Doc/lib/libimp.tex

   1 \section{Built-in Module \sectcode{imp}}
   2 \bimodindex{imp}
   3 \index{import}
   4
   5 This module provides an interface to the mechanisms used to implement
   6 the \code{import} statement.  It defines the following constants and
   7 functions:
   8
   9 \renewcommand{\indexsubitem}{(in module imp)}
  10
  11 \begin{funcdesc}{get_magic}{}
  12 Return the magic string value used to recognize byte-compiled code
  13 files (``\code{.pyc} files'').
  14 \end{funcdesc}
  15
  16 \begin{funcdesc}{get_suffixes}{}
  17 Return a list of triples, each describing a particular type of file.
  18 Each triple has the form \code{(\var{suffix}, \var{mode},
  19 \var{type})}, where \var{suffix} is a string to be appended to the
  20 module name to form the filename to search for, \var{mode} is the mode
  21 string to pass to the built-in \code{open} function to open the file
  22 (this can be \code{'r'} for text files or \code{'rb'} for binary
  23 files), and \var{type} is the file type, which has one of the values
  24 \code{PY_SOURCE}, \code{PY_COMPILED} or \code{C_EXTENSION}, defined
  25 below.  (System-dependent values may also be returned.)
  26 \end{funcdesc}
  27
  28 \begin{funcdesc}{find_module}{name\, \optional{path}}
  29 Try to find the module \var{name} on the search path \var{path}.  The
  30 default \var{path} is \code{sys.path}.  The return value is a triple
  31 \code{(\var{file}, \var{pathname}, \var{description})} where
  32 \var{file} is an open file object positioned at the beginning,
  33 \var{pathname} is the pathname of the
  34 file found, and \var{description} is a triple as contained in the list
  35 returned by \code{get_suffixes} describing the kind of file found.
  36 \end{funcdesc}
  37
  38 \begin{funcdesc}{init_builtin}{name}
  39 Initialize the built-in module called \var{name} and return its module
  40 object.  If the module was already initialized, it will be initialized
  41 {\em again}.  A few modules cannot be initialized twice --- attempting
  42 to initialize these again will raise an \code{ImportError} exception.
  43 If there is no
  44 built-in module called \var{name}, \code{None} is returned.
  45 \end{funcdesc}
  46
  47 \begin{funcdesc}{init_frozen}{name}
  48 Initialize the frozen module called \var{name} and return its module
  49 object.  If the module was already initialized, it will be initialized
  50 {\em again}.  If there is no frozen module called \var{name},
  51 \code{None} is returned.  (Frozen modules are modules written in
  52 Python whose compiled byte-code object is incorporated into a
  53 custom-built Python interpreter by Python's \code{freeze} utility.
  54 See \code{Tools/freeze} for now.)
  55 \end{funcdesc}
  56
  57 \begin{funcdesc}{is_builtin}{name}
  58 Return \code{1} if there is a built-in module called \var{name} which can be
  59 initialized again.  Return \code{-1} if there is a built-in module
  60 called \var{name} which cannot be initialized again (see
  61 \code{init_builtin}).  Return \code{0} if there is no built-in module
  62 called \var{name}.
  63 \end{funcdesc}
  64
  65 \begin{funcdesc}{is_frozen}{name}
  66 Return \code{1} if there is a frozen module (see \code{init_frozen})
  67 called \var{name}, \code{0} if there is no such module.
  68 \end{funcdesc}
  69
  70 \begin{funcdesc}{load_compiled}{name\, pathname\, \optional{file}}
  71 Load and initialize a module implemented as a byte-compiled code file
  72 and return its module object.  If the module was already initialized,
  73 it will be initialized {\em again}.  The \var{name} argument is used
  74 to create or access a module object.  The \var{pathname} argument
  75 points to the byte-compiled code file.  The optional \var{file}
  76 argument is the byte-compiled code file, open for reading in binary
  77 mode, from the beginning --- if not given, the function opens
  78 \var{pathname}.  It must currently be a real file object, not a
  79 user-defined class emulating a file.
