问题

I am following this tutorial to see why Python doesn't recognize an environment variable that was set at the Conda prompt (using CMD syntax, as this is an Anaconda installation on Windows 10).

It requires the os module, and as part of my getting familiar with Python, I decided to test whether os was already imported. Testing the presence of a module requires the sys module (as described here).

Strangely, right after importing sys, I found that os was imported without me having to do so. I find this odd, as most of my googling shows that you have to import them individually, e.g., here.

Does importing sys also impart os, as it seems to? If so, why is it common to import both individually?

I can't test for the presence of os before importing sys, as I need sys to test for the presence of modules.

Here is the code that shows the apparent presence of os from importing sys, formatted for readability. It starts from the Conda prompt in Windows. The Conda environment is "py39", which is for Python 3.9:

(py39) C:\Users\User.Name > python

   Python 3.9.16 (main, Mar  8 2023, 10:39:24)
   [MSC v.1916 64 bit (AMD64)] on win32

   Type "help", "copyright", "credits", or "license"
   for more information.

>>> import sys

>>> "os" in sys.modules

   True

Afternote: Thanks to Zero's answer, I found this code to be more what I'm looking for. After loading sys, the appropriate test is ( 'os' in sys.modules ) and ( 'os' in dir() ):

(py39) C:\Users\User.Name > python
'os' in dir() # False

import sys
'os' in sys.modules , 'os' in dir() # (True, False)
( 'os' in sys.modules ) and ( 'os' in dir() ) # False

import os
'os' in sys.modules , 'os' in dir() # (True, True)
( 'os' in sys.modules ) and ( 'os' in dir() ) # True

sys.modules shows whether the module has been imported anywhere (presumably in the code that the Python interpreter has executed) while dir() indicates whether the module name is in the current namespace. Thanks to Carcigenicate for clarifying this point, and I hope that I understood it properly.

(py39) C:\Users\User.Name > python

   Python 3.9.16 (main, Mar  8 2023, 10:39:24)
   [MSC v.1916 64 bit (AMD64)] on win32

   Type "help", "copyright", "credits" or "license"
   for more information.

>>> import sys

>>> "os" in sys.modules

   True

(py39) C:\Users\User.Name > python
'os' in dir() # False

import sys
'os' in sys.modules , 'os' in dir() # (True, False)
( 'os' in sys.modules ) and ( 'os' in dir() ) # False

import os
'os' in sys.modules , 'os' in dir() # (True, True)
( 'os' in sys.modules ) and ( 'os' in dir() ) # True

答案1

得分: 2

不，`sys` 不会导入 `os`。然而，[`os` 会导入 `sys`](https://github.com/python/cpython/blob/ce091c96cfb9a5cd6c7120dc077606da993680b4/Lib/os.py#L26)。

```python
>>> import os
>>> os.sys
<module 'sys' (built-in)>

实际的 os 模块对象在典型的安装中可能会被冻结，也就是说它将被一个 FrozenImporter 从一个自动生成的 C 版本中找到，而不是从 os.py 文件中。sys 模块可能会被一个 BuiltinImporter 载入，并且实际上被编译进解释器中（与另外一些模块一起，目前不包括 os）。

请注意，它们两者在解释器启动时都会被导入，如下所示：

python3 -v -c '' 

当你从交互式解释器中导入 sys 和/或 os 时，你只是将它们绑定到了你的本地命名空间中 - 这两个模块已经完全加载，并且不会再次被导入。

<details>
<summary>英文:</summary>

No, `sys` does not import `os`. However, [`os` does import `sys`](https://github.com/python/cpython/blob/ce091c96cfb9a5cd6c7120dc077606da993680b4/Lib/os.py#L26).

>>> import os
>>> os.sys
<module 'sys' (built-in)>

The actual `os` module object will likely be frozen on a typical installation, i.e. it will be found by a [`FrozenImporter`](https://docs.python.org/3/library/importlib.html#importlib.machinery.FrozenImporter) from an [autogenerated C version](https://github.com/python/cpython/blob/3fb7c608e5764559a718ce8cb81350d7a3df0356/Python/frozen.c#L122), rather than from the `os.py` file. The `sys` module will likely be loaded by a [`BuiltinImporter`](https://docs.python.org/3/library/importlib.html#importlib.machinery.BuiltinImporter), and is actually compiled into the interpreter (along with [a handful of others](https://docs.python.org/3/library/sys.html#sys.builtin_module_names), not currently including `os`).

Note that they're _both_ imported during interpreter startup, as you can see by executing:

    python3 -v -c ''

When you import `sys` and/or `os` from an interactive interpreter, all you do is bind the names in your local namespace - both modules have already been fully loaded and will not be imported again.

</details>



# 答案2
**得分**: -1

`sys` 使用了 `os` 模块，但没有将其导入到您的代码中。您可以通过以下代码进行确认：

```python
import sys

然后您可以使用以下代码来获取当前工作目录：

import os
os.getcwd()

In [1]: import sys

In [2]: os.getcwd()
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
Cell In [2], line 1
----> 1 os.getcwd()

NameError: name 'os' is not defined