The try and except Blocks

15 min · Beginner

Wrap code that might fail inside a try block. If an error occurs, Python jumps immediately to the matching except block, allowing your program to recover gracefully.

Basic try-except block

Exception handling is writing code that catches an error and responds to it, so the program keeps running instead of crashing.

Place potentially failing statements in try:. If an exception occurs, the except: block executes instead of crashing.

Python

try:    number = int("abc")except ValueError:    print("Conversion failed: not a valid integer")

Example 1 · Reading a quantity

Python

1quantity_text = "three"2
3try:4    quantity = int(quantity_text)5    print(f"Ordered {quantity} items")6except ValueError:7    print("Quantity must be a whole number")

Output

Quantity must be a whole number

What this code does

int("three") raises ValueError, so the print inside try is skipped.
Python jumps to the matching except and prints the message.
The program then continues normally.

Because 'abc' is invalid, execution skips the rest of the try block and prints the friendly recovery message.

Capturing error details with as err

Exception object is the value Python creates when an error happens. Writing except ... as err gives it a name so you can print its message.

Capture the exception instance using as variable_name to inspect the error message.

Python

try:    res = 10 / 0except ZeroDivisionError as err:    print(f"Caught error: {err}")

Example 2 · Reading the error message

Python

1stock = {"pen": 40, "book": 12}2
3try:4    print(stock["bag"])5except KeyError as err:6    print(f"No such product: {err}")

Output

No such product: 'bag'

What this code does

stock["bag"] raises KeyError because there is no such key.
as err stores the exception object, and printing it shows the missing key.

The console displays Caught error: division by zero. You can log err to telemetry or monitoring services.

Handling multiple distinct exceptions

Multiple except blocks are a chain of except clauses on one try. Python checks them from top to bottom and runs the first one that matches.

Attach multiple except blocks to handle different failure modes distinctly.

Python

1def process_data(items, index):2    try:3        val = int(items[index])4        return 100 // val5    except IndexError:6        return "Error: Index out of range"7    except ValueError:8        return "Error: Non-numeric data"9    except ZeroDivisionError:10        return "Error: Cannot divide by zero"

Example 3 · Dividing safely

Python

1user_input = "100"2try:3    val = int(user_input)4    result = 500 // val5    print(f"Result: {result}")6except (ValueError, ZeroDivisionError) as err:7    print(f"Calculation error: {err}")

Output

Result: 5

What this code does

Attempts to parse input and divide 500 by value.
Executes successfully, printing 'Result: 5'.

Python checks each except block from top to bottom, executing only the first matching handler.

Where an uncaught error goes

Call stack is the list of function calls that are active right now. An unhandled error moves up this list until some function has a matching except.

A function does not need its own try. If it has none, the error moves up to the function that called it.

Python

1def parse_price(text):2    return float(text)3
4def load_row(row):5    return parse_price(row)6
7try:8    load_row("abc")9except ValueError:10    print("Bad price")

The error stops at the first function that has a matching except.

Three function calls: main calls load_row, which calls parse_price. parse_price raises a ValueError and has no except block, so the error moves up to load_row, which has none either, and then up to main, where a try and except catches it.

The error starts in parse_price, passes through load_row, and is caught in the top-level try.

Next step

Check your understanding

8 quick questions

1def process_data(items, index):2 try:3 val = int(items[index])4 return 100 // val5 except IndexError:6 return "Error: Index out of range"7 except ValueError:8 return "Error: Non-numeric data"9 except ZeroDivisionError:10 return "Error: Cannot divide by zero"