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| 1 | +\section{Built-in module \sectcode{imp}} |
| 2 | +\bimodindex{imp} |
| 3 | +\index{import} |
| 4 | + |
| 5 | +This module provides an interface to the mechanisms use to implement |
| 6 | +the \code{import} statement. It defines the following constants and |
| 7 | +functions: |
| 8 | + |
| 9 | +\renewcommand{\indexsubitem}{(in module struct)} |
| 10 | + |
| 11 | +\begin{funcdesc}{get_magic}{} |
| 12 | +Return the magic string used to recognize value 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. |
| 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 | +corresponding to the file found, \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 exception. If there is no |
| 43 | +built-in module called \var{name}, \code{None} is returned. |
| 44 | +\end{funcdesc} |
| 45 | + |
| 46 | +\begin{funcdesc}{init_frozen}{name} |
| 47 | +Initialize the frozen module called \var{name} and return its module |
| 48 | +object. If the module was already initialized, it will be initialized |
| 49 | +{\em again}. If there is no frozen module called \var{name}, |
| 50 | +\code{None} is returned. (Frozen modules are modules written in |
| 51 | +Python whose compiled byte-code object is incorporated into a |
| 52 | +custom-built Python interpreter by Python's \code{freeze} utility. |
| 53 | +See \code{Demo/freeze} for now.) |
| 54 | +\end{funcdesc} |
| 55 | + |
| 56 | +\begin{funcdesc}{is_builtin}{name} |
| 57 | +Return \code{1} if there is a built-in module called \var{name} which can be |
| 58 | +initialized again. Return \code{-1} if there is a built-in module |
| 59 | +called \var{name} which cannot be initialized again (see |
| 60 | +\code{init_builtin}). Return \code{0} if there is no built-in module |
| 61 | +called \var{name}. |
| 62 | +\end{funcdesc} |
| 63 | + |
| 64 | +\begin{funcdesc}{is_frozen}{name} |
| 65 | +Return \code{1} if there is a frozen module (see \code{init_frozen}) |
| 66 | +called \var{name}, \code{0} if there is no such module. |
| 67 | +\end{funcdesc} |
| 68 | + |
| 69 | +\begin{funcdesc}{load_compiled}{name\, pathname\, \optional{file}} |
| 70 | +Load and initialize a module implemented as a byte-compiled code file |
| 71 | +and return its module object. If the module was already initialized, |
| 72 | +it will be initialized {\em again}. The \var{name} argument is used |
| 73 | +to create or access a module object. The \var{pathname} argument |
| 74 | +points to the byte-compiled code file. The optional \var{file} |
| 75 | +argument is the byte-compiled code file, open for reading in binary |
| 76 | +mode, from the beginning -- if not given, the function opens |
| 77 | +\var{pathname}. It must currently be a real file object, not a |
| 78 | +user-defined class emulating a file. |
| 79 | +\end{funcdesc} |
| 80 | + |
| 81 | +\begin{funcdesc}{load_dynamic}{name\, pathname\, \optional{file}} |
| 82 | +Load and initialize a module implemented as a dynamically loadable |
| 83 | +shared library and return its module object. If the module was |
| 84 | +already initialized, it will be initialized {\em again}. Some modules |
| 85 | +don't like that and may raise an exception. The \var{pathname} |
| 86 | +argument must point to the shared library. The \var{name} argument is |
| 87 | +used to construct the name of the initialization function: an external |
| 88 | +C function called \code{init\var{name}()} in the shared library is |
| 89 | +called. The optional \var{file} argment is ignored. (Note: using |
| 90 | +shared libraries is highly system dependent, and not all systems |
| 91 | +support it.) |
| 92 | +\end{funcdesc} |
| 93 | + |
