When a submodule is loaded using any mechanism (e.g. importlib APIs, the import or import-from statements, or built-in __import__()) a binding is placed in the parent module’s namespace to the submodule object. For example, if package spam has a submodule foo, after importing spam.foo, spam will have an attribute foo which is bound to the submodule.
...
Given Python’s familiar name binding rules this might seem surprising, but it’s actually a fundamental feature of the import system. The invariant holding is that if you have sys.modules['spam'] and sys.modules['spam.foo'] (as you would after the above import), the latter must appear as the foo attribute of the former.
— 5. The import system — Python 3.10.5 documentation
另外，对于形如 import A.B.C 的导入，A、A.B、A.B.C 都会被挂载到本 module 上。然而，from A.B import C 却只会挂载 C，而 import A.B.C as D 也只会挂载 D ，即使 A、A.B 都被执行且都在 sys.modules 里。
sys.path
A list of strings that specifies the search path for modules. Initialized from the environment variable PYTHONPATH, plus an installation-dependent default.
As initialized upon program startup, the first item of this list, path[0], is the directory containing the script that was used to invoke the Python interpreter. If the script directory is not available (e.g. if the interpreter is invoked interactively or if the script is read from standard input), path[0] is the empty string, which directs Python to search modules in the current directory first. Notice that the script directory is inserted before the entries inserted as a result of PYTHONPATH.
A program is free to modify this list for its own purposes. Only strings and bytes should be added to sys.path; all other data types are ignored during import.
Search sys.path for the named module and execute its contents as the __main__ module.
Since the argument is a module name, you must not give a file extension (.py). The module name should be a valid absolute Python module name, but the implementation may not always enforce this (e.g. it may allow you to use a name that includes a hyphen).
Package names (including namespace packages) are also permitted. When a package name is supplied instead of a normal module, the interpreter will execute <pkg>.__main__ as the main module. This behaviour is deliberately similar to the handling of directories and zipfiles that are passed to the interpreter as the script argument.
Note
This option cannot be used with built-in modules and extension modules written in C, since they do not have Python module files. However, it can still be used for precompiled modules, even if the original source file is not available.
If this option is given, the first element of sys.argv will be the full path to the module file (while the module file is being located, the
first element will be set to "-m"). As with the -c option, the current directory will be added to the start of sys.path.
-I option can be used to run the script in isolated mode where sys.path contains neither the current directory nor the user’s site-packages directory. All PYTHON* environment variables are ignored, too.
Many standard library modules contain code that is invoked on their execution as a script. An example is the timeit module:
The first place checked during import search is sys.modules. This mapping serves as a cache of all modules that have been previously imported, including the intermediate paths. So if foo.bar.baz was previously imported, sys.modules will contain entries for foo, foo.bar, and foo.bar.baz. Each key will have as its value the corresponding module object.
During import, the module name is looked up in sys.modules and if present, the associated value is the module satisfying the import, and the process completes. However, if the value is None, then a ModuleNotFoundError is raised. If the module name is missing, Python will continue searching for the module.
sys.modules is writable. Deleting a key may not destroy the associated module (as other modules may hold references to it),
but it will invalidate the cache entry for the named module, causing Python to search anew for the named module upon its next
import. The key can also be assigned to None, forcing the next import of the module to result in a ModuleNotFoundError.
Beware though, as if you keep a reference to the module object, invalidate its cache entry in sys.modules, and then re-import the named module, the two module objects will not be the same. By contrast, importlib.reload() will reuse the same module object, and simply reinitialise the module contents by rerunning the module’s code.
— 5. The import system — Python 3.10.5 documentation
When a submodule is loaded using any mechanism (e.g. importlib APIs, the import or import-from statements, or built-in __import__()) a binding is placed in the parent module’s namespace to the submodule object. For example, if package spam has a submodule foo, after importing spam.foo, spam will have an attribute foo which is bound to the submodule.
...
Given Python’s familiar name binding rules this might seem surprising, but it’s actually a fundamental feature of the import system. The invariant holding is that if you have sys.modules['spam'] and sys.modules['spam.foo'] (as you would after the above import), the latter must appear as the foo attribute of the former.
— 5. The import system — Python 3.10.5 documentation
另外，对于形如 import A.B.C 的导入，A、A.B、A.B.C 都会被挂载到本 module 上。然而，from A.B import C 却只会挂载 C，而 import A.B.C as D 也只会挂载 D ，即使 A、A.B 都被执行且都在 sys.modules 里。
sys.path
A list of strings that specifies the search path for modules. Initialized from the environment variable PYTHONPATH, plus an installation-dependent default.
As initialized upon program startup, the first item of this list, path[0], is the directory containing the script that was used to invoke the Python interpreter. If the script directory is not available (e.g. if the interpreter is invoked interactively or if the script is read from standard input), path[0] is the empty string, which directs Python to search modules in the current directory first. Notice that the script directory is inserted before the entries inserted as a result of PYTHONPATH.
A program is free to modify this list for its own purposes. Only strings and bytes should be added to sys.path; all other data types are ignored during import.
Search sys.path for the named module and execute its contents as the __main__ module.
Since the argument is a module name, you must not give a file extension (.py). The module name should be a valid absolute Python module name, but the implementation may not always enforce this (e.g. it may allow you to use a name that includes a hyphen).
Package names (including namespace packages) are also permitted. When a package name is supplied instead of a normal module, the interpreter will execute <pkg>.__main__ as the main module. This behaviour is deliberately similar to the handling of directories and zipfiles that are passed to the interpreter as the script argument.
Note
This option cannot be used with built-in modules and extension modules written in C, since they do not have Python module files. However, it can still be used for precompiled modules, even if the original source file is not available.
If this option is given, the first element of sys.argv will be the full path to the module file (while the module file is being located, the
first element will be set to "-m"). As with the -c option, the current directory will be added to the start of sys.path.
-I option can be used to run the script in isolated mode where sys.path contains neither the current directory nor the user’s site-packages directory. All PYTHON* environment variables are ignored, too.
Many standard library modules contain code that is invoked on their execution as a script. An example is the timeit module:
The first place checked during import search is sys.modules. This mapping serves as a cache of all modules that have been previously imported, including the intermediate paths. So if foo.bar.baz was previously imported, sys.modules will contain entries for foo, foo.bar, and foo.bar.baz. Each key will have as its value the corresponding module object.
During import, the module name is looked up in sys.modules and if present, the associated value is the module satisfying the import, and the process completes. However, if the value is None, then a ModuleNotFoundError is raised. If the module name is missing, Python will continue searching for the module.
sys.modules is writable. Deleting a key may not destroy the associated module (as other modules may hold references to it),
but it will invalidate the cache entry for the named module, causing Python to search anew for the named module upon its next
import. The key can also be assigned to None, forcing the next import of the module to result in a ModuleNotFoundError.
Beware though, as if you keep a reference to the module object, invalidate its cache entry in sys.modules, and then re-import the named module, the two module objects will not be the same. By contrast, importlib.reload() will reuse the same module object, and simply reinitialise the module contents by rerunning the module’s code.
— 5. The import system — Python 3.10.5 documentation
