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.
In this context, to ``initialize'' a built-in or extension module means to call an initialization function that the module must provide for the purpose (in the reference implementation, the function's name is obtained by prepending string ``init'' to the module's name); to ``initialize'' a Python-coded module means to execute the module's body.
The system maintains a table of modules that have been or are being
indexed by module name. This table is
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
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. If the module name is followed by
as, the name following as is used as the local
name for the module. To avoid confusion, you cannot import modules
with dotted names as a different local name. So
module as m is legal, but
import module.submod as s is not.
The latter should be written as
from module import submod as s;
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. As with the first form of import, an alternate local name can be supplied by specifying "as localname". If a name is not found, ImportError is raised. If the list of identifiers is replaced by a star ("*"), all public names defined in the module are bound in the local namespace of the import statement..
The public names defined by a module are determined by checking
the module's namespace for a variable named
defined, it must be a sequence of strings which are names defined or
imported by that module. The names given in
__all__ are all
considered public and are required to exist. If
__all__ is not
defined, the set of public names includes all names found in the
module's namespace which do not begin with an underscore character
The from form with "*" may only occur in a module scope. If the wild card form of import -- "import *" -- is used in a function and the function contains or is a nested block with free variables, the compiler will raise a SyntaxError.
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.]
The built-in function __import__() is provided to support applications that determine which modules need to be loaded dynamically; refer to Built-in Functions in the Python Library Reference for additional information.
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