Python中的Ellipsis（...）有三個主要用途：1.作為代碼存根或未完成邏輯的占位符，例如在函數(shù)或類中暫時留空結(jié)構(gòu)；2.在多維數(shù)組切片（如NumPy）中表示所有前導(dǎo)維度，簡化高維數(shù)據(jù)訪問；3.在Python 3.9 的類型提示中用于表示可變或未指定參數(shù)，如與typing.Concatenate結(jié)合使用。這些用途分別對應(yīng)開發(fā)初期的結(jié)構(gòu)預(yù)留、高維數(shù)據(jù)切片簡化和復(fù)雜類型聲明，雖不常用但特定場景下非常實用。

In Python, the Ellipsis object (...) might seem like a mysterious or rarely used feature, but it does have some practical purposes, especially in specific contexts like slicing and type hinting.

1. Placeholder in Stubs and Incomplete Code

One of the most common uses for ... is as a placeholder when writing stubs or incomplete code. It’s especially handy during development when you want to define a function or class structure without implementing the logic yet.

For example:

def process_data(data):
    ...

This tells anyone reading the code (including future you) that this function is intentionally left blank for now. It's cleaner than using pass sometimes, and in some IDEs or tools, it might be treated differently.

You could also use it in classes:

class MyClass:
    ...

This avoids syntax errors while allowing you to build out the structure first.

2. Multidimensional Array Slicing (Especially in NumPy)

Another major use case appears when working with multidimensional arrays — particularly in libraries like NumPy.

Here’s how it looks:

import numpy as np

arr = np.random.rand(5, 5, 5)
print(arr[..., 0])  # Equivalent to arr[:, :, 0]

In this context, ... stands for "all preceding dimensions." So instead of typing out multiple colons (:), you can use ... to refer to all dimensions except the last one (or whichever comes after).

It makes your slicing more readable, especially when dealing with higher-dimensional data like images or tensors.

3. Type Hinting in Python 3.9

With the evolution of Python's type system, ... also plays a role in type hints.

For instance, it's used in typing.Unpack, typing.Concatenate, and other advanced type constructs introduced in later versions of Python.

A simple example:

from typing import Callable, Concatenate

def decorator(func: Callable[Concatenate[int, ...], None]) -> None:
    ...

Here, ... means that the function can take any additional arguments after the int. It helps express variable-length argument lists in a flexible way.

Summary

• Use ... as a lightweight placeholder in functions and classes during early development.
• Leverage it in array slicing to simplify access to high-dimensional data.
• Utilize it in complex type hints to represent variable or unspecified parameters.

It's not something you'll use every day, but when you need it, it fits just right.

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