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:sys.path
that has a
file __init__.py.
[XXX Also should mention __import__().]