← Python

Files – Error Handling (try/except)

A try/except block around file operations catches FileNotFoundError and other IO errors

What it does: Catching FileNotFoundError specifically handles the most common file error. The finally block runs whether or not an exception occurred — great for cleanup. FileNotFoundError is specific to missing files — ValueError is for bad values. Files are outside your program's control — they can be missing, locked, or unreadable. with handles closing; try/except handles the error — use both together. PermissionError means the OS denied access — WriteError doesn't exist in Python. Stack multiple except blocks, or catch a tuple: except (TypeError, ValueError):. The except block only runs when the specific error is raised.

Common mistakes: Missing colon after try. Error names are capitalized: FileNotFoundError.

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Learning

Duo

What error occurs when reading a missing file?

FileNotFoundErrorValueError

Matches

❌ open('missing.txt')

↓

Error: FileNotFoundError

Applying

Meanings

Term

FileNotFoundError

Meaning

Error when trying to read a file that doesn't exist.

Practice

try:

Testing

Gaps

except :

Check Answer

Spots

1try

2 with open('f.txt') as f:

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Quick Questions

1. What error occurs when reading a missing file?

A. FileNotFoundError ✓

B. ValueError

FileNotFoundError is specific to missing files — ValueError is for bad values

2. Why use try/except with files?

A. It makes code faster

B. Files might not exist or be inaccessible ✓

Files are outside your program's control — they can be missing, locked, or unreadable

3. What does 'finally' do?

A. Always runs, even if there's an error ✓

B. Only runs on error

finally runs whether the try succeeded or failed — good for cleanup

4. Which is safer: try/except or with?

A. try/except alone

B. Both together is safest ✓

with handles closing; try/except handles the error — use both together

5. What error if you can't write to a file?

A. PermissionError ✓

B. WriteError

PermissionError means the OS denied access — WriteError doesn't exist in Python

Match the Pairs

open('missing.txt') → Error: FileNotFoundError

try: f = open('f.txt') except: pass f.read() → Error: f may not be defined

try: f = open('f.txt') except FileNotFoundError: print('Not found') → Valid: Handle missing file

try: open('f.txt') finally: print('done') → Ok: finally always runs

try: open('x.txt') except FileNotFoundError: print('Missing') → Output: Missing

try: open('x.txt') except Exception as e: print(type(e).__name__) → Output: FileNotFoundError

Key Term

FileNotFoundError

Error when trying to read a file that doesn't exist.~~PermissionError

Fill in the Blanks

1. try: with open('f.txt') as f: print(f.read()) except FileNotFoundError: print('Not found')

2. try: with open('f.txt') as f: print(f.read()) except Exception as e: print(e)

3. try: f = open('f.txt') except FileNotFoundError: print('Missing')

4. try: with open('f.txt') as f: print(f.read()) except PermissionError: print('No access')

5. try: open('f.txt') except FileNotFoundError: print('Missing') finally: print('Done')

Spot the Error

Question 1

A try

B with open('f.txt') as f:

C print(f.read())

D except FileNotFoundError:

E print('Missing')

Line A — Missing colon after try

Question 2

A try:

B with open('f.txt') as f:

C print(f.read())

D except filenotfounderror:

E print('Missing')

Line D — Error names are capitalized: FileNotFoundError

Question 3

A except:

B print('Error')

C try:

D with open('f.txt') as f:

E print(f.read())

Line A — try must come before except

Question 4

A try:

B with open('f.txt') as f:

C print(f.read())

D catch FileNotFoundError:

E print('Missing')

Line D — Python uses except, not catch

Question 5

A try:

B with open('f.txt') as f:

C print(f.read())

D except:

E pass # silently ignore

Line E — Bare except with pass silently hides ALL errors. Catch specific errors.

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