使用sys.path添加了绝对路径，但仍然无法import相应的自己编写的.py文件

Check if it is correct path to your import module when you execute your script.

os.path.abspath(r"/Users/mason-liu/Downloads/Machine_Learning/Fish_book/dataset")

You can find it’s wrong path to your user module mnist

os.path.abspath
on most platforms, this is equivalent to calling the function normpath() as follows:
os.path.normpath(join(os.getcwd(), path)).

d:\>python
Python 3.8.10 (tags/v3.8.10:3d8993a, May  3 2021, 11:48:03) [MSC v.1928 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> file = r"/Users/mason-liu/Downloads/Machine_Learning/Fish_book/dataset"
>>> import os, sys
>>> file = r"/Users/mason-liu/Downloads/Machine_Learning/Fish_book/dataset"
>>> path1 = os.path.abspath(file)
>>> path1     # Wrong path to your module
'd:\\Users\\mason-liu\\Downloads\\Machine_Learning\\Fish_book\\dataset'
>>> path2 = os.path.dirname(path1)
>>> path2
'd:\\Users\\mason-liu\\Downloads\\Machine_Learning\\Fish_book'
>>> sys.path
['', 'C:\\Software\\Python\\python39.zip', 'C:\\Software\\Python\\DLLs', 'C:\\Software\\Python\\lib', 'C:\\Software\\Python', 'C:\\Software\\Python\\lib\\site-packages', 'C:\\Software\\Python\\lib\\site-packages\\win32', 'C:\\Software\\Python\\lib\\site-packages\\win32\\lib', 'C:\\Software\\Python\\lib\\site-packages\\Pythonwin', 'D:\\Python\\Project\\OK', 'd:\\', 'C:\\Software\\Python\\python38.zip']
>>> sys.path.append(path2)
>>> sys.path
['', 'C:\\Software\\Python\\python39.zip', 'C:\\Software\\Python\\DLLs', 'C:\\Software\\Python\\lib', 'C:\\Software\\Python', 'C:\\Software\\Python\\lib\\site-packages', 'C:\\Software\\Python\\lib\\site-packages\\win32', 'C:\\Software\\Python\\lib\\site-packages\\win32\\lib', 'C:\\Software\\Python\\lib\\site-packages\\Pythonwin', 'D:\\Python\\Project\\OK', 'd:\\', 'C:\\Software\\Python\\python38.zip', 'd:\\Users\\mason-liu\\Downloads\\Machine_Learning\\Fish_book']

Thanks for your reply. I dont understand how to modify yet. Thats's correct address.

@Jason990420