Let's break down exactly why this happens—it all comes down to how Python handles module imports and how PyCryptodome's package structure is set up.

• When you run import Crypto.publickey
  This only imports the top-level publickey package (specifically, it executes the __init__.py file in the Crypto/publickey/ directory). By default, Python doesn't automatically import submodules of a package unless explicitly told to do so. Since PyCryptodome's Crypto/publickey/__init__.py doesn't include an import for the RSA submodule, the RSA attribute doesn't exist in the Crypto.publickey namespace yet. So when you try to access Crypto.publickey.RSA, Python throws that "no attribute" error.

• When you run import Crypto.publickey.RSA
  This explicitly tells Python to load the RSA submodule. Once imported, Python adds the RSA reference to the Crypto.publickey namespace, making it accessible via Crypto.publickey.RSA (or directly via RSA if you use from Crypto.publickey import RSA).

Think of it like a folder structure:

• Crypto/ is a main folder, publickey/ is a subfolder inside it, and RSA.py is a file inside publickey/.
• Just opening the publickey/ folder (importing Crypto.publickey) doesn't automatically open the RSA.py file—you have to specifically grab that file (import Crypto.publickey.RSA) to use its contents.

If you prefer a shorter alternative to the full import, you can also use:

from Crypto.publickey import RSA
# Then use RSA directly, like RSA.importKey(base64.b64decode(PRIVATE_KEY))

内容的提问来源于stack exchange，提问作者SparksofFire