B2.5 File processing

The program must add a new reading each minute without losing any previously saved readings. This means each run of the write operation must preserve existing file content.

Append mode ("a") opens the file and positions the write pointer at the end, so new data is added without disturbing anything already stored. If the file doesn't exist yet, it is created.

Write mode ("w") would erase all previous readings every minute — catastrophic for a logging scenario. Read mode ("r") does not allow writing at all.

The correct mode is "a" (append) because it adds new data to the end of the file each time, preserving the full history of readings.

The question asks which file mode preserves existing data while still allowing new data to be written every minute.

"r" (read mode) does not allow writing. Attempting to write in read mode would raise an error, so this option is incorrect.

"w" (write mode) silently deletes all existing content before writing. Every minute, all previous readings would be lost — this contradicts the requirement.

"rw" is not a standard Python file mode. Python uses "r+" for read-write, and it is not the appropriate pattern for simple sequential logging.

"a" (append mode) is the only option that adds new content to the end of the file without erasing existing data, making it the correct choice.

This question tests understanding of Python's context manager (with statement) and why it is the recommended way to open files.

Python's with statement implements a context manager protocol. When the indented block exits — whether normally or due to an exception — the file's __exit__ method is called, which automatically closes the file handle.

Without with, if an exception occurs before f.close() is called manually, the file remains open. This can cause resource leaks and potential data loss, particularly in Java where buffered writes may not be flushed.

The correct answer is that the with statement automatically closes the file when the block exits, even if an exception is thrown inside the block.

The question asks for the primary advantage of the with statement over manually opening a file — focusing on resource management.

"reads the entire file faster" is incorrect. The with statement is a resource management tool — it does not change the speed of file I/O operations.

"allows both reading and writing simultaneously" is incorrect. The mode (read or write) is determined by the mode string argument ("r", "w", etc.), not by the with statement.

"prevents FileNotFoundError" is incorrect. The with statement does not suppress exceptions. A FileNotFoundError will still be raised if the file doesn't exist — you still need try/except for that.

The correct answer is automatic file closing — the with statement guarantees the file is closed when the block exits, even if an error occurs inside it.