Mercurial > cpython-lru_cache-weakref

annotate Doc/library/struct.rst @ 78069:a8aa77458e12 3.2

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Add c_size_t and c_ssize_t to the ctypes summary table.
author Antoine Pitrou <solipsis@pitrou.net>
date Thu, 12 Jul 2012 20:31:50 +0200
parents 90cdf403132e
children db3e15017172
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1 :mod:`struct` --- Interpret bytes as packed binary data
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2 =======================================================
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3
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4 .. module:: struct
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5 :synopsis: Interpret bytes as packed binary data.
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6
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7 .. index::
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8 pair: C; structures
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9 triple: packing; binary; data
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10
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11 This module performs conversions between Python values and C structs represented
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12 as Python :class:`bytes` objects. This can be used in handling binary data
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13 stored in files or from network connections, among other sources. It uses
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14 :ref:`struct-format-strings` as compact descriptions of the layout of the C
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15 structs and the intended conversion to/from Python values.
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16
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17 .. note::
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18
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19 By default, the result of packing a given C struct includes pad bytes in
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20 order to maintain proper alignment for the C types involved; similarly,
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21 alignment is taken into account when unpacking. This behavior is chosen so
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22 that the bytes of a packed struct correspond exactly to the layout in memory
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23 of the corresponding C struct. To handle platform-independent data formats
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24 or omit implicit pad bytes, use ``standard`` size and alignment instead of
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25 ``native`` size and alignment: see :ref:`struct-alignment` for details.
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26
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27 Functions and Exceptions
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28 ------------------------
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29
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30 The module defines the following exception and functions:
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31
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32
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33 .. exception:: error
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34
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35 Exception raised on various occasions; argument is a string describing what
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36 is wrong.
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39 .. function:: pack(fmt, v1, v2, ...)
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41 Return a bytes object containing the values *v1*, *v2*, ... packed according
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42 to the format string *fmt*. The arguments must match the values required by
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43 the format exactly.
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46 .. function:: pack_into(fmt, buffer, offset, v1, v2, ...)
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48 Pack the values *v1*, *v2*, ... according to the format string *fmt* and
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49 write the packed bytes into the writable buffer *buffer* starting at
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50 position *offset*. Note that *offset* is a required argument.
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53 .. function:: unpack(fmt, buffer)
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55 Unpack from the buffer *buffer* (presumably packed by ``pack(fmt, ...)``)
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56 according to the format string *fmt*. The result is a tuple even if it
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57 contains exactly one item. The buffer must contain exactly the amount of
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58 data required by the format (``len(bytes)`` must equal ``calcsize(fmt)``).
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61 .. function:: unpack_from(fmt, buffer, offset=0)
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62
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63 Unpack from *buffer* starting at position *offset*, according to the format
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64 string *fmt*. The result is a tuple even if it contains exactly one
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65 item. *buffer* must contain at least the amount of data required by the
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66 format (``len(buffer[offset:])`` must be at least ``calcsize(fmt)``).
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68
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69 .. function:: calcsize(fmt)
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71 Return the size of the struct (and hence of the bytes object produced by
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72 ``pack(fmt, ...)``) corresponding to the format string *fmt*.
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73
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74 .. _struct-format-strings:
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75
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76 Format Strings
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77 --------------
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78
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79 Format strings are the mechanism used to specify the expected layout when
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80 packing and unpacking data. They are built up from :ref:`format-characters`,
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81 which specify the type of data being packed/unpacked. In addition, there are
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82 special characters for controlling the :ref:`struct-alignment`.
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83
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84
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85 .. _struct-alignment:
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86
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87 Byte Order, Size, and Alignment
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88 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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89
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90 By default, C types are represented in the machine's native format and byte
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91 order, and properly aligned by skipping pad bytes if necessary (according to the
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92 rules used by the C compiler).
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93
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94 Alternatively, the first character of the format string can be used to indicate
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95 the byte order, size and alignment of the packed data, according to the
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96 following table:
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97
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98 +-----------+------------------------+----------+-----------+
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99 | Character | Byte order | Size | Alignment |
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100 +===========+========================+==========+===========+
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101 | ``@`` | native | native | native |
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102 +-----------+------------------------+----------+-----------+
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103 | ``=`` | native | standard | none |
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104 +-----------+------------------------+----------+-----------+
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105 | ``<`` | little-endian | standard | none |
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106 +-----------+------------------------+----------+-----------+
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107 | ``>`` | big-endian | standard | none |
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108 +-----------+------------------------+----------+-----------+
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109 | ``!`` | network (= big-endian) | standard | none |
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110 +-----------+------------------------+----------+-----------+
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111
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112 If the first character is not one of these, ``'@'`` is assumed.
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113
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114 Native byte order is big-endian or little-endian, depending on the host
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115 system. For example, Intel x86 and AMD64 (x86-64) are little-endian;
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116 Motorola 68000 and PowerPC G5 are big-endian; ARM and Intel Itanium feature
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117 switchable endianness (bi-endian). Use ``sys.byteorder`` to check the
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118 endianness of your system.
