Python Tutorial - Readline and Readlines Functions

As mentioned in the previous chapter, if you want to read the contents of a file opened with the open function, you can use the readline and readlines functions in addition to the read function.

Unlike the read function, these two functions use "line" as the reading unit, which reads one line in the target file at a time. For reading files opened in text format, reading one line is well understood; for reading files opened in binary format, they will use "\ n" as a sign to read one line.

Python Readline Function

The readline function is used to read a line in the file, including the last newline character "\ n". The basic syntax of this function is:


Among them, file is an open file object; size is an optional parameter used to specify the maximum number of characters (bytes) to be read at a time when each line is read.

As with the read function, the prerequisite for this function to successfully read the file data is that the use of the "Python open function" specifies that the mode of opening the file must be readable (including r, rb, r +, and rb + 4).

Taking the file a.txt created in the previous chapter as an example, the file has the following two lines of data:

Python tutorial:

The following program demonstrates the specific use of the readline function:

f = open ("a.txt")
#Read a line of data
byt = f.readline ()
print (byt)
#close the file

The output is:

Python tutorial:

Because the readline () function reads the last newline character "\ n" when reading the content of a line in the file, plus the print function outputs a newline by default, so you will see more in the output. A blank line.

Not only that, you can also limit the maximum number of characters (bytes) that can be read when reading line by line, for example:

#Open the specified file in binary form
f = open ("a.txt", 'rb')
byt = f.readline (6)
print (byt)
#close the file

The output is:


Compared with the output of the previous example, because there is no complete read of one line of data, no newline character will be read.

Python Readlines Function

The readlines function is used to read all the lines in the file. It is similar to calling the read function without specifying the size parameter, except that the function returns a list of strings, where each element is a line in the file.

Like the readline function, the readlines function reads each line with a newline character at the end of the line.

The basic syntax of the readlines function is as follows:


Where file is an open file object. Like the read and readline functions, it requires that the mode of opening the file must be readable (including r, rb, r +, and rb + 4).


f = open("a.txt",'rb')
byt = f.readlines()

The output is:

b'Python tutorial: \r\n', b''


More Tutorials:

Python Installation - Linux (Ubuntu)
Python Installation - Mac OS
Integrated Development Environment - IDE
Python - Basic Variables
Python - Sequence Introduction
Python - Output Formatting
Python - Escape Character
Python - Type Conversion
Python - Numbers
Python – Arithmetic Operators
Python – Assignment Operators
Python – Comparison Operators
Python – Logical Operators
Python – Precedence and Associativity Operators
Python – Bytes Type and Usage
Python – Long & Raw Strings
Python – Concatenate Function
Python – Slice Function
Python – Length and Split Function
Python – Join and Count Function
Python – Find Function
Python – Index Function
Python – Alignment Function
Python – Startswith and Endswith Function
Python – String Case Conversion
Python – Remove Specified Character
Python – Encode and Decode Function
Python – dir and help Function
Python – Input Output Introduction
Python – Basic Operation
Python – Open Function
Python – Read Function
Python – Readline Function
Python – Write Function
Python – Close Function
Python – Seek and Tell Function
Python – Pickle Module
Python - File Input Module and Linecache Module
Python - Pathlib Module
Python - Pathlib Module
Python - os.path Module
Python - fnmatch Module
Python - Tuple Introduction
Python - List Introduction
Python - List Add Element
Python - List Delete Element
Python - List Modification Element
Python - List Find Element
Python - Dictionary Introduction
Python - Dictionary Basic Operation
Python - Dictionary Method Guide
Python - Set Collection
Python - Set Basic Operation
Python - Set Basic Method
Python - Frozenset Method
Python - If Condition I
Python - If Condition II
Python - While loop
Python - For loop
Python - Pass Statement
Python - Break Statement
Python - Zip Reverse Function
Python - Function Introduction
Python - Positional Parameters
Python - Key Arguments
Python - None and Return
Python - Variable Scope
Python - Local Function
Python - Closure Method
Python - Lamdba Expression

More Python Exercises:

Python String Exercises
Python List Exercises
Python Library Exercises
Python Sets Exercises
Python Array Exercises
Python Condition Statement Exercises