|Python Language Reference Manual|
by Guido van Rossum and Fred L. Drake, Jr.
Paperback (6"x9"), 120 pages
RRP £12.95 ($19.95)
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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 the 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,
If a file is found, it is parsed,
yielding an executable code block. If a syntax error occurs,
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.
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
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
__all__ should contain the entire public API.
It is intended to avoid accidentally exporting items that are not part
of the API (such as library modules which were imported and used within
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
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
The built-in function
__import__() is provided to support
applications that determine which modules need to be loaded
dynamically; refer to the Built-in Functions section in the
Python Library Reference Manual for additional
|ISBN 0954161785||Python Language Reference Manual||See the print edition|