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119
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120 Native size and alignment are determined using the C compiler's
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121 ``sizeof`` expression. This is always combined with native byte order.
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122
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123 Standard size depends only on the format character; see the table in
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124 the :ref:`format-characters` section.
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125
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126 Note the difference between ``'@'`` and ``'='``: both use native byte order, but
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127 the size and alignment of the latter is standardized.
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128
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129 The form ``'!'`` is available for those poor souls who claim they can't remember
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130 whether network byte order is big-endian or little-endian.
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131
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132 There is no way to indicate non-native byte order (force byte-swapping); use the
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133 appropriate choice of ``'<'`` or ``'>'``.
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134
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135 Notes:
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136
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137 (1) Padding is only automatically added between successive structure members.
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138 No padding is added at the beginning or the end of the encoded struct.
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139
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140 (2) No padding is added when using non-native size and alignment, e.g.
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141 with '<', '>', '=', and '!'.
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142
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143 (3) To align the end of a structure to the alignment requirement of a
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144 particular type, end the format with the code for that type with a repeat
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145 count of zero. See :ref:`struct-examples`.
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146
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147
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148 .. _format-characters:
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149
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150 Format Characters
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151 ^^^^^^^^^^^^^^^^^
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152
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153 Format characters have the following meaning; the conversion between C and
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154 Python values should be obvious given their types. The 'Standard size' column
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155 refers to the size of the packed value in bytes when using standard size; that
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156 is, when the format string starts with one of ``'<'``, ``'>'``, ``'!'`` or
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157 ``'='``. When using native size, the size of the packed value is
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158 platform-dependent.
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159
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160 +--------+--------------------------+--------------------+----------------+------------+
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161 | Format | C Type | Python type | Standard size | Notes |
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162 +========+==========================+====================+================+============+
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163 | ``x`` | pad byte | no value | | |
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164 +--------+--------------------------+--------------------+----------------+------------+
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165 | ``c`` | :c:type:`char` | bytes of length 1 | 1 | |
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166 +--------+--------------------------+--------------------+----------------+------------+
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167 | ``b`` | :c:type:`signed char` | integer | 1 | \(1),\(3) |
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168 +--------+--------------------------+--------------------+----------------+------------+
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169 | ``B`` | :c:type:`unsigned char` | integer | 1 | \(3) |
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170 +--------+--------------------------+--------------------+----------------+------------+
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171 | ``?`` | :c:type:`_Bool` | bool | 1 | \(1) |
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172 +--------+--------------------------+--------------------+----------------+------------+
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173 | ``h`` | :c:type:`short` | integer | 2 | \(3) |
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174 +--------+--------------------------+--------------------+----------------+------------+
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175 | ``H`` | :c:type:`unsigned short` | integer | 2 | \(3) |
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176 +--------+--------------------------+--------------------+----------------+------------+
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177 | ``i`` | :c:type:`int` | integer | 4 | \(3) |
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178 +--------+--------------------------+--------------------+----------------+------------+
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179 | ``I`` | :c:type:`unsigned int` | integer | 4 | \(3) |
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180 +--------+--------------------------+--------------------+----------------+------------+
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181 | ``l`` | :c:type:`long` | integer | 4 | \(3) |
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182 +--------+--------------------------+--------------------+----------------+------------+
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183 | ``L`` | :c:type:`unsigned long` | integer | 4 | \(3) |
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184 +--------+--------------------------+--------------------+----------------+------------+
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185 | ``q`` | :c:type:`long long` | integer | 8 | \(2), \(3) |
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186 +--------+--------------------------+--------------------+----------------+------------+
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187 | ``Q`` | :c:type:`unsigned long | integer | 8 | \(2), \(3) |
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188 | | long` | | | |
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189 +--------+--------------------------+--------------------+----------------+------------+
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190 | ``f`` | :c:type:`float` | float | 4 | \(4) |
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191 +--------+--------------------------+--------------------+----------------+------------+
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192 | ``d`` | :c:type:`double` | float | 8 | \(4) |
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193 +--------+--------------------------+--------------------+----------------+------------+
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194 | ``s`` | :c:type:`char[]` | bytes | | |
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195 +--------+--------------------------+--------------------+----------------+------------+
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196 | ``p`` | :c:type:`char[]` | bytes | | |
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197 +--------+--------------------------+--------------------+----------------+------------+
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198 | ``P`` | :c:type:`void \*` | integer | | \(5) |
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199 +--------+--------------------------+--------------------+----------------+------------+
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200
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201 Notes:
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202
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203 (1)
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204 The ``'?'`` conversion code corresponds to the :c:type:`_Bool` type defined by
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205 C99. If this type is not available, it is simulated using a :c:type:`char`. In
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206 standard mode, it is always represented by one byte.
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207
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208 (2)
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209 The ``'q'`` and ``'Q'`` conversion codes are available in native mode only if
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210 the platform C compiler supports C :c:type:`long long`, or, on Windows,
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211 :c:type:`__int64`. They are always available in standard modes.