| 94 | +\begin{funcdesc}{load_source}{name\, pathname\, \optional{file}} |
| 95 | +Load and initialize a module implemented as a Python source file and |
| 96 | +return its module object. If the module was already initialized, it |
| 97 | +will be initialized {\em again}. The \var{name} argument is used to |
| 98 | +create or access a module object. The \var{pathname} argument points |
| 99 | +to the source file. The optional \var{file} argument is the source |
| 100 | +file, open for reading as text, from the beginning -- if not given, |
| 101 | +the function opens \var{pathname}. It must currently be a real file |
| 102 | +object, not a user-defined class emulating a file. Note that if a |
| 103 | +properly matching byte-compiled file (with suffix \code{.pyc}) exists, |
| 104 | +it will be used instead of parsing the given source file. |
| 105 | +\end{funcdesc} |
| 106 | + |
| 107 | +\begin{funcdesc}{new_module}{name} |
| 108 | +Return a new empty module object called \var{name}. This object is |
| 109 | +{\em not} inserted in \code{sys.modules}. |
| 110 | +\end{funcdesc} |
| 111 | + |
| 112 | +The following constants with integer values, defined in the module, |
| 113 | +are used to indicate the search result of \code{imp.find_module}. |
| 114 | + |
| 115 | +\begin{datadesc}{SEARCH_ERROR} |
| 116 | +The module was not found. |
| 117 | +\end{datadesc} |
| 118 | + |
| 119 | +\begin{datadesc}{PY_SOURCE} |
| 120 | +The module was found as a source file. |
| 121 | +\end{datadesc} |
| 122 | + |
| 123 | +\begin{datadesc}{PY_COMPILED} |
| 124 | +The module was found as a compiled code object file. |
| 125 | +\end{datadesc} |
| 126 | + |
| 127 | +\begin{datadesc}{C_EXTENSION} |
| 128 | +The module was found as dynamically loadable shared library. |
| 129 | +\end{datadesc} |
| 130 | + |
| 131 | +\subsection{Examples} |
| 132 | +The following function emulates the default import statement: |
| 133 | + |
| 134 | +\begin{verbatim} |
| 135 | +import imp |
| 136 | +from sys import modules |
| 137 | +
|
| 138 | +def __import__(name): |
| 139 | + # Fast path: let's see if it's already in sys.modules. |
| 140 | + # Two speed optimizations are worth mentioning: |
| 141 | + # - We use 'modules' instead of 'sys.modules'; this saves a |
| 142 | + # dictionary look-up per call. |
| 143 | + # - It's also faster to use a try-except statement than |
| 144 | + # to use modules.has_key(name) to check if it's there. |
| 145 | + try: |
| 146 | + return modules[name] |
| 147 | + except KeyError: |
| 148 | + pass |
| 149 | +
|
| 150 | + # See if it's a built-in module |
| 151 | + m = imp.init_builtin(name) |
| 152 | + if m: |
| 153 | + return m |
| 154 | +
|
| 155 | + # See if it's a frozen module |
| 156 | + m = imp.init_frozen(name) |
| 157 | + if m: |
| 158 | + return m |
| 159 | +
|
| 160 | + # Search the default path (i.e. sys.path). |
| 161 | + # If this raises an exception, the module is not found -- |
| 162 | + # let the caller handle the exception. |
| 163 | + fp, pathname, (suffix, mode, type) = imp.find_module(name) |
| 164 | +
|
| 165 | + # See what we got. |
| 166 | + # Note that fp will be closed automatically when we return. |
| 167 | + if type == imp.C_EXTENSION: |
| 168 | + return imp.load_dynamic(name, pathname) |
| 169 | + if type == imp.PY_SOURCE: |
| 170 | + return imp.load_source(name, pathname, fp) |
| 171 | + if type == imp.PY_COMPILED: |
| 172 | + return imp.load_source(name, pathname, fp) |
| 173 | +
|
| 174 | + # Shouldn't get here at all. |
| 175 | + raise ImportError, '%s: unknown module type (%d)' % (name, type) |
| 176 | +\end{verbatim} |
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