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annotate Doc/lib/libxdrlib.tex @ 5817:48b94de85bf3 legacy-trunk

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author Guido van Rossum <guido@python.org>
date Thu, 17 Jul 1997 16:29:42 +0000
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1 \section{Standard module \sectcode{xdrlib}}
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2 \stmodindex{xdrlib}
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3 \index{XDR}
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4
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5 \renewcommand{\indexsubitem}{(in module xdrlib)}
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6
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7
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8 The \code{xdrlib} module supports the External Data Representation
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9 Standard as described in RFC 1014, written by Sun Microsystems,
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10 Inc. June 1987. It supports most of the data types described in the
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11 RFC, although some, most notably \code{float} and \code{double} are
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12 only supported on those operating systems that provide an XDR
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13 library.
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14
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15 The \code{xdrlib} module defines two classes, one for packing
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16 variables into XDR representation, and another for unpacking from XDR
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17 representation. There are also two exception classes.
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18
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19
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20 \subsection{Packer Objects}
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21
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22 \code{Packer} is the class for packing data into XDR representation.
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23 The \code{Packer} class is instantiated with no arguments.
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24
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25 \begin{funcdesc}{get_buffer}{}
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26 Returns the current pack buffer as a string.
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27 \end{funcdesc}
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28
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29 \begin{funcdesc}{reset}{}
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30 Resets the pack buffer to the empty string.
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31 \end{funcdesc}
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32
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33 In general, you can pack any of the most common XDR data types by
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34 calling the appropriate \code{pack_\var{type}} method. Each method
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35 takes a single argument, the value to pack. The following simple data
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36 type packing methods are supported: \code{pack_uint}, \code{pack_int},
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37 \code{pack_enum}, \code{pack_bool}, \code{pack_uhyper},
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38 and \code{pack_hyper}.
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39
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40 The following methods pack floating point numbers, however they
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41 require C library support. Without the optional C built-in module,
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42 both of these methods will raise an \code{xdrlib.ConversionError}
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43 exception. See the note at the end of this chapter for details.
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44
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45 \begin{funcdesc}{pack_float}{value}
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46 Packs the single-precision floating point number \var{value}.
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47 \end{funcdesc}
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48
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49 \begin{funcdesc}{pack_double}{value}
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50 Packs the double-precision floating point number \var{value}.
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51 \end{funcdesc}
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52
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53 The following methods support packing strings, bytes, and opaque data:
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54
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55 \begin{funcdesc}{pack_fstring}{n\, s}
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56 Packs a fixed length string, \var{s}. \var{n} is the length of the
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57 string but it is \emph{not} packed into the data buffer. The string
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58 is padded with null bytes if necessary to guaranteed 4 byte alignment.
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59 \end{funcdesc}
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60
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61 \begin{funcdesc}{pack_fopaque}{n\, data}
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62 Packs a fixed length opaque data stream, similarly to
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63 \code{pack_fstring}.
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64 \end{funcdesc}
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65
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66 \begin{funcdesc}{pack_string}{s}
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67 Packs a variable length string, \var{s}. The length of the string is
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68 first packed as an unsigned integer, then the string data is packed
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69 with \code{pack_fstring}.
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70 \end{funcdesc}
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71
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72 \begin{funcdesc}{pack_opaque}{data}
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73 Packs a variable length opaque data string, similarly to
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74 \code{pack_string}.
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75 \end{funcdesc}
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76
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77 \begin{funcdesc}{pack_bytes}{bytes}
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78 Packs a variable length byte stream, similarly to \code{pack_string}.
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79 \end{funcdesc}
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80
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81 The following methods support packing arrays and lists:
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82
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83 \begin{funcdesc}{pack_list}{list\, pack_item}
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84 Packs a \var{list} of homogeneous items. This method is useful for
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85 lists with an indeterminate size; i.e. the size is not available until
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86 the entire list has been walked. For each item in the list, an
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87 unsigned integer \code{1} is packed first, followed by the data value
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88 from the list. \var{pack_item} is the function that is called to pack
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89 the individual item. At the end of the list, an unsigned integer
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90 \code{0} is packed.
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91 \end{funcdesc}
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92
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93 \begin{funcdesc}{pack_farray}{n\, array\, pack_item}
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94 Packs a fixed length list (\var{array}) of homogeneous items. \var{n}
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95 is the length of the list; it is \emph{not} packed into the buffer,
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96 but a \code{ValueError} exception is raised if \code{len(array)} is not
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97 equal to \var{n}. As above, \var{pack_item} is the function used to
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98 pack each element.
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99 \end{funcdesc}
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100
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101 \begin{funcdesc}{pack_array}{list\, pack_item}
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102 Packs a variable length \var{list} of homogeneous items. First, the
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103 length of the list is packed as an unsigned integer, then each element
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104 is packed as in \code{pack_farray} above.
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105 \end{funcdesc}
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106
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107 \subsection{Unpacker Objects}
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108
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109 \code{Unpacker} is the complementary class which unpacks XDR data
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110 values from a string buffer, and has the following methods:
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111
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112 \begin{funcdesc}{__init__}{data}
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113 Instantiates an \code{Unpacker} object with the string buffer
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114 \var{data}.
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115 \end{funcdesc}
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116
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117 \begin{funcdesc}{reset}{data}
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118 Resets the string buffer with the given \var{data}.
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119 \end{funcdesc}
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120
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121 \begin{funcdesc}{get_position}{}
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122 Returns the current unpack position in the data buffer.
