try / except — handling errors gracefully

The pattern

try:
    n = int(user_input)
except ValueError:
    n = 0
    print("Couldn't parse the number, defaulting to 0.")

Python runs the try block. If a ValueError is raised, control jumps to the except block. If no error, the except is skipped.

The big four exception types

• ValueError — the value was the right type but wrong content (e.g. int("abc")).
• TypeError — the value was the wrong type.
• KeyError — that key didn't exist in the dict.
• FileNotFoundError — couldn't find that file.

Catch specific, not generic

# OK but lazy — swallows bugs you'd want to know about
try:
    do_thing()
except:
    pass

# Better — explicit about what you expect
try:
    do_thing()
except FileNotFoundError:
    print("File missing, skipping.")

The finally block — cleanup that always runs

f = open("data.txt")
try:
    work_with(f)
except Exception as e:
    print(f"Failed: {e}")
finally:
    f.close()      # runs whether we succeeded or not

Walkthrough: process a folder of CSVs that might be broken

import pandas as pd
from pathlib import Path

results = []
for path in Path("monthly_files").glob("*.csv"):
    try:
        df = pd.read_csv(path)
        results.append((path.name, len(df)))
    except Exception as e:
        print(f"Skipped {path.name}: {e}")

print(f"Processed {len(results)} files successfully")