🐍 Unit 1 – Python Revision & Functions · Lesson 04
Exception Handling (try-except)
Errors: syntax errors vs exceptions
Syntax error
- Code breaks grammar rules of Python
- Detected before the program runs
- Example: missing colon, bracket, quote
- Must be fixed in the code
Exception (run-time error)
- Code is valid, but something goes wrong while running
- Example: 10/0, int("abc"), missing file
- Program crashes if not handled
- Can be handled with try-except
Python
a = 10
b = 0
print(a / b)
print('This line never runs')ZeroDivisionError: division by zero
When an exception is raised and not handled, Python stops the program and prints a traceback whose last line names the exception.
Built-in exceptions
| Exception | Raised when | Example |
|---|---|---|
| ZeroDivisionError | Division or modulus by zero | 10 / 0 |
| ValueError | Right type but wrong value | int('abc') |
| TypeError | Operation on the wrong type | '2' + 2 |
| IndexError | Index out of range | [1, 2][5] |
| KeyError | Dictionary key not found | {'a': 1}['b'] |
| NameError | Name not defined | print(undefined_var) |
| AttributeError | Object has no such attribute/method | (5).append(1) |
| ModuleNotFoundError / ImportError | Module cannot be imported | import mathh |
| FileNotFoundError | File does not exist | open('nofile.txt') |
| EOFError | input()/pickle.load() reaches end of file | reading past the last record |
| OverflowError | Result too large | 2.0 ** 10000 |
| IndentationError | Wrong indentation (a syntax error) | missing indented block |
Why handle exceptions?
Exception handling lets a program respond to problems gracefully instead of crashing — show a friendly message, ask again, use a default, close files safely. The code that might fail goes in try; the code that handles the problem goes in except.
try and except
Python
try:
a = 10
b = 0
print(a / b)
except ZeroDivisionError:
print('Cannot divide by zero!')
print('Program continues normally')Cannot divide by zero!
Program continues normally
try, except, else, finally
Syntax
try:
statements that may raise an exception
except ExceptionName:
handle that exception
else:
runs only if NO exception occurred
finally:
runs ALWAYS (clean-up)Python
def safe_divide(x, y):
try:
result = int(x) / int(y)
except ZeroDivisionError:
print('Error: divisor is zero')
except ValueError:
print('Error: enter whole numbers only')
else:
print('Result =', result) # runs only if no exception
finally:
print('-- finished --') # always runs
safe_divide('20', '4')
safe_divide('20', '0')
safe_divide('20', 'x')Result = 5.0
-- finished --
Error: divisor is zero
-- finished --
Error: enter whole numbers only
-- finished --
Handling several exceptions
Use several except blocks, or one block with a tuple of exception names. as e gives the exception object, whose message can be printed.
Python
data = [10, 20, 30]
for idx in ['1', '5', 'two']:
try:
print(data[int(idx)])
except (IndexError, ValueError) as e: # one block for many types
print(type(e).__name__, '->', e)20
IndexError -> list index out of range
ValueError -> invalid literal for int() with base 10: 'two'
A bare except: or except Exception: catches almost everything — put it last, after specific handlers:
Python
try:
x = [1, 2, 3]
print(x[3])
except Exception as e: # catches almost every error
print('Something went wrong:', e)Something went wrong: list index out of range
raise and assert
raise lets your code signal an error. assert condition, message raises AssertionError when the condition is False — handy for checking inputs.
Python
def set_marks(m):
if m < 0 or m > 100:
raise ValueError('Marks must be between 0 and 100')
return m
for m in (88, 140):
try:
print('Saved', set_marks(m))
except ValueError as err:
print('Error:', err)Saved 88
Error: Marks must be between 0 and 100
Exceptions with files and input
Python
try:
f = open('marks_missing.txt')
print(f.read())
f.close()
except FileNotFoundError:
print('File not found – please check the name')File not found – please check the name
Keep asking until valid input is given:
Python
while True:
try:
n = int(input('Enter a positive number: '))
assert n > 0, 'number must be positive'
break
except ValueError:
print('Not a number, try again')
except AssertionError as e:
print('Invalid:', e)
print('You entered', n)Enter a positive number: abc
Not a number, try again
Enter a positive number: -4
Invalid: number must be positive
Enter a positive number: 12
You entered 12
Flow of control
- No exception in try → else runs → finally runs.
- Exception in try → rest of try is skipped → first matching except runs → finally runs.
- Exception with no matching except → finally runs → program stops with the traceback.
Summary & practice
- Syntax errors are found before running; exceptions happen while running.
- try–except catches exceptions; else runs if none; finally always runs.
- Catch specific exceptions first; use raise / assert for your own checks.
✅ Board-style question
What is printed? try: print('A'); x = int('5a'); print('B') except ValueError: print('C') else: print('D') finally: print('E')
Answer: A, C, E (each on a new line).
Frequently Asked Questions
What is the difference between an error and an exception?
A syntax error stops the program before it starts because the code breaks Python’s rules. An exception is an error detected while the program is running (like division by zero); it can be caught and handled with try-except.
When does the else block of try run?
Only when the try block finishes without raising any exception.
Does finally run even if there is no exception?
Yes. finally always runs — whether an exception occurred or not — so it is used for clean-up like closing files.
