items = []
items.append("apple")
items.append("orange")
items.append("banana")

# FAKE METHOD::
items.amount()  # Should return 3

我如何获得列表中的元素数量？

---

len()函数可以与Python中的很多类型一起使用 - 包括内置类型和库类型。

>>> len([1,2,3])
3

---

How to get the size of a list?

To find the size of a list, use the builtin function, len :

items = []
items.append("apple")
items.append("orange")
items.append("banana")

And now:

len(items)

returns 3.

From the docs

len(s)

Return the length (the number of items) of an object. The argument may be a sequence (such as a string, bytes, tuple, list, or range) or a collection (such as a dictionary, set, or frozen set).

len is implemented with __len__ , from the data model docs:

object.__len__(self)

Called to implement the built-in function len() . Should return the length of the object, an integer >= 0. Also, an object that doesn't define a __nonzero__() [in Python 2 or __bool__() in Python 3] method and whose __len__() method returns zero is considered to be false in a Boolean context.

And we can also see that __len__ is a method of lists:

items.__len__()

returns 3.

Builtin types you can get the len (length) of

And in fact we see we can get this information for all of the described types:

>>> all(hasattr(cls, '__len__') for cls in (str, bytes, tuple, list, 
                                            xrange, dict, set, frozenset))
True

Do not use len as a value for a condition

Do not do:

if len(items): 
    ...

Instead, do:

if items:
    ...

I explain why here but in short, it is more readable and more performant.

---

While this may not be useful due to the fact that it'd make a lot more sense as being "out of the box" functionality, a fairly simple hack would be to build a class with a length property:

class slist(list):
    @property
    def length(self):
        return len(self)

You can use it like so:

>>> l = slist(range(10))
>>> l.length
10
>>> print l
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Essentially, it's exactly identical to a list object, with the added benefit of having an OOP-friendly length property.

As always, your mileage may vary.

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