Error Handling

Catching multiple exception types

Real code can fail in more than one way. Learn how to attach several except blocks to a single try, group exceptions in a tuple, and use a generic Exception as a last resort.

Real code can fail in more than one way. A single try block can be paired with several except clauses, each handling a different exception type.

Here we react differently depending on what went wrong:

try:
	x = int("abc") / 2
except ValueError:
	x = "bad text"
except ZeroDivisionError:
	x = "no divide"
print(x)

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Output

Python checks each except in order. The first one that matches the exception runs, the rest are skipped.

What will be the output?

try:
	x = 10 / 0
except ValueError:
	x = "v"
except ZeroDivisionError:
	x = "z"
print(x)

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When two exceptions deserve the same response, group them in a tuple after a single except keyword.

One block, two exception types:

def safe(value):
	try:
		return int(value) + 1
	except (ValueError, TypeError):
		return -1
print(safe("oops"))
print(safe(None))

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Output

What will be the output?

try:
	x = "5" + 5
except (ValueError, TypeError):
	x = 0
print(x)

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Python exceptions form a hierarchy. IndexError and KeyError both inherit from LookupError, so catching the parent catches both.

A LookupError except catches an IndexError:

try:
	items = [1, 2]
	print(items[9])
except LookupError:
	print("missing")

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Output

And the same except also catches a KeyError, since KeyError is a child of LookupError too.

Same except, different exception:

data = {"a": 1}
try:
	print(data["z"])
except LookupError:
	print("missing")

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Output

What will be the output?

try:
	nums = [1, 2, 3]
	print(nums[5])
except LookupError:
	print("oops")

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At the top of the hierarchy sits Exception. Catching it grabs almost any error. Use it as a last resort — it can hide real bugs.

A generic Exception with as captures the message:

try:
	x = 1 / 0
except Exception as e:
	msg = str(e)
print(msg)

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Output

What will be the output?

try:
	n = int("x")
except Exception:
	n = 42
print(n)

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What will be the output?

try:
	x = [][0]
except KeyError:
	x = "k"
except IndexError:
	x = "i"
print(x)

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What will be the output?

def parse(v):
	try:
		return int(v)
	except (ValueError, TypeError):
		return 0
print(parse("3"))
print(parse(None))

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What will be the output?

try:
	d = {"a": 1}
	print(d["b"])
except LookupError:
	print("none")

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