are going to make use of read style i.eastward. (r).

Read a File Line-by-Line in Python

The readline() method helps to read just one line at a time, and information technology returns the first line from the file given.

Here, we will make use of readline() to read all the lines from the file given. The file that volition read is demo.txt. The contents of the file are:

Save the file demo.txt and use the location of demo.txt inside open() function.

Testing - FirstLine Testing - SecondLine Testing - Third Line Testing - Fourth Line Testing - Fifth Line          

Using readline() inside while-loop will accept care of reading all the lines present in the file demo.txt.

myfile = open up("demo.txt", "r") myline = myfile.readline() while myline:     print(myline)     myline = myfile.readline() myfile.close()          

Output:

Testing - FirstLine Testing - SecondLine Testing - Tertiary Line Testing - Fourth Line Testing - Fifth Line          

How to read all lines in a file at once?

To read all the lines from a given file, you lot can make use of Python readlines() function. The specialty of Python readlines() function is to read all the contents from the given file and save the output in a listing.

The readlines() office reads until the Stop of the file, making use of readline() function internally and returns a list with all the lines read from the file.

Hither is a working example to read all the lines from the file using readlines().

The file that nosotros are going to make use of to read is test.txt. The contents of the file test.txt are as follows:

test.txt: Save the file test.txt and use the location of test.txt inside open() function.

Line No 1 Line No 2 Line No 3 Line No 4 Line No v          

myfile = open("examination.txt", "r") mylist = myfile.readlines() impress(mylist) myfile.close()          

Output:

['Line No 1\n', 'Line No 2\n', 'Line No 3\n', 'Line No iv\n', 'Line No 5']          

How to read a File line-by-line using for loop?

Following are the steps to read a line-by-line from a given file using for-loop:

Step1 :

Commencement, open the file using Python open() part in read way.

Step 2:

The open() part will return a file handler. Employ the file handler inside your for-loop and read all the lines from the given file line-by-line.

Step three:

One time done, close the file handler using the close() role.

Here is a working example of using for-loop to read line-by-line from a given file. The file that we are going to use hither is test.txt.

The contents of test.txt are every bit shown below. Save the file exam.txt and use the location of test.txt within an open() function.

Line No 1 Line No 2 Line No 3 Line No 4 Line No five          

myfile = open up("test.txt", "r") for line in myfile:     print(line) myfile.close()          

Output:

Line No 1 Line No 2 Line No iii Line No 4 Line No five          

How to read a File line-by-line using a while loop?

You tin can brand utilise of a while loop and read the contents from the given file line-by-line. To do that, first, open the file in read way using open() function. The file handler returned from open(), use it inside while –loop to read the lines.

Python readline() function is used within while-loop to read the lines. In the case of for-loop, the loop terminates when the end of the file is encountered. But the same is non the case with a while-loop, and yous need to keep a check to encounter if the file is done reading. So in one case the readline() function returns an empty string, you tin make utilize of the suspension statement to stop from the while –loop.

Here is a working example to read a file line by line using a while-loop.

The file that we are going to brand use is test.txt .Save the file test.txt and use the location of test.txt inside open up() function.

Line No 1 Line No two Line No 3 Line No 4 Line No 5          

myfile = open("test.txt", "r") while myfile:     line  = myfile.readline()     impress(line)     if line == "":         suspension myfile.close()          

Output:

Line No 1 Line No ii Line No iii Line No 4 Line No 5          

Summary

• Python readline() is a file method that helps to read 1 complete line from the given file. It has a abaft newline ("\due north") at the end of the string returned.
• You tin can besides brand employ of the size parameter to get a specific length of the line. The size parameter is optional, and by default, the unabridged line will be returned.
• The readline() method helps to read simply one line at a time, and information technology returns the kickoff line from the file given. We will make use of readline() to read all the lines from the file given.
• To read all the lines from a given file, you lot tin can brand employ of Python readlines() function. The specialty of Python readlines() function is that it reads all the contents from the given file and saves the output in a list.
• The readlines() part reads till the End of the file making apply of readline() office internally and returns a list that has all the lines read from the file.
• It is possible to read a file line by line using for loop. To do that, start, open the file using Python open() function in read mode. The open() function will render a file handler. Use the file handler inside your for-loop and read all the lines from the given file line by line. Once done,close the file handler using close() function.
• You can brand use of a while loop and read the contents from the given file line by line. To do that, offset, open the file in read mode using open() function. The file handler returned from open(), use it inside while –loop to read the lines. Python readline() function is used within while-loop to read the lines.

brandonhationlove.blogspot.com

Source: https://www.guru99.com/python-file-readline.html

Share this post