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Python Language Reference Manual
by Guido van Rossum and Fred L. Drake, Jr.
Paperback (6"x9"), 120 pages
ISBN 0954161785
RRP £12.95 ($19.95)

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6.12 The import statement

import_stmt "import" module ["as" name]
( "," module ["as" name] )* | "from" module "import" identifier ["as" name] ( "," identifier ["as" name] )* | "from" module "import" "(" identifier ["as" name] ( "," identifier ["as" name] )* [","] ")" | "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.

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 initialized, indexed by module name. This table is 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. If the module name is followed by as, the name following as is used as the local name for the module.

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 __all__; if 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 the module).

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’. (16)

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 information.

ISBN 0954161785Python Language Reference ManualSee the print edition