Suggest an edit

Improve this article

Refine the answer for “Give an example of LSP compliance”. Your changes go to moderation before they’re published.

Approval required

Content

What you’re changing

Title (EN)

Short answer (EN)

Shown above the full answer for quick recall.

Answer (EN)

An example of LSP compliance is a situation where a subclass **extends the parent's behavior without violating its contract**, and can fully substitute for the base class in client code.

Below is a correct example: the base class describes a notification interface, and subclasses implement different ways of sending it.

---

### Base class (contract)

```python
class Notifier:
    def send(self, message: str):
        raise NotImplementedError
```

---

### Subclasses that follow LSP

```python
class EmailNotifier(Notifier):
    def send(self, message: str):
        print(f"Sending email: {message}")

class SmsNotifier(Notifier):
    def send(self, message: str):
        print(f"Sending SMS: {message}")
```

---

### Why this follows LSP:

1. **The subclasses do not change the meaning of the** `send()` **method**
They simply implement it in their own way.
The base contract: "send a message".
Both subclasses follow this contract.
2. **The subclasses do not tighten input restrictions**
The method accepts any string - both Email and SMS accept any string.
3. **The subclasses do not weaken the guarantees of the result**
The base class's promise: *"the send operation is performed"*.
Both implementations fulfill this.
4. **There is no need to check the type**

Client code:

```python
def notify(notifier: Notifier, msg):
    notifier.send(msg)
```

Any subclass can be substituted in:

```python
notify(EmailNotifier(), "Hello!")
notify(SmsNotifier(), "Hello!")
```

And the program will work **correctly** and **predictably**, with no changes.

5. **Invariants are not violated**
The base class has no state at all, so descendants cannot break anything.

---

### Summary

The `EmailNotifier` and `SmsNotifier` classes *fully preserve the contract* of `Notifier`, do not change the meaning of the behavior, and can safely be used wherever the base type is expected - this is exactly what correct LSP compliance looks like.

Markdown · drag & drop images · ⌘B / ⌘I shortcuts275 words

For the reviewer

Note to the moderator (optional)

Visible only to the moderator. Helps review go faster.