**Answer** A good way to write a test‑case for a `while` loop that reads a file is to 1. **Create a temporary file** that contains the data you want to test. 2. **Pass the file name to the program** (or redirect the file into the program). 3. **Run the program** and capture its output. 4. **Assert that the output is what you expect**. Below is a concrete example that uses the `tempfile` module to create a temporary file, writes some test data to it, runs the program that contains the `while` loop, and checks that the output is correct. The example uses `pytest` and `subprocess` to execute the program. ```python # test_read_file.py import subprocess import tempfile import os def test_read_file_loop(tmp_path): """ Test that the program correctly reads a file line by line and processes each line. """ # ------------------------------------------------------------------ # 1. Create a temporary file with known content # ------------------------------------------------------------------ tmp_file = tmp_path / "input.txt" tmp_file.write_text( "first line\n" "second line\n" "third line\n" ) # ------------------------------------------------------------------ # 2. Run the program that contains the while loop # ------------------------------------------------------------------ # Assume the program is called read_file.py and is in the same # directory as this test. The program should read the file # whose name is given as the first command‑line argument. # # Example content of read_file.py: # # import sys # def main(): # with open(sys.argv[1], 'r') as f: # while True: # line = f.readline() # if not line: # break # # For demonstration we just print the line # print(line.rstrip()) # if __name__ == "__main__": # main() # # The test below will run that script. # cmd = ["python", "read_file.py", str(tmp_file)] result = subprocess.run( cmd, capture_output=True, text=True, # decode bytes -> str check=True # raise CalledProcessError on non‑zero exit ) # ------------------------------------------------------------------ # 3. Verify the output # ------------------------------------------------------------------ expected_output = "first line\nsecond line\nthird line\n" assert result.stdout == expected_output, ( f"Unexpected output.\n" f"Expected:\n{expected_output!r}\n" f"Got:\n{result.stdout!r}" ) ``` ### How the test works | Step | What happens | Why it matters | |------|--------------|----------------| | 1 | `tmp_path / "input.txt"` creates a *temporary* file that is automatically deleted after the test. | Keeps the test isolated from the real filesystem. | | 2 | The file is written with three lines. | These are the data that the `while` loop will read. | | 3 | `subprocess.run()` executes the program that contains the `while` loop, passing the temporary file name as an argument. | The program reads the file line by line and prints each line. | | 4 | The test captures the program’s standard output (`stdout`). | We can compare it to the expected result. | | 5 | `assert` checks that the output matches the three lines we wrote. | If the loop fails (e.g., it stops early, skips a line, or mis‑parses the file), the assertion will fail. | ### Adapting the test * **Different processing logic** – If the program does something other than printing each line (e.g., counts words, transforms the text, or writes to another file), change the `expected_output` accordingly or read the output file and compare its contents. * **Multiple files or edge cases** – Create additional temporary files (empty file, very large file, file with special characters) and repeat the test to cover more scenarios. * **Unit‑testing the loop directly** – If the loop is inside a function, you can import that function and call it directly from the test, passing a file‑like object created with `io.StringIO`. That way you avoid launching a separate process. This pattern gives you a clean, repeatable way to test any `while` loop that reads a file, ensuring that the loop behaves correctly for the data you provide.