what is __len__ in python? Explain with example
In Python, `__len__` is a special method (also known as a dunder method, short for "double underscore") that is used to define the behavior of the built-in `len()` function for instances of a class. When you implement `__len__` in your custom class, you allow Python to know how to compute the length of instances of that class.
Here's a breakdown of how `__len__` works:
- When you call `len(instance)` on an instance of a class, Python implicitly calls the instance's `__len__()` method.
- The `__len__()` method should return an integer that represents the length or size of the object.
### Example
Let's create a simple class called `MyList` that will have its own behavior for the `len()` function by implementing `__len__`.
```python
class MyList:
def __init__(self, values):
self.values = values # Store a list of values
def __len__(self):
return len(self.values) # Return the length of the stored values
# Create an instance of MyList
my_list = MyList([1, 2, 3, 4, 5])
# Use the len() function
print(len(my_list)) # Output: 5
```
### Explanation of the Example:
1. **Initialization**: The `MyList` class has an `__init__` method that takes a list of values and stores it in an instance variable called `values`.
2. **Defining `__len__`**: The `__len__` method is defined to return the length of the list stored in `values`. It uses the built-in `len()` function to achieve this.
3. **Creating an Instance**: An instance of `MyList` is created with a list of integers.
4. **Getting Length**: When `len(my_list)` is called, it invokes `my_list.__len__()`, which returns `5`, the number of elements in the list.
Hence, by implementing `__len__`, you can control how the length of your custom objects is computed, making your classes more Pythonic and intuitive to use.
### Additional Note:
If you don't implement the `__len__` method in your custom class and you try to use `len()` on an instance of that class, it will raise a `TypeError`.