要自定義Python中l(wèi)en()函數(shù)返回的對象長度，需在類中實現(xiàn)__len__方法，並確保其返回一個非負整數(shù)。 1. __len__方法不接受除self外的參數(shù)，用於定義對象的“長度”概念；2. 若未實現(xiàn)該方法，調用len()會拋出TypeError；3. 返回值必須為整數(shù)，且應具實際意義，如表示容器內元素數(shù)量；4. 常見於類似容器或序列行為的類，如購物車、自定義列表等；5. 返回非整數(shù)或負數(shù)會引發(fā)錯誤或不符合規(guī)範；6. 實現(xiàn)時應考慮性能和一致性，使類更具“Pythonic”風格。

When you want to define what len() should return for your custom object in Python, you implement the __len__ method. It's straightforward — just make sure it returns a non-negative integer.

How __len__ Works in a Custom Class

If you've created a class and want to use len(instance_of_your_class) , you need to define __len__ inside that class. This method doesn't take any arguments except self .

For example:

 class MyCollection:
    def __init__(self, items):
        self.items = items

    def __len__(self):
        return len(self.items)

Now when you do this:

 coll = MyCollection([1, 2, 3])
print(len(coll)) # Outputs: 3

It works because __len__ is returning the length of the internal list.

Keep in mind:

• If you don't define __len__ , calling len() on an instance will raise a TypeError .
• The return value must be an integer and ideally should represent something meaningful about your object.

Common Use Cases and Examples

You'll often see __len__ used in classes that behave like containers or sequences. For example:

• A custom list wrapper
• A database query result that supports pagination
• A file reader that tracks how many lines are loaded

Here's another practical case: say you have a shopping cart class.

 class ShoppingCart:
    def __init__(self):
        self.products = []

    def add_product(self, product):
        self.products.append(product)

    def __len__(self):
        return len(self.products)

Now len(cart) gives you the number of products added — very intuitive behavior.

This helps especially if you're writing code that other developers might use. Having len() work as expected makes your class feel more "Pythonic".

What Happens If You Return Something Invalid?

The __len__ method must return an integer. If you return anything else — like a float, string, or even None — Python will throw a TypeError .

Try this:

 def __len__(self):
    return 'not-an-int'

And then call len(obj) — you'll get an error like:

 TypeError: 'str' object cannot be interpreted as an integer

Also, if you return a negative number, Python won't stop you directly, but it's considered bad practice. Built-in types usually avoid that, so it's best not to surprise users of your class.

Tips for Implementing __len__

• Think about what “l(fā)ength” means for your object. Is it the number of items, characters, entries, or something else?
• Make sure it's consistent with how other container-like objects behave.
• Don't overcomplicate the logic unless necessary — keep performance in mind if computing length is expensive.

In short, implementing __len__ is simple, but choosing a logical and useful definition of length matters most.

基本上就這些。

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