Python File I/O - Read and Write Files

In Python, the IO module provides methods of three types of IO operations; raw binary files, buffered binary files, and text files. The canonical way to create a file object is by using the open() function.

Any file operations can be performed in the following three steps:

1. Open the file to get the file object using the built-in open() function. There are different access modes, which you can specify while opening a file using the open() function.
2. Perform read, write, append operations using the file object retrieved from the open() function.
3. Close and dispose the file object.

Reading File

File object includes the following methods to read data from the file.

• read(chars): reads the specified number of characters starting from the current position.
• readline(): reads the characters starting from the current reading position up to a newline character.
• readlines(): reads all lines until the end of file and returns a list object.

The following C:\myfile.txt file will be used in all the examples of reading and writing files.

This is the first line. 
This is the second line.
This is the third line.

The following example performs the read operation using the read(chars) method.

f = open('C:myfile.txt') # opening a file
lines = f.read() # reading a file
print(lines)  #'This is the first line. 
This is the second line.
This is the third line.'
f.close() # closing file object

Above, f = open('C:\myfile.txt') opens the myfile.txt in the default read mode from the current directory and returns a file object.f.read() function reads all the content until EOF as a string. If you specify the char size argument in the read(chars) method, then it will read that many chars only.f.close() will flush and close the stream.

Reading a Line

The following example demonstrates reading a line from the file.

f = open('C:myfile.txt') # opening a file
line1 = f.readline() # reading a line
print(line1)  #'This is the first line. 
'

line2 = f.readline() # reading a line
print(line2)  #'This is the second line.
'

line3 = f.readline() # reading a line
print(line3)  #'This is the third line.'

line4 = f.readline() # reading a line
print(line4)  #''

f.close() # closing file object

As you can see, we have to open the file in 'r' mode. The readline() method will return the first line, and then will point to the second line in the file.

Reading All Lines

The following reads all lines using the readlines() function.

f = open('C:myfile.txt') # opening a file
lines = f.readlines() # reading all lines
print(lines)  #'This is the first line. 
This is the second line.
This is the third line.'
f.close() # closing file object

The file object has an inbuilt iterator. The following program reads the given file line by line until StopIteration is raised, i.e., the EOF is reached.

f=open('C:myfile.txt')
while True:
    try:
        line=next(f)
        print(line)
    except StopIteration:
        break
f.close()

Use the for loop to read a file easily.

f=open('C:myfile.txt')
for line in f:
    print(line)
f.close()

This is the first line. This is the second line. This is the third line.

Reading Binary File

Use the 'rb' mode in the open() function to read a binary files, as shown below.

f = open('C:myimg.png', 'rb') # opening a binary file
content = f.read() # reading all lines
print(content) #print content
f.close() # closing file object

Writing to a File

The file object provides the following methods to write to a file.

• write(s): Write the string s to the stream and return the number of characters written.
• writelines(lines): Write a list of lines to the stream. Each line must have a separator at the end of it.

Create a new File and Write

The following creates a new file if it does not exist or overwrites to an existing file.

f = open('C:myfile.txt','w')
f.write("Hello") # writing to file
f.close()

# reading file
f = open('C:myfile.txt','r') 
f.read()  #'Hello'
f.close()

In the above example, the f=open("myfile.txt","w") statement opens myfile.txt in write mode, the open() method returns the file object and assigns it to a variable f.'w' specifies that the file should be writable. Next, f.write("Hello") overwrites an existing content of the myfile.txt file. It returns the number of characters written to a file, which is 5 in the above example. In the end, f.close() closes the file object.

Appending to an Existing File

The following appends the content at the end of the existing file by passing 'a' or 'a+' mode in the open() method.

f = open('C:myfile.txt','a')
f.write(" World!")
f.close()

# reading file
f = open('C:myfile.txt','r') 
f.read()  #'Hello World!'
f.close()

Write Multiple Lines

Python provides the writelines() method to save the contents of a list object in a file. Since the newline character is not automatically written to the file, it must be provided as a part of the string.

lines=["Hello world.
", "Welcome to TutorialsTeacher.
"]
f=open("D:myfile.txt", "w")
f.writelines(lines)
f.close()

Opening a file with "w" mode or "a" mode can only be written into and cannot be read from. Similarly "r" mode allows reading only and not writing. In order to perform simultaneous read/append operations, use "a+" mode.

Writing to a Binary File

The open() function opens a file in text format by default. To open a file in binary format, add 'b' to the mode parameter. Hence the "rb" mode opens the file in binary format for reading, while the "wb" mode opens the file in binary format for writing. Unlike text files, binary files are not human-readable. When opened using any text editor, the data is unrecognizable.

The following code stores a list of numbers in a binary file. The list is first converted in a byte array before writing. The built-in function bytearray() returns a byte representation of the object.

f=open("binfile.bin","wb")
num=[5, 10, 15, 20, 25]
arr=bytearray(num)
f.write(arr)
f.close()