Inplace Editing using Python FileInput

Inplace editing is a technique that allows us to modify the contents of a file directly, without the need for creating a new file or loading the entire file into memory. It offers a streamlined approach to file manipulation by allowing us to make changes directly to the existing file, making it an efficient and resource-friendly method.

To facilitate inplace editing in Python, the fileinput module comes into play. The fileinput module, part of the Python standard library, provides a high-level interface for reading and writing files, streamlining the process of inplace editing. With the fileinput module, we can open a file for inplace editing, iterate over its lines, make modifications, and save the changes directly to the file.

The Fileinput Module

The fileinput module in Python provides a convenient and efficient way to perform inplace editing of files. It abstracts the complexities of file handling and iteration, making it easier to modify file contents directly.

Before we can start using the fileinput module, we need to import it into our Python script. We can do this by including the following import statement at the beginning of our code −

import fileinput

Opening a File for Inplace Editing

To open a file for inplace editing, we use the fileinput.input() function. This function takes the file path as an argument and returns an instance of the fileinput.FileInput class, which provides the necessary methods for reading and modifying the file.

To open a file for inplace editing, we need to provide the file path as an argument to the fileinput.input() function. For example, let's say we have a file named "example.txt" that we want to edit in place. We can open it for inplace editing using the following code −

with fileinput.input('example.txt', inplace=True) as f:
   for line in f:
      # Modify the line as needed

In the code above, we open the file "example.txt" for inplace editing by passing its path to the fileinput.input() function. The inplace=True argument tells the fileinput module to modify the file in place.

Suppose we have the following content in the "example.txt" file −

This is a sample file.
It contains some foo words.

Let's perform inplace editing using Python.

Example 1 

Adding a prefix to each line

with fileinput.input('example.txt', inplace=True) as f:
   for line in f:
      modified_line = "Prefix: " + line
      print(modified_line, end='')

Output

Prefix: This is a sample file.
Prefix: It contains some foo words.
Prefix: Let's perform inplace editing using Python.

Example 2 

Removing leading and trailing whitespace

with fileinput.input('example.txt', inplace=True) as f:
   for line in f:
      modified_line = line.strip()
      print(modified_line, end='')

Output

This is a sample file.
It contains some foo words.
Let's perform inplace editing using Python.

Modifying the File

Once the file is opened for inplace editing, we can iterate over its lines using a for loop. The fileinput.FileInput object returned by fileinput.input() behaves like a regular file object and can be used in a loop to access each line of the file.

Here's an example of iterating over the lines in the file −

with fileinput.input('example.txt', inplace=True) as f:
   for line in f:
      # Modify the line as needed

For example, let's say we want to replace all occurrences of the word "foo" with "bar" in each line. We can achieve this by using the replace() method:

with fileinput.input('example.txt', inplace=True) as f:
   for line in f:
      modified_line = line.replace('foo', 'bar')

In the code above, we replace occurrences of the string 'foo' with 'bar' in each line, storing the modified line in the modified_line variable. You can modify the line using any desired logic or operations, depending on your specific use case. Here are a few examples to illustrate some common modifications.

with fileinput.input('example.txt', inplace=True) as f:
   for line in f:
      # Example 1: Adding a prefix to each line
      modified_line = "Prefix: " + line

      # Example 2: Removing leading and trailing whitespace
      modified_line = line.strip()

      # Example 3: Reversing the line
      modified_line = line[::-1]

      # Example 4: Converting to uppercase
      modified_line = line.upper()

Suppose we use the same content in the "example.txt" file as previously taken −

This is a sample file.
It contains some foo words.
Let's perform inplace editing using Python.

Example 3

Reversing the line

with fileinput.input('example.txt', inplace=True) as f:
   for line in f:
      modified_line = line[::-1]
      print(modified_line, end='')

Output

elif elpmis elgnitedi ecnetneidni repmetS
.sdrow oof emos sniatnoc tnuocnoc sI
.enilpmeb saw elpmis a si sihT

Example 4

 Converting to uppercase

with fileinput.input('example.txt', inplace=True) as f:
   for line in f:
      modified_line = line.upper()
      print(modified_line, end='')

Output

THIS IS A SAMPLE FILE.
IT CONTAINS SOME FOO WORDS.
LET'S PERFORM INPLACE EDITING USING PYTHON.

These examples demonstrate different modifications that can be applied to each line of the file using inplace editing.

Saving the Modified File

Once we have made the necessary modifications to each line, we need to save the changes back to the file. The fileinput module provides a convenient way to achieve this using the print() function.

To save the changes made to a line, we use the print() function and pass the modified line as an argument. By default, the print() function writes the output to the standard output (usually the console). However, in the context of inplace editing, we want to redirect the output back to the file.

To accomplish this, we need to pass the print() function an additional argument: end=''. This argument ensures that no additional newline characters are added to the output, which would interfere with the file's original formatting.

Here's an example of using print() to write the modified line back to the file −

with fileinput.input('example.txt', inplace=True) as f:
   for line in f:
      modified_line = line.replace('foo', 'bar')
      print(modified_line, end='')

In the code above, the print(modified_line, end='') statement writes the modified line back to the file opened for inplace editing.

It's essential to note that when using the print() function to write the modified line back to the file, we need to ensure proper handling of newline characters. By default, the print() function adds a newline character at the end of each line. In inplace editing, we want to preserve the original line endings.

To achieve this, we use the end='' argument, which instructs print() not to add any additional characters at the end of the line. This way, we maintain the original line endings as they were in the file.

Conclusion

In this article, we looked at the various forms of inplace editing using the fileinput module in Python. We explored how to open a file for inplace editing, make modifications to its contents, and save the modified file. Now, you will have a solid understanding of how to leverage inplace editing efficiently, empowering you to handle file manipulation tasks more effectively. However, it's important to exercise caution when using inplace editing, as any mistakes or unintended changes can have permanent effects on the original file.