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123 \end{funcdesc}
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124
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125 \begin{funcdesc}{set_position}{position}
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126 Sets the data buffer unpack position to \var{position}. You should be
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127 careful about using \code{get_position()} and \code{set_position()}.
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128 \end{funcdesc}
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129
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130 \begin{funcdesc}{get_buffer}{}
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131 Returns the current unpack data buffer as a string.
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132 \end{funcdesc}
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133
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134 \begin{funcdesc}{done}{}
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135 Indicates unpack completion. Raises an \code{xdrlib.Error} exception
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136 if all of the data has not been unpacked.
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137 \end{funcdesc}
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138
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139 In addition, every data type that can be packed with a \code{Packer},
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140 can be unpacked with an \code{Unpacker}. Unpacking methods are of the
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141 form \code{unpack_\var{type}}, and take no arguments. They return the
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142 unpacked object. The same caveats apply for \code{unpack_float} and
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143 \code{unpack_double} as above.
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144
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145 \begin{funcdesc}{unpack_float}{}
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146 Unpacks a single-precision floating point number.
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147 \end{funcdesc}
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148
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149 \begin{funcdesc}{unpack_double}{}
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150 Unpacks a double-precision floating point number, similarly to
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151 \code{unpack_float}.
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152 \end{funcdesc}
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153
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154 In addition, the following methods unpack strings, bytes, and opaque
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155 data:
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156
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157 \begin{funcdesc}{unpack_fstring}{n}
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158 Unpacks and returns a fixed length string. \var{n} is the number of
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159 characters expected. Padding with null bytes to guaranteed 4 byte
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160 alignment is assumed.
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161 \end{funcdesc}
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162
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163 \begin{funcdesc}{unpack_fopaque}{n}
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164 Unpacks and returns a fixed length opaque data stream, similarly to
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165 \code{unpack_fstring}.
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166 \end{funcdesc}
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167
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168 \begin{funcdesc}{unpack_string}{}
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169 Unpacks and returns a variable length string. The length of the
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170 string is first unpacked as an unsigned integer, then the string data
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171 is unpacked with \code{unpack_fstring}.
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172 \end{funcdesc}
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173
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174 \begin{funcdesc}{unpack_opaque}{}
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175 Unpacks and returns a variable length opaque data string, similarly to
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176 \code{unpack_string}.
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177 \end{funcdesc}
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178
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179 \begin{funcdesc}{unpack_bytes}{}
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180 Unpacks and returns a variable length byte stream, similarly to
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181 \code{unpack_string}.
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182 \end{funcdesc}
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183
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184 The following methods support unpacking arrays and lists:
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185
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186 \begin{funcdesc}{unpack_list}{unpack_item}
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187 Unpacks and returns a list of homogeneous items. The list is unpacked
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188 one element at a time
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189 by first unpacking an unsigned integer flag. If the flag is \code{1},
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190 then the item is unpacked and appended to the list. A flag of
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191 \code{0} indicates the end of the list. \var{unpack_item} is the
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192 function that is called to unpack the items.
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193 \end{funcdesc}
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194
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195 \begin{funcdesc}{unpack_farray}{n\, unpack_item}
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196 Unpacks and returns (as a list) a fixed length array of homogeneous
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197 items. \var{n} is number of list elements to expect in the buffer.
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198 As above, \var{unpack_item} is the function used to unpack each element.
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199 \end{funcdesc}
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200
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201 \begin{funcdesc}{unpack_array}{unpack_item}
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202 Unpacks and returns a variable length \var{list} of homogeneous items.
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203 First, the length of the list is unpacked as an unsigned integer, then
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204 each element is unpacked as in \code{unpack_farray} above.
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205 \end{funcdesc}
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206
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207 \subsection{Exceptions}
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208 \nodename{Exceptions in xdrlib module}
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209
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210 Exceptions in this module are coded as class instances:
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211
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212 \begin{excdesc}{Error}
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213 The base exception class. \code{Error} has a single public data
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214 member \code{msg} containing the description of the error.
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215 \end{excdesc}
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216
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217 \begin{excdesc}{ConversionError}
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218 Class derived from \code{Error}. Contains no additional instance
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219 variables.
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220 \end{excdesc}
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221
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222 Here is an example of how you would catch one of these exceptions:
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223
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224 \begin{verbatim}
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225 import xdrlib
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226 p = xdrlib.Packer()
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227 try:
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228 p.pack_double(8.01)
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229 except xdrlib.ConversionError, instance:
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230 print 'packing the double failed:', instance.msg
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231 \end{verbatim}
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232
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233 \subsection{Supporting Floating Point Data}
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234
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235 Packing and unpacking floating point data,
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236 i.e. \code{Packer.pack_float}, \code{Packer.pack_double},
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237 \code{Unpacker.unpack_float}, and \code{Unpacker.unpack_double}, are
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238 only supported with the helper built-in \code{_xdr} module, which
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239 relies on your operating system having the appropriate XDR library
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240 routines.
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241
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242 If you have built the Python interpeter with the \code{_xdr} module,
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243 or have built the \code{_xdr} module as a shared library,
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244 \code{xdrlib} will use these to pack and unpack floating point
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245 numbers. Otherwise, using these routines will raise a
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246 \code{ConversionError} exception.
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247
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248 See the Python installation instructions for details on building the
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249 \code{_xdr} module.