  80 \end{funcdesc}
  81
  82 \begin{funcdesc}{load_dynamic}{name\, pathname\, \optional{file}}
  83 Load and initialize a module implemented as a dynamically loadable
  84 shared library and return its module object.  If the module was
  85 already initialized, it will be initialized {\em again}.  Some modules
  86 don't like that and may raise an exception.  The \var{pathname}
  87 argument must point to the shared library.  The \var{name} argument is
  88 used to construct the name of the initialization function: an external
  89 C function called \code{init\var{name}()} in the shared library is
  90 called.  The optional \var{file} argment is ignored.  (Note: using
  91 shared libraries is highly system dependent, and not all systems
  92 support it.)
  93 \end{funcdesc}
  94
  95 \begin{funcdesc}{load_source}{name\, pathname\, \optional{file}}
  96 Load and initialize a module implemented as a Python source file and
  97 return its module object.  If the module was already initialized, it
  98 will be initialized {\em again}.  The \var{name} argument is used to
  99 create or access a module object.  The \var{pathname} argument points
 100 to the source file.  The optional \var{file} argument is the source
 101 file, open for reading as text, from the beginning --- if not given,
 102 the function opens \var{pathname}.  It must currently be a real file
 103 object, not a user-defined class emulating a file.  Note that if a
 104 properly matching byte-compiled file (with suffix \code{.pyc}) exists,
 105 it will be used instead of parsing the given source file.
 106 \end{funcdesc}
 107
 108 \begin{funcdesc}{new_module}{name}
 109 Return a new empty module object called \var{name}.  This object is
 110 {\em not} inserted in \code{sys.modules}.
 111 \end{funcdesc}
 112
 113 The following constants with integer values, defined in the module,
 114 are used to indicate the search result of \code{imp.find_module}.
 115
 116 \begin{datadesc}{SEARCH_ERROR}
 117 The module was not found.
 118 \end{datadesc}
 119
 120 \begin{datadesc}{PY_SOURCE}
 121 The module was found as a source file.
 122 \end{datadesc}
 123
 124 \begin{datadesc}{PY_COMPILED}
 125 The module was found as a compiled code object file.
 126 \end{datadesc}
 127
 128 \begin{datadesc}{C_EXTENSION}
 129 The module was found as dynamically loadable shared library.
 130 \end{datadesc}
 131
 132 \subsection{Examples}
 133 The following function emulates the default import statement:
 134
 135 \begin{verbatim}
 136 import imp
 137 import sys
 138
 139 def __import__(name, globals=None, locals=None, fromlist=None):
 140     # Fast path: see if the module has already been imported.
 141     if sys.modules.has_key(name):
 142         return sys.modules[name]
 143
 144     # If any of the following calls raises an exception,
 145     # there's a problem we can't handle -- let the caller handle it.
 146
 147     # See if it's a built-in module.
 148     m = imp.init_builtin(name)
 149     if m:
 150         return m
 151
 152     # See if it's a frozen module.
 153     m = imp.init_frozen(name)
 154     if m:
 155         return m
 156
 157     # Search the default path (i.e. sys.path).
 158     fp, pathname, (suffix, mode, type) = imp.find_module(name)
 159
 160     # See what we got.
 161     try:
 162         if type == imp.C_EXTENSION:
 163             return imp.load_dynamic(name, pathname)
 164         if type == imp.PY_SOURCE:
 165             return imp.load_source(name, pathname, fp)
 166         if type == imp.PY_COMPILED:
 167             return imp.load_compiled(name, pathname, fp)
 168
 169         # Shouldn't get here at all.
 170         raise ImportError, '%s: unknown module type (%d)' % (name, type)
 171     finally:
 172         # Since we may exit via an exception, close fp explicitly.
 173         fp.close()
 174 \end{verbatim}