\section{\module{imp} ---         Access the \keyword{import} internals} \declaremodule{builtin}{imp}\modulesynopsis{Access the implementation of the \keyword{import} statement.}  This\stindex{import} module provides an interface to the mechanismsused to implement the \keyword{import} statement.  It defines thefollowing constants and functions:  \begin{funcdesc}{get_magic}{}\indexii{file}{byte-code}Return the magic string value used to recognize byte-compiled codefiles (\file{.pyc} files).  (This value may be different for eachPython version.)\end{funcdesc} \begin{funcdesc}{get_suffixes}{}Return a list of triples, each describing a particular type of module.Each triple has the form \code{(\var{suffix}, \var{mode},\var{type})}, where \var{suffix} is a string to be appended to themodule name to form the filename to search for, \var{mode} is the modestring to pass to the built-in \function{open()} function to open thefile (this can be \code{'r'} for text files or \code{'rb'} for binaryfiles), and \var{type} is the file type, which has one of the values\constant{PY_SOURCE}, \constant{PY_COMPILED}, or\constant{C_EXTENSION}, described below.\end{funcdesc} \begin{funcdesc}{find_module}{name\optional{, path}}Try to find the module \var{name} on the search path \var{path}.  If\var{path} is a list of directory names, each directory is searchedfor files with any of the suffixes returned by \function{get_suffixes()}above.  Invalid names in the list are silently ignored (but all listitems must be strings).  If \var{path} is omitted or \code{None}, thelist of directory names given by \code{sys.path} is searched, butfirst it searches a few special places: it tries to find a built-inmodule with the given name (\constant{C_BUILTIN}), then a frozen module(\constant{PY_FROZEN}), and on some systems some other places are lookedin as well (on the Mac, it looks for a resource (\constant{PY_RESOURCE});on Windows, it looks in the registry which may point to a specificfile). If search is successful, the return value is a triple\code{(\var{file}, \var{pathname}, \var{description})} where\var{file} is an open file object positioned at the beginning,\var{pathname} is the pathname of thefile found, and \var{description} is a triple as contained in the listreturned by \function{get_suffixes()} describing the kind of module found.If the module does not live in a file, the returned \var{file} is\code{None}, \var{filename} is the empty string, and the\var{description} tuple contains empty strings for its suffix andmode; the module type is as indicate in parentheses above.  If thesearch is unsuccessful, \exception{ImportError} is raised.  Otherexceptions indicate problems with the arguments or environment. This function does not handle hierarchical module names (namescontaining dots).  In order to find \var{P}.\var{M}, that is, submodule\var{M} of package \var{P}, use \function{find_module()} and\function{load_module()} to find and load package \var{P}, and then use\function{find_module()} with the \var{path} argument set to\code{\var{P}.__path__}.  When \var{P} itself has a dotted name, applythis recipe recursively.\end{funcdesc} \begin{funcdesc}{load_module}{name, file, filename, description}Load a module that was previously found by \function{find_module()} (or byan otherwise conducted search yielding compatible results).  Thisfunction does more than importing the module: if the module wasalready imported, it is equivalent to a\function{reload()}\bifuncindex{reload}!  The \var{name} argumentindicates the full module name (including the package name, if this isa submodule of a package).  The \var{file} argument is an open file,and \var{filename} is the corresponding file name; these can be\code{None} and \code{''}, respectively, when the module is not beingloaded from a file.  The \var{description} argument is a tuple, aswould be returned by \function{get_suffixes()}, describing what kindof module must be loaded. If the load is successful, the return value is the module object;otherwise, an exception (usually \exception{ImportError}) is raised. \strong{Important:} the caller is responsible for closing the\var{file} argument, if it was not \code{None}, even when an exceptionis raised.  This is best done using a \keyword{try}... \keyword{finally} statement.\end{funcdesc} \begin{funcdesc}{new_module}{name}Return a new empty module object called \var{name}.  This object is\emph{not} inserted in \code{sys.modules}.\end{funcdesc} \begin{funcdesc}{lock_held}{}Return \code{True} if the import lock is currently held, else \code{False}.On platforms without threads, always return \code{False}. On platforms with threads, a thread executing an import holds an internallock until the import is complete.This lock blocks other threads from doing an import until the originalimport completes, which in turn prevents other threads from seeingincomplete module objects constructed by the original thread while inthe process of completing its import (and the imports, if any,triggered by that).\end{funcdesc} \begin{funcdesc}{acquire_lock}{}Acquires the interpreter's import lock for the current thread.  This lockshould be used by import hooks to ensure thread-safety when importing modules.On platforms without threads, this function does nothing.\versionadded{2.3}\end{funcdesc} \begin{funcdesc}{release_lock}{}Release the interpreter's import lock.On platforms without threads, this function does nothing.\versionadded{2.3}\end{funcdesc} The following constants with integer values, defined in this module,are used to indicate the search result of \function{find_module()}. \begin{datadesc}{PY_SOURCE}The module was found as a source file.\end{datadesc} \begin{datadesc}{PY_COMPILED}The module was found as a compiled code object file.\end{datadesc} \begin{datadesc}{C_EXTENSION}The module was found as dynamically loadable shared library.\end{datadesc} \begin{datadesc}{PY_RESOURCE}The module was found as a Mac OS 9 resource.  