Python: Reading and Writing Files

Reading and Writing Files

Python has several functions for creating, reading, updating, and deleting files.

Syntax

To open a file for reading it is enough to specify the name of the file:

f = open("file.txt")

The code above is the same as:

f = open("file.txt", "rt")

Because “r” for read, and “t” for text are the default values, you do not need to specify them.

Mode can be

• “r” - Read - Default value. Opens a file for reading, error if the file does not exist
• “a” - Append - Opens a file for appending, creates the file if it does not exist
• “w” - Write - Opens a file for writing, creates the file if it does not exist
• “x” - Create - Creates the specified file, returns an error if the file exists

In addition you can specify if the file should be handled as binary or text mode

• “t” - Text - Default value. Text mode
• “b” - Binary - Binary mode (e.g. images)

It is good practice to use the with keyword when dealing with file objects. The advantage is that the file is properly closed after its suite finishes.

>>> with open('file.txt') as f:
...     read_data = f.read()

If you’re not using the with keyword, then you should call f.close() to close the file and immediately free up any system resources used by it.

Methods of File Objects

To read a file’s contents, call f.read().

>>> f = open('device')
>>> print(f.read())
192.168.10.10
192.168.10.11
>>> f.close()

f.readline() reads a single line from the file.

>>> f = open('device')
>>> print(f.readline())
192.168.10.10
>>> print(f.readline())
192.168.10.11
>>> f.readline()
# empty line
>>> f.close()

For reading lines from a file, you can loop over the file object. This is memory efficient, fast, and leads to simple code:

>>> f = open('device')
>>> for ip in f:
...     print(ip)
... 
192.168.10.10

192.168.10.11

If you want to read all the lines of a file in a list you can also use f.readlines().

>>> f = open('device')
>>> print(f.readlines())
['192.168.10.10\n', '192.168.10.11\n', '192.168.10.12\n'] # return a list

Write to an Existing File

To write to an existing file, you must add a parameter to the open() function:

• “a” - Append - will append to the end of the file
• “w” - Write - will overwrite any existing content

f.write(string) writes the contents of string to the file, returning the number of characters written.

>>> f = open('device', 'a') # file will open in append mode
>>> f.write('192.168.10.12') # this line will add as last line in file
>>> f.close()

>>> f = open('device')
>>> for ip in f:
...     print(ip)
... 
192.168.10.10

192.168.10.11

192.168.10.12

Create a New File

To create a new file in Python, use the open() method, with one of the following parameters:

• “x” - Create - will create a file, returns an error if the file exist
• “a” - Append - will create a file if the specified file does not exist
• “w” - Write - will create a file if the specified file does not exist

Example

Create a file called “file.txt”:

f = open("file.txt", "x")

Result: a new empty file is created!

Create a new file if it does not exist:

f = open("file.txt", "w")

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