Q.Write a program to accept string/sentences from the user till the user enters "END". Save the data in a text file and then display only those sentences which begin with an uppercase alphabet.

This question requires writing a Python program to handle file input/output and apply a filtering condition.

def process_sentences():
    file_name = "sentences.txt"

    # Part 1: Accept input and save to file
    print("Enter sentences. Type 'END' (in uppercase) to stop.")
    with open(file_name, 'w') as file:
        while True:
            sentence = input("> ")
            if sentence == "END":
                break
            file.write(sentence + '\n')
    print(f"\nAll entered sentences saved to '{file_name}'.")

    # Part 2: Read from file and display filtered sentences
    print("\nSentences beginning with an uppercase alphabet:")
    try:
        with open(file_name, 'r') as file:
            found_any = False
            for line in file:
                stripped_line = line.strip() # Remove leading/trailing whitespace, including newline
                if stripped_line and stripped_line[0].isupper():
                    print(stripped_line)
                    found_any = True
            if not found_any:
                print("No sentences found that begin with an uppercase alphabet.")
    except FileNotFoundError:
        print(f"Error: The file '{file_name}' was not found.")
    except Exception as e:
        print(f"An error occurred while reading the file: {e}")

# Call the function to run the program
process_sentences()

Explanation of Key Lines:

1. with open(file_name, 'w') as file:: This line opens the file named sentences.txt in write mode ('w'). The with statement ensures the file is automatically closed even if errors occur. If the file doesn't exist, it will be created; if it exists, its content will be truncated (emptied) before writing.
2. sentence = input("> "): This prompts the user to enter a sentence and stores it in the sentence variable.
3. if sentence == "END": break: This checks if the user input is exactly "END". If it is, the while loop terminates, stopping further input.
4. file.write(sentence + '\n'): This writes the entered sentence to the file. '\n' is appended to ensure each sentence is written on a new line in the file.
5. with open(file_name, 'r') as file:: This re-opens the same file, sentences.txt, but this time in read mode ('r') to retrieve its contents.
6. for line in file:: This loop iterates through each line in the opened file. Each line variable will include the newline character (\n) at the end.
7. stripped_line = line.strip(): The strip() method is crucial here. It removes any leading or trailing whitespace characters (including spaces, tabs, and the newline character \n) from the line. This ensures that the check for the first character is accurate and the printed output is clean. …

When working with files in programming, a common task is to read data from the user, store it persistently, and then process that stored data. This problem combines several fundamental concepts:

1. File Input/Output (I/O): How to open a file, write data to it, and read data from it.
2. User Input: How to continuously accept input from the user until a specific sentinel value is provided.
3. String Manipulation: How to check properties of a string, specifically its first character.

The core idea is to first gather all the user's input and commit it to a file. This ensures that even if the program stops, the data remains. Then, in a separate step, we read back from this file, line by line, and apply a filtering condition (starting with an uppercase letter) before displaying the result.

Here's the Python program to achieve this:

# Define the filename for storing sentences
filename = "user_sentences.txt"

# --- Part 1: Accept input from the user and save to file ---

print("Please enter sentences. Type 'END' (in uppercase) when you are finished.")

# Open the file in write mode ('w').
# The 'with' statement ensures the file is automatically closed even if errors occur.
with open(filename, 'w') as file_writer:
    while True:
        user_input = input() # Accept a line of input from the user
        if user_input == "END":
            break # Exit the loop if the user types "END"
        
        # Write the accepted sentence to the file, followed by a newline character.
        # This ensures each sentence is on a new line in the file.
        file_writer.write(user_input + '\n')

print(f"\nAll sentences (excluding 'END') have been saved to '{filename}'.")

# --- Part 2: Read from the file and display sentences starting with an uppercase alphabet ---

print("\nDisplaying sentences from the file that begin with an uppercase alphabet:")

# Open the file in read mode ('r').
with open(filename, 'r') as file_reader:
    for line in file_reader:
        # The .strip() method removes leading/trailing whitespace, including the newline character
        # that is typically read from the end of each line in a text file.
        sentence = line.strip()
        
        # Check if the sentence is not empty and its first character is an uppercase letter.
        # An empty string check (`if sentence`) prevents an IndexError if a line was just a newline.
        if sentence and sentence[0].isupper():
            print(sentence)

Explanation of Key Lines:

1. filename = "user_sentences.txt":

   • This line defines a string variable filename to store the name of the text file. Using a variable makes it easy to change the filename later if needed.

2. with open(filename, 'w') as file_writer::

   • This is the standard and recommended way to open files in Python.
   • open() function is used to establish a connection to the file.
   • filename specifies which file to open.
   • 'w' is the mode for opening the file. It stands for "write mode". If the file user_sentences.txt does not exist, it will be created. If it already exists, its contents will be truncated (erased) before writing new data.
   • as file_writer assigns the file object to the variable file_writer.
   • The with statement acts as a context manager. It automatically handles closing the file (file_writer.close()) once the block of code inside with is exited, even if an error occurs. This prevents resource leaks.

3. user_input = input():

   • The input() function reads a line of text entered by the user from the console and returns it as a string.

4. if user_input == "END": break:

   • This condition checks if the user has typed the specific string "END". If true, the break statement immediately terminates the while loop, stopping further input collection.

5. file_writer.write(user_input + '\n'):

   • The write() method of the file object writes the given string to the file.
   • We append '\n' (newline character) to user_input. This is crucial because input() does not include the newline character, and without it, all sentences would be written on a single line in the file, making them difficult to read back as individual sentences.

6. with open(filename, 'r') as file_reader::

   • This opens the same file, but this time in 'r' (read mode). This mode allows us to read the contents of the file without modifying it.

7. for line in file_reader:: …