导入我自己的 python 模块时出现 ModuleNotFoundError

问：

所以问题是，当我尝试从我创建的模块导入某些内容时，我遇到了 ModuleNotFoundError：没有名为“<my_module>”的模块。

我的项目结构是这样的：

├── first.py
└── some_dir
    ├── second.py
    └── third.py

您可以使用以下 3 个文件复制问题：

third.py

""" This is the third file and we store some variable here
    that will be imported to the second """

a = 69

second.py

"""This is the second file. We import 
    a variable from the third and calculate
    the sum of a and b"""

from third import a
b = 10
sum = a + b

first.py

"""This is the first and final file 
    where everything comes together"""
from some_dir.second import c

print(c)

当我运行 first.py 时，我收到错误：

Traceback (most recent call last):
  File "/home/username/moo/goo/foo/first.py", line 3, in <module>
    from some_dir.second import c
  File "/home/username/moo/goo/foo/first.py", line 5, in <module>
    from third import a
ModuleNotFoundError: No module named 'third'