Python match Case Guards: Advanced Conditions

Use a guard to add a Boolean rule after a structural pattern matches.

What You Will Learn

In case pattern if condition, Python first tries to match and bind names; only if that succeeds does it evaluate the guard. This lets you distinguish values that have the same shape but different business rules. A guard can use captured names, but you should not rely on side effects or variable bindings from unsuccessful cases.

Guards are ordinary Python expressions and should be kept easy to read.

Step-by-Step Example

The example below makes the behavior visible with small input values that you can change safely.

def shipping_band(order):
    match order:
        case {"total": amount} if amount >= 100:
            return "Free shipping"
        case {"total": amount} if amount > 0:
            return "Standard shipping"
        case _:
            return "Invalid total"

for order in ({"total": 120}, {"total": 20}, {"total": -3}):
    print(shipping_band(order))

How the Example Works

The function shipping_band defines reusable behavior; its body executes when it is called, not when it is declared.
The loop demonstrates when each value is processed or when cleanup and exception handling take effect.
The print() calls make the return values or state visible so you can compare them with the explanation and check your prediction.

Expected output from the example:

Free shipping
Standard shipping
Invalid total

When You Would Use This

Pattern matching can simplify dispatching application commands or decoding structured messages from a trusted parser. It chooses a branch based on shape and selected values, but application-level validation is still necessary. Compare it with an if/elif chain if the problem only involves simple Boolean expressions.

Practice: Modify the Code

Add a guard requiring the order to contain an express delivery flag.

Verify the changed output against the behavior described above, including the relevant invalid or boundary case.

For complete parameters and version details, consult the official Python documentation.

Common Beginner Mistakes

Putting a guard before the pattern instead of after it.
Assuming a guard can refer to a name that was never captured by its case pattern.

Key Takeaways

Use a guard to add a Boolean rule after a structural pattern matches.
In case pattern if condition, Python first tries to match and bind names; only if that succeeds does it evaluate the guard.
Guards are ordinary Python expressions and should be kept easy to read.