This value can only bereturned on a Mac OS 9 or earlier Macintosh.\end{datadesc} \begin{datadesc}{PKG_DIRECTORY}The module was found as a package directory.\end{datadesc} \begin{datadesc}{C_BUILTIN}The module was found as a built-in module.\end{datadesc} \begin{datadesc}{PY_FROZEN}The module was found as a frozen module (see \function{init_frozen()}).\end{datadesc} The following constant and functions are obsolete; their functionalityis available through \function{find_module()} or \function{load_module()}.They are kept around for backward compatibility: \begin{datadesc}{SEARCH_ERROR}Unused.\end{datadesc} \begin{funcdesc}{init_builtin}{name}Initialize the built-in module called \var{name} and return its moduleobject.  If the module was already initialized, it will be initialized\emph{again}.  A few modules cannot be initialized twice --- attemptingto initialize these again will raise an \exception{ImportError}exception.  If there is nobuilt-in module called \var{name}, \code{None} is returned.\end{funcdesc} \begin{funcdesc}{init_frozen}{name}Initialize the frozen module called \var{name} and return its moduleobject.  If the module was already initialized, it will be initialized\emph{again}.  If there is no frozen module called \var{name},\code{None} is returned.  (Frozen modules are modules written inPython whose compiled byte-code object is incorporated into acustom-built Python interpreter by Python's \program{freeze} utility.See \file{Tools/freeze/} for now.)\end{funcdesc} \begin{funcdesc}{is_builtin}{name}Return \code{1} if there is a built-in module called \var{name} whichcan be initialized again.  Return \code{-1} if there is a built-inmodule called \var{name} which cannot be initialized again (see\function{init_builtin()}).  Return \code{0} if there is no built-inmodule called \var{name}.\end{funcdesc} \begin{funcdesc}{is_frozen}{name}Return \code{True} if there is a frozen module (see\function{init_frozen()}) called \var{name}, or \code{False} if there isno such module.\end{funcdesc} \begin{funcdesc}{load_compiled}{name, pathname, \optional{file}}\indexii{file}{byte-code}Load and initialize a module implemented as a byte-compiled code fileand return its module object.  If the module was already initialized,it will be initialized \emph{again}.  The \var{name} argument is usedto create or access a module object.  The \var{pathname} argumentpoints to the byte-compiled code file.  The \var{file}argument is the byte-compiled code file, open for reading in binarymode, from the beginning.It must currently be a real file object, not auser-defined class emulating a file.\end{funcdesc} \begin{funcdesc}{load_dynamic}{name, pathname\optional{, file}}Load and initialize a module implemented as a dynamically loadableshared library and return its module object.  If the module wasalready initialized, it will be initialized \emph{again}.  Some modulesdon't like that and may raise an exception.  The \var{pathname}argument must point to the shared library.  The \var{name} argument isused to construct the name of the initialization function: an externalC function called \samp{init\var{name}()} in the shared library iscalled.  The optional \var{file} argument is ignored.  (Note: usingshared libraries is highly system dependent, and not all systemssupport it.)\end{funcdesc} \begin{funcdesc}{load_source}{name, pathname\optional{, file}}Load and initialize a module implemented as a Python source file andreturn its module object.  If the module was already initialized, itwill be initialized \emph{again}.  The \var{name} argument is used tocreate or access a module object.  The \var{pathname} argument pointsto the source file.  The \var{file} argument is the sourcefile, open for reading as text, from the beginning.It must currently be a real fileobject, not a user-defined class emulating a file.  Note that if aproperly matching byte-compiled file (with suffix \file{.pyc} or\file{.pyo}) exists, it will be used instead of parsing the givensource file.\end{funcdesc} \begin{classdesc}{NullImporter}{path_string}The \class{NullImporter} type is a \pep{302} import hook that handlesnon-directory path strings by failing to find any modules.  Calling thistype with an existing directory or empty string raises\exception{ImportError}.  Otherwise, a \class{NullImporter} instance isreturned. Python adds instances of this type to \code{sys.path_importer_cache} forany path entries that are not directories and are not handled by any otherpath hooks on \code{sys.path_hooks}.  Instances have only one method: \begin{methoddesc}{find_module}{fullname \optional{, path}}This method always returns \code{None}, indicating that the requestedmodule could not be found.\end{methoddesc} \versionadded{2.5}\end{classdesc} \subsection{Examples}\label{examples-imp} The following function emulates what was the standard import statementup to Python 1.4 (no hierarchical module names).  (This\emph{implementation} wouldn't work in that version, since\function{find_module()} has been extended and\function{load_module()} has been added in 1.4.) \begin{verbatim}import impimport sys def __import__(name, globals=None, locals=None, fromlist=None):    # Fast path: see if the module has already been imported.    try:        return sys.modules[name]    except KeyError:        pass     # If any of the following calls raises an exception,    # there's a problem we can't handle -- let the caller handle it.     fp, pathname, description = imp.find_module(name)     try:        return imp.load_module(name, fp, pathname, description)    finally:        # Since we may exit via an exception, close fp explicitly.        if fp:            fp.close()\end{verbatim} A more complete example that implements hierarchical module names andincludes a \function{reload()}\bifuncindex{reload} function can befound in the module \module{knee}\refmodindex{knee}.  The\module{knee} module can be found in \file{Demo/imputil/} in thePython source distribution.