Python 如何 import 模块？

平时写的每一个 py 文件都会 import 其他模块，但其原理我也是刚刚搞懂。

在使用 Python 的 import 导入模块时，会首先从缓存中检索 Python 内建的模块，比如 os、sys 等。

如果要使用的模块不是内建模块，之后会从指定的路径中依次查找。

sys.path 可以列出这些路径，它包含三部分内容：

1. 当前 py 文件的目录；如果是在交互式环境，是当前终端的目录。
2. 操作系统中的当前环境变量 PYTHONPATH 保存的所有路径
3. 当前虚拟环境的路径，用来查找 pip 或 conda 安装的包。

~ % pwd
/Users/junsircoding

~ % ipython
In [1]: import sys
In [2]: sys.path
Out[2]: 
['/Users/junsircoding/miniforge3/bin',
 '/Users/junsircoding/miniforge3/lib/python39.zip',
 '/Users/junsircoding/miniforge3/lib/python3.9',
 '/Users/junsircoding/miniforge3/lib/python3.9/lib-dynload',
 '',
 '/Users/junsircoding/miniforge3/lib/python3.9/site-packages']

~ % python
>>> import sys
>>> import pprint as pp
>>> pp.pprint(sys.path)
['',
 '/Users/junsircoding/miniforge3/lib/python39.zip',
 '/Users/junsircoding/miniforge3/lib/python3.9',
 '/Users/junsircoding/miniforge3/lib/python3.9/lib-dynload',
 '/Users/junsircoding/miniforge3/lib/python3.9/site-packages']
>>> exit()

~ % export PYTHONPATH=/test/python_path
~ % ipython
In [1]: import sys
In [2]: sys.path
Out[2]: 
['/Users/junsircoding/miniforge3/bin',
 '/test/python_path',
 '/Users/junsircoding/miniforge3/lib/python39.zip',
 '/Users/junsircoding/miniforge3/lib/python3.9',
 '/Users/junsircoding/miniforge3/lib/python3.9/lib-dynload',
 '',
 '/Users/junsircoding/miniforge3/lib/python3.9/site-packages']

~ % python
>>> import sys
>>> import pprint as pp
>>> pp.pprint(sys.path)
['',
 '/test/python_path',
 '/Users/junsircoding/miniforge3/lib/python39.zip',
 '/Users/junsircoding/miniforge3/lib/python3.9',
 '/Users/junsircoding/miniforge3/lib/python3.9/lib-dynload',
 '/Users/junsircoding/miniforge3/lib/python3.9/site-packages']
>>> exit()