为什么需要使用'import'语句显式导入*内置* python模块'sys'？

Question

The python documentation says:

"One particular module deserves some attention: sys, which is built into every Python interpreter."

My understanding is that if a module is built into the Python interpreter itself, then there is no need for an explicit import statement. If the sys module is built-in the Python interpreter, then why is an explicit import statement required for the sys module?

Answer 1

sys is imported at Python startup. So when you import sys , it does not actually do anything except bind a variable name to the already-existing module.

When creating a module instance, there is no reason to have the sys name bound in the module scope when many (probably most) modules don't need to use sys . So, that name is not in scope by default.

Answer 2

import performs two functions:

1. It loads the module from disk, initializing and executing it.
2. It adds the module to the local namespace.

With "built-in" modules item 1 is not an issue, but item 2 is still important; without it the code would throw a NameError .