Python file readline() function can read a line of file, in this tutorial, we will introduce how to use function and some tips on using it.
Syntax
Python file readline() function is defined as:
fileObject.readline( size );
Parameter
size: the count of bytes from the line you want to read once.
If you do not set a size, fileObject.readline() will only read a whole line once.
Create a txt file
We create a filedemo.txt firstly.
line 1 line 2 line 3 line 4 line 5
Read a line without setting line size
line = ''
with open('filedemo.txt', 'r') as fin:
line = fin.readline()
print(type(line))
print(line)
The line content is:
<class 'str'> line 1
From the line content we will find, if you do not set a line size, you will read a whole line content.
Read line with setting line size
line = ''
with open('filedemo.txt', 'r') as fin:
size = 3
line = fin.readline(size)
print(type(line))
print(line)
The line content is:
<class 'str'> lin
From the line content we will find: if you have set the size = 3, readline(size) will only read 3 bytes from the beginning of a line.
Read a line with negative size
line = ''
with open('filedemo.txt', 'r') as fin:
size = -1
line = fin.readline(size)
print(type(line))
print(line)
The line content is:
<class 'str'> line 1
From the result, we will find if line size is a negative (-1) number, this function will return the whole line.
How to read all lines of a file
fileObject.readline() only can read one line of a file, how to read all lines?
Here is an example:
with open('filedemo.txt', 'r') as fin:
for line in fin:
print(type(line))
print(line)
The file content is:
<class 'str'> line 1 <class 'str'> line 2 <class 'str'> line 3 <class 'str'> line 4 <class 'str'> line 5
Here are some tips you should notice:
1.If you read a file with a binary mode, fileObject.readline() function will return a byte like object, not a string object.
line = ''
with open('filedemo.txt', 'rb') as fin:
line = fin.readline()
print(type(line))
print(line)
The first line is:
<class 'bytes'> b'line 1\r\n'
2.When this function return the whole line, the new line symbol (\r or \n) is also be returned. You can find the truth from the above example.