import_stmt: "import" module ("," module)* | "from" module "import" identifier ("," identifier)* | "from" module "import" "*" module: (identifier ".")* identifier
Import statements are executed in two steps: (1) find a module, and initialize it if necessary; (2) define a name or names in the local namespace (of the scope where the import statement occurs). The first form (without from) repeats these steps for each identifier in the list. The form with from performs step (1) once, and then performs step (2) repeatedly.
The system maintains a table of modules that have been initialized, indexed by module name. This table table accessible as sys.modules. When a module name is found in this table, step (1) is finished. If not, a search for a module definition is started. When a module is found, it is loaded. Details of the module searching and loading process are implementation and platform specific. It generally involves searching for a ``built-in'' module with the given name and then searching a list of locations given as sys.path.
If a built-in module is found, its built-in initialization code is executed and step (1) is finished. If no matching file is found, ImportError is raised. If a file is found, it is parsed, yielding an executable code block. If a syntax error occurs, SyntaxError is raised. Otherwise, an empty module of the given name is created and inserted in the module table, and then the code block is executed in the context of this module. Exceptions during this execution terminate step (1).
When step (1) finishes without raising an exception, step (2) can begin.
The first form of import statement binds the module name in the local namespace to the module object, and then goes on to import the next identifier, if any. The from form does not bind the module name: it goes through the list of identifiers, looks each one of them up in the module found in step (1), and binds the name in the local namespace to the object thus found. If a name is not found, ImportError is raised. If the list of identifiers is replaced by a star (*), all names defined in the module are bound, except those beginning with an underscore(_).
Names bound by import statements may not occur in global statements in the same scope.
The from form with * may only occur in a module scope.
(The current implementation does not enforce the latter two restrictions, but programs should not abuse this freedom, as future implementations may enforce them or silently change the meaning of the program.)
Hierarchical module names: when the module names contains one or more dots, the module search path is carried out differently. The sequence of identifiers up to the last dot is used to find a ``package''; the final identifier is then searched inside the package. A package is generally a subdirectory of a directory on sys.path that has a file "__init__.py".[XXX Can't be bothered to spell this out right now; see the URL http://www.python.org/doc/essays/packages.html for more details, also about how the module search works from inside a package.]
[XXX Also should mention __import__().]