Question

Write a program that replaces each line of a file with its reverse. For example, if you run python reverse.py hello.py then the contents of hello.py are changed to \- margorp nohtyp tsrif y )"Idiroin , o11et"(tnirp Of course, if you run Reverse twice on the same file, you get back the original file.

Short answer

The program reads each line of the file, reverses it, and writes it back to the file.

Step-by-step solution

Open File

Begin by opening the file that you need to reverse. Use Python's built-in function `open()`, which will allow you to read the lines one by one. For example: ```python with open('hello.py', 'r') as file: lines = file.readlines() ``` This reads all lines from 'hello.py' into a list called `lines`.

Reverse Each Line

For each line in the list `lines`, reverse the characters. You can achieve this by using Python slicing. For example: ```python reversed_lines = [line[::-1] for line in lines] ``` This creates a new list `reversed_lines` where each element is the reverse of the corresponding element in `lines`.

Write Reversed Lines Back to File

Open the file again in write mode and overwrite it with the reversed lines. Use `open()` with the write mode ('w') and use the `writelines()` method to write the reversed content back to the file. ```python with open('hello.py', 'w') as file: file.writelines(reversed_lines) ``` This writes all the reversed lines back to 'hello.py', effectively reversing the contents of the file.

Run the Program

Save your Python script, e.g., `reverse.py`. Run the program using Python in the terminal or command prompt by executing: ```shell python reverse.py hello.py ``` This command will modify 'hello.py', replacing each line with its reversed form, as explained in earlier steps.

Key concepts

File Reading

Reading from a file in Python is a straightforward process that involves using the built-in `open()` function. When dealing specifically with reading tasks, you typically open the file in 'read' mode using 'r'. This allows you to access the file's contents without making any modifications.

Here's how reading works in the context of our exercise:

• First, we open the file using a `with` statement. This ensures that the file is properly closed after its suite finishes execution, even if an exception is raised.
• The `readlines()` method is used to read all the lines of the file at once. This method returns a list of strings, where each string represents a line in the file.

For instance: ```python with open('hello.py', 'r') as file: lines = file.readlines() ``` This snippet opens the file 'hello.py' and reads each line into a list called `lines`. This approach provides a simple way to handle multiple lines and helps us manipulate file content easily.

File Writing

Writing back to a file is as crucial as reading from it. Python provides seamless support for writing data to files using the `open()` function, configured in 'write' mode ('w'). When you open a file in write mode, it clears existing contents, so anything written to the file overwrites previous data.

In the exercise, after reversing the lines, you need to open the file for writing:

• Use the `open()` function with 'w' to enable writing.
• The `writelines()` method then writes each line from our list back into the file.

Here's an example based on our solution: ```python with open('hello.py', 'w') as file: file.writelines(reversed_lines) ``` This code snippet opens 'hello.py' and writes the list `reversed_lines` back to it. Each item in the list, representing a reversed line, is written to the file, effectively overwriting its contents with the new, reversed content. It's essential to remember that any previous data is lost when using 'write' mode.

String Manipulation

String manipulation is a fundamental skill in Python, particularly useful when transforming text data. For our exercise, the primary string manipulation task involves reversing strings, which can be efficiently accomplished using slicing.

Python slicing offers a concise syntax to reverse a string:

• The slice notation `[start:stop:step]` allows us to define how to access elements in a sequence.
• By specifying a step of `-1`, Python iterates over the string in reverse order.

Consider the example: ```python reversed_line = line[::-1] ``` This line of code takes an existing `line`, and the slice `[::-1]` returns a new string where the characters are in reverse order. This technique is not only elegant but also efficient for reversing lines, as it is done in a single line of code.

Mastering string slicing for reversing strings is a handy tool, especially when you need quick string transformations like the one used in our file reversal exercise.