pytest: Catching Multiple Exception Types with Parentheses
In the realm of software testing, the ability to anticipate and handle exceptions is crucial for ensuring code robustness. pytest, a popular testing framework for Python, provides powerful mechanisms to gracefully manage exceptional scenarios. One common need is to test if your code raises specific exception types, and pytest elegantly handles this with its raises context manager.
However, what if you want to check if your code raises multiple specific exception types? Here's where parentheses play a key role.
The Problem: Unparenthesized Exception Types
Imagine you have a function that might throw either a TypeError or a ValueError. You want to write a pytest test to verify this behavior. A naive attempt might look like this:
def test_my_function():
with pytest.raises(TypeError, ValueError):
my_function(invalid_input)
This code will not work as intended. Why? pytest's raises context manager expects a single exception type. Trying to pass multiple exception types without parentheses will result in an error:
TypeError: raises() argument must be a subclass of BaseException, not tuple
The Solution: Parenthesized Exception Types
To correctly test for multiple exception types, you need to use parentheses to create a tuple containing the exception types:
def test_my_function():
with pytest.raises((TypeError, ValueError)):
my_function(invalid_input)
This modification tells pytest to expect any of the exceptions listed within the tuple. Now, the test will pass if my_function throws either a TypeError or a ValueError.
Example: Catching Multiple Exceptions
Let's illustrate this with a concrete example:
def divide(a, b):
if b == 0:
raise ZeroDivisionError("Cannot divide by zero")
return a / b
def test_divide():
with pytest.raises((ZeroDivisionError, TypeError)):
divide(10, 0) # Raises ZeroDivisionError
divide("hello", 2) # Raises TypeError
In this test, divide can raise either ZeroDivisionError or TypeError depending on the input. The pytest.raises context manager with the tuple (ZeroDivisionError, TypeError) will catch both cases, ensuring the test passes.
Beyond Two Exceptions
You can extend this approach to handle any number of exception types. Just list them within the tuple:
with pytest.raises((ValueError, TypeError, KeyError)):
# ... your code here ...
Important Considerations
- Order Matters: The order of exception types within the tuple does not influence the test's outcome.
- Subclasses: The
raisescontext manager also allows for catching exceptions based on inheritance. If a subclass of an exception type is raised, it will also be caught. For instance, if you have a custom exception class that inherits fromValueError, it will be caught by theValueErrorwithin the tuple.
Conclusion
Understanding how to handle multiple exception types with pytest.raises is vital for comprehensive testing. By using parentheses to create a tuple of exception types, you can gracefully test for scenarios where various exceptions might arise. This approach contributes to robust and reliable code, allowing you to catch potential issues early in the development process.