Comments on: The Internals of Python’s IMPORT_NAME Bytecode http://www.vector-seven.com/2008/04/14/the-internals-of-pythons-import_name-bytecode/ My software development thoughts, experiments and discoveries Thu, 24 Jul 2008 04:25:36 +0000 http://wordpress.org/?v=2.5.1 By: Brett http://www.vector-seven.com/2008/04/14/the-internals-of-pythons-import_name-bytecode/#comment-1054 Brett Mon, 14 Apr 2008 18:47:47 +0000 http://www.vector-seven.com/?p=59#comment-1054 The fromlist is needed by __import__ to perform potential imports if the module being imported is a package. Consider importing pkg/__init__.py as ``from pkg import stuff``. Stuff is expected to be a module, either pkg/stuff.py or pkg/stuff/__init__.py . __import__ handles this import and then sets the module as an attribute on the pkg module itself. I know it seems convoluted, but it helps keep the bytecode simple. The fromlist is needed by __import__ to perform potential imports if the module being imported is a package. Consider importing pkg/__init__.py as “from pkg import stuff“. Stuff is expected to be a module, either pkg/stuff.py or pkg/stuff/__init__.py . __import__ handles this import and then sets the module as an attribute on the pkg module itself.

I know it seems convoluted, but it helps keep the bytecode simple.

]]> By: Paul Bonser http://www.vector-seven.com/2008/04/14/the-internals-of-pythons-import_name-bytecode/#comment-1053 Paul Bonser Mon, 14 Apr 2008 17:42:07 +0000 http://www.vector-seven.com/?p=59#comment-1053 Thanks for the response, Thomas. Somehow at 2AM it doesn't always occur to me to do the obvious thing like look at the Python documentation. After looking at the docs for __import__, it's interesting to see that the values in fromlist aren't actually used, but it is significant whether the fromlist is empty or non-empty. <blockquote> When the name variable is of the form package.module, normally, the top-level package (the name up till the first dot) is returned, not the module named by name. However, when a non-empty fromlist argument is given, the module named by name is returned. This is done for compatibility with the bytecode generated for the different kinds of import statement; when using "import spam.ham.eggs", the top-level package spam must be placed in the importing namespace, but when using "from spam.ham import eggs", the spam.ham subpackage must be used to find the eggs variable. </blockquote> I suppose the reason that fromlist isn't just changed to a boolean is that it might be significant in a setup where __import__ is redefined for custom importing, like you said. Thanks for the response, Thomas. Somehow at 2AM it doesn’t always occur to me to do the obvious thing like look at the Python documentation.

After looking at the docs for __import__, it’s interesting to see that the values in fromlist aren’t actually used, but it is significant whether the fromlist is empty or non-empty.

When the name variable is of the form package.module, normally, the top-level package (the name up till the first dot) is returned, not the module named by name. However, when a non-empty fromlist argument is given, the module named by name is returned. This is done for compatibility with the bytecode generated for the different kinds of import statement; when using “import spam.ham.eggs”, the top-level package spam must be placed in the importing namespace, but when using “from spam.ham import eggs”, the spam.ham subpackage must be used to find the eggs variable.

I suppose the reason that fromlist isn’t just changed to a boolean is that it might be significant in a setup where __import__ is redefined for custom importing, like you said.

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