Exceptions and Files: Interview Questions

50 min · Beginner

Interviewers use exceptions and files to see whether you write code that fails safely. Short, exact answers on finally, bare except and with show that habit quickly.

Recap: try, except, else and finally

An exception is an object that Python raises when something goes wrong. If no except block handles it, the program stops and prints a traceback.

Each block has one job. try holds the risky line, except handles a named error, else runs only if nothing failed, and finally always runs.

Java

1def read_qty(text):2    try:3        qty = int(text)4    except ValueError:5        print("bad number")6        return 07    else:8        print("ok")9        return qty10    finally:11        print("done")12
13print(read_qty("4"))14print(read_qty("x"))

Example 1 · else and finally in order

Python

1def divide(a, b):2    try:3        result = a / b4    except ZeroDivisionError:5        print("cannot divide by zero")6        return None7    else:8        print("division worked")9        return result10    finally:11        print("finished")12
13print(divide(10, 4))14print(divide(1, 0))

Output

division worked
finished
2.5
cannot divide by zero
finished
None

What this code does

else runs only when try raised nothing. finally runs in both calls, just before each return hands back its value.
Each print of the result comes after that call's finished line.

Each call prints done last, even though both return early. Catch the specific error you expect, not every error.

Recap: raise, custom errors and the hierarchy

The exception hierarchy is the tree of exception classes. BaseException is the root, Exception is the parent of normal errors, and an except block also catches all subclasses.

raise throws an exception. A custom error is a small class that extends Exception. Use raise ... from err to keep the original cause.

Java

1class InsufficientFunds(Exception):2    pass3
4def withdraw(balance, amount):5    if amount > balance:6        raise InsufficientFunds(f"need {amount - balance} more")7    return balance - amount8
9try:10    withdraw(100, 150)11except InsufficientFunds as err:12    print(err)

Example 2 · Specific errors and chaining

Python

1def parse_mark(text):2    try:3        return int(text)4    except ValueError as err:5        raise ValueError(f"invalid mark: {text!r}") from err6
7try:8    parse_mark("ab")9except ValueError as err:10    print(err)11    print(type(err.__cause__).__name__)12    print(issubclass(FileNotFoundError, OSError))

Output

invalid mark: 'ab'
ValueError
True

What this code does

raise ... from err stores the original error as __cause__, so the full story survives in a traceback.
FileNotFoundError is a subclass of OSError, so except OSError also catches it.

except Exception catches this error, but never KeyboardInterrupt or SystemExit, because those extend BaseException directly. A bare except: catches them too, which is why it is a bad habit.

Recap: files and the with statement

A context manager is an object used with with. It sets something up on entry and always cleans up on exit, even if an exception is raised inside the block.

open() gives a file object that must be closed. with open(...) closes it for you. Name the encoding so the text reads the same on every computer.

Java

1import tempfile, os2
3folder = tempfile.mkdtemp()4path = os.path.join(folder, "orders.txt")5with open(path, "w", encoding="utf-8") as f:6    f.write("A100\nA101\n")7with open(path, encoding="utf-8") as f:8    for line in f:9        print(line.strip())10print(f.closed)

Example 3 · Write, append and read a file safely

Python

1import tempfile2from pathlib import Path3
4with tempfile.TemporaryDirectory() as folder:5    path = Path(folder) / "marks.txt"6    path.write_text("Asha 80\nRavi 72\n", encoding="utf-8")7    with open(path, "a", encoding="utf-8") as f:8        f.write("Meena 91\n")9    with open(path, encoding="utf-8") as f:10        total = sum(int(line.split()[1]) for line in f)11    print(total)12    try:13        open(Path(folder) / "missing.txt")14    except FileNotFoundError:15        print("no such file")

Output

243
no such file

What this code does

Mode "a" adds to the end, while "w" would erase the old content first.
Opening a missing file raises FileNotFoundError; catch that exact error to handle it. The temporary folder is removed when the outer with ends.

Looping over the file reads one line at a time, so even a huge file fits in memory. f.closed is True after the block, because with closed it.

Next step

Check your understanding

12 quick questions

1def read_qty(text):2 try:3 qty = int(text)4 except ValueError:5 print("bad number")6 return 07 else:8 print("ok")9 return qty10 finally:11 print("done")12 13print(read_qty("4"))14print(read_qty("x"))

1def divide(a, b):2 try:3 result = a / b4 except ZeroDivisionError:5 print("cannot divide by zero")6 return None7 else:8 print("division worked")9 return result10 finally:11 print("finished")12 13print(divide(10, 4))14print(divide(1, 0))

Recap: raise, custom errors and the hierarchy

The exception hierarchy is the tree of exception classes. BaseException is the root, Exception is the parent of normal errors, and an except block also catches all subclasses.

raise throws an exception. A custom error is a small class that extends Exception. Use raise ... from err to keep the original cause.

1class InsufficientFunds(Exception):2 pass3 4def withdraw(balance, amount):5 if amount > balance:6 raise InsufficientFunds(f"need {amount - balance} more")7 return balance - amount8 9try:10 withdraw(100, 150)11except InsufficientFunds as err:12 print(err)

1def parse_mark(text):2 try:3 return int(text)4 except ValueError as err:5 raise ValueError(f"invalid mark: {text!r}") from err6 7try:8 parse_mark("ab")9except ValueError as err:10 print(err)11 print(type(err.__cause__).__name__)12 print(issubclass(FileNotFoundError, OSError))

Recap: files and the with statement

A context manager is an object used with with. It sets something up on entry and always cleans up on exit, even if an exception is raised inside the block.

open() gives a file object that must be closed. with open(...) closes it for you. Name the encoding so the text reads the same on every computer.

1import tempfile, os2 3folder = tempfile.mkdtemp()4path = os.path.join(folder, "orders.txt")5with open(path, "w", encoding="utf-8") as f:6 f.write("A100\nA101\n")7with open(path, encoding="utf-8") as f:8 for line in f:9 print(line.strip())10print(f.closed)

1import tempfile2from pathlib import Path3 4with tempfile.TemporaryDirectory() as folder:5 path = Path(folder) / "marks.txt"6 path.write_text("Asha 80\nRavi 72\n", encoding="utf-8")7 with open(path, "a", encoding="utf-8") as f:8 f.write("Meena 91\n")9 with open(path, encoding="utf-8") as f:10 total = sum(int(line.split()[1]) for line in f)11 print(total)12 try:13 open(Path(folder) / "missing.txt")14 except FileNotFoundError:15 print("no such file")