Python File Operation

Files

Files are objects on a computer that stores data. They are used to permanently store data in non-volatile memory (hard disk).

As we know, RAM (Random Access Memory) is volatile, which loses its data when the computer is shut down. This is why we use files to store data permanently.

Before reading from or writing to a file, we need to open it first. After finishing our operation with a file, we need to close it so that the resources attached to the file are released.

In Python, to interact with a file, we need to follow the following steps:

1. Open a file
2. Read from or write to a file
3. Close the file

Opening Files in Python

The built-in open() function is used to open a file. It returns a file used to read or modify the file accordingly.

In the following example, we will open a file by specifying the file in the current directory and the full path of the file.

>>> f = open("demo.txt")  # open file in current directory
>>> f = open("/home/expectocode/example.txt") # specifying full path

The open() function takes two parameters, filename and mode.

There are four different modes for opening a file.

Also, you can specify if the file should be handled as binary or text mode.

Syntax

You can specify only the name of the file to open it.

>>> f = open("demo.txt")

The above code is the same as:

>>> f = open("demo.txt", "rt")

The above code uses the "r" (read) and "t" (text), which are the default mode values, so we do not need to specify them.

Note:

• When opening a file with "r" (read) mode, make sure the file exists, or else you will get an error.
• When opening a file with "x" (create) mode, make sure the file does not exist, or else you will get an error.

File encodig

In Python, the character d does not imply the number 100 until it is encoded using ASCII (or other equivalent encodings).

The default encoding in python is platform-dependent. In Linux is utf-8, and in Windows is cp1252.

So when working with files in text mode, it is recommended to specify the encoding type.

f = open("demo.txt", mode='r', encoding='utf-8')

Note: You must not rely on the default encoding, or else your code will behave differently in different platforms.

Closing Files in Python

After finishing working with a file, we need to close the file correctly.

After closing a file, the resources that were attached to the file become free.

To close a file, python provides the close() method.

Python has a garbage collector to clean up unreferenced objects, how you must not rely on it to close the file.

f = open("demo.txt", encoding = "utf-8")
# close a file
f.close()

When interacting with a file, an exception can occur. When it does, the code will exists without closing the file.

To overcome exceptions raised when interacting with files, we use the try...finally block.

try:
    f = open("demo.txt", encoding = "utf-8")
finally:
    f.close()

In the above code, using the try...finally block, we ensure that the file is closed correctly even if an exception is raised.

The best approach to close a file is by using the with statement. This guarantees that the file is closed when the block inside the with statement is exited.

Using the with statement, we do not need to explicitly call the close() method.

with open("demo.txt", encoding = "utf-8") as f:
    # perform file operations

Writing to Files in Python

Before writing to a file, we need to open the file with one of the following modes: w (write), a append, or exclusive x (creation) mode.

Opening a file with the w mode will overwrite the file if it already exists. So be cantions using this mode because all the previous data will be rased.

To write a string or sequence of bytes (for binary files) into a file, we use the write() method. The write() method returns the number of characters written to the file.

with open("demo.txt", "w", encoding = "utf-8") as f:
    f.write("the first line\n")
    f.write("the second line\n")
    f.write("the last line\n")

In the above code, we created a file named demo.txt in the current directory. We used the w mode to open the file, so if the file does not exist, it will be created; otherwise, if it does exist, it will be overwritten.

We can also notice that we used the \n to include a new line.

Reading Files in Python

Before reading a file in Python, we must open the file with r (reading) mode.

Python provides different methods to read a file. We can use the read(size) method to read in the size number of data. If the size parameter is not specified, the read() method will read and return up to the end of the file.

In the following example, we will read the demo.txt file we created in the above section.

with open("demo.txt", "r", encoding = "utf-8") as f:
    print(f.read(3)) # read the first 3 data
    print(f.read(6)) # read the next 6 data
    print(f.read()) # read in the rest to the end of file
    print(f.read()) # further reading returns empty string

Output

the
 first
 line
the second line
the last line

As we can see above, the read() method returns a newline as it encounters a \n. After reaching the end of the file, we get an empty string on further reading.

To change the current file cursor (position), we can use the seek() method. The tell() method returns our current position (in a number of bytes).

with open("demo.txt", "r", encoding = "utf-8") as f:
    print(f.tell()) # get the current file position
    print(f.read())
    print(f.tell())
    print(f.seek(0)) # bring the cursor to the initial position

Output

0
the first line
the second line
the last line

45
0

We can also read a file using a for loop, which is a fast and efficient way to read a file.

with open("demo.txt", "r", encoding = "utf-8") as f:
    for line in f:
        print(line, end = "")

Output

the first line
the second line
the last line

In the above code, the lines in the file include a newline character \n. This is why we used the end parameter of the print() function to avoid two newlines when printing.

We can also use the readline() method to read individual lines of a file. The readline() method reads a file to the newline, including the newline character.

with open("demo.txt", "r", encoding = "utf-8") as f:
    print(f.readline(), end = "")
    print(f.readline(), end = "")
    print(f.readline(), end = "")

Output

the first line
the second line
the last line

The readlines() method returns a list of the remaining lines of the entire file.

with open("demo.txt", "r", encoding = "utf-8") as f:
  print(f.readlines())

Output

['the first line\n', 'the second line\n', 'the last line\n']

Note: All the reading methods return empty values when the end of file EOF is reached.

Python File Methods

Python provides various methods to interact with a file.

Here is a complete list of methods in text mode with a short description.