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212
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213 (3)
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214 When attempting to pack a non-integer using any of the integer conversion
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215 codes, if the non-integer has a :meth:`__index__` method then that method is
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216 called to convert the argument to an integer before packing.
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217
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218 .. versionchanged:: 3.2
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219 Use of the :meth:`__index__` method for non-integers is new in 3.2.
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220
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221 (4)
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222 For the ``'f'`` and ``'d'`` conversion codes, the packed representation uses
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223 the IEEE 754 binary32 (for ``'f'``) or binary64 (for ``'d'``) format,
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224 regardless of the floating-point format used by the platform.
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225
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226 (5)
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227 The ``'P'`` format character is only available for the native byte ordering
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228 (selected as the default or with the ``'@'`` byte order character). The byte
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229 order character ``'='`` chooses to use little- or big-endian ordering based
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230 on the host system. The struct module does not interpret this as native
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231 ordering, so the ``'P'`` format is not available.
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232
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233
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234 A format character may be preceded by an integral repeat count. For example,
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235 the format string ``'4h'`` means exactly the same as ``'hhhh'``.
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236
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237 Whitespace characters between formats are ignored; a count and its format must
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238 not contain whitespace though.
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239
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240 For the ``'s'`` format character, the count is interpreted as the length of the
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241 bytes, not a repeat count like for the other format characters; for example,
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242 ``'10s'`` means a single 10-byte string, while ``'10c'`` means 10 characters.
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243 If a count is not given, it defaults to 1. For packing, the string is
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244 truncated or padded with null bytes as appropriate to make it fit. For
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245 unpacking, the resulting bytes object always has exactly the specified number
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246 of bytes. As a special case, ``'0s'`` means a single, empty string (while
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247 ``'0c'`` means 0 characters).
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248
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249 When packing a value ``x`` using one of the integer formats (``'b'``,
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250 ``'B'``, ``'h'``, ``'H'``, ``'i'``, ``'I'``, ``'l'``, ``'L'``,
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251 ``'q'``, ``'Q'``), if ``x`` is outside the valid range for that format
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252 then :exc:`struct.error` is raised.
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253
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254 .. versionchanged:: 3.1
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255 In 3.0, some of the integer formats wrapped out-of-range values and
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256 raised :exc:`DeprecationWarning` instead of :exc:`struct.error`.
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257
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258 The ``'p'`` format character encodes a "Pascal string", meaning a short
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259 variable-length string stored in a *fixed number of bytes*, given by the count.
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260 The first byte stored is the length of the string, or 255, whichever is
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261 smaller. The bytes of the string follow. If the string passed in to
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262 :func:`pack` is too long (longer than the count minus 1), only the leading
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263 ``count-1`` bytes of the string are stored. If the string is shorter than
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264 ``count-1``, it is padded with null bytes so that exactly count bytes in all
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265 are used. Note that for :func:`unpack`, the ``'p'`` format character consumes
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266 ``count`` bytes, but that the string returned can never contain more than 255
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267 bytes.
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268
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269 For the ``'?'`` format character, the return value is either :const:`True` or
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270 :const:`False`. When packing, the truth value of the argument object is used.
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271 Either 0 or 1 in the native or standard bool representation will be packed, and
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272 any non-zero value will be True when unpacking.
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273
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274
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275
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276 .. _struct-examples:
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277
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278 Examples
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279 ^^^^^^^^
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280
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281 .. note::
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282 All examples assume a native byte order, size, and alignment with a
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283 big-endian machine.
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284
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285 A basic example of packing/unpacking three integers::
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286
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287 >>> from struct import *
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288 >>> pack('hhl', 1, 2, 3)
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289 b'\x00\x01\x00\x02\x00\x00\x00\x03'
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290 >>> unpack('hhl', b'\x00\x01\x00\x02\x00\x00\x00\x03')
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291 (1, 2, 3)
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292 >>> calcsize('hhl')
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293 8
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294
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295 Unpacked fields can be named by assigning them to variables or by wrapping
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296 the result in a named tuple::
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297
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298 >>> record = b'raymond \x32\x12\x08\x01\x08'
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299 >>> name, serialnum, school, gradelevel = unpack('<10sHHb', record)
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300
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301 >>> from collections import namedtuple
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302 >>> Student = namedtuple('Student', 'name serialnum school gradelevel')
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303 >>> Student._make(unpack('<10sHHb', record))
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304 Student(name=b'raymond ', serialnum=4658, school=264, gradelevel=8)
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305
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306 The ordering of format characters may have an impact on size since the padding
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307 needed to satisfy alignment requirements is different::
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308
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309 >>> pack('ci', b'*', 0x12131415)
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310 b'*\x00\x00\x00\x12\x13\x14\x15'
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311 >>> pack('ic', 0x12131415, b'*')
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312 b'\x12\x13\x14\x15*'
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313 >>> calcsize('ci')
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314 8
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315 >>> calcsize('ic')
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316 5
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317
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318 The following format ``'llh0l'`` specifies two pad bytes at the end, assuming
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319 longs are aligned on 4-byte boundaries::
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320
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321 >>> pack('llh0l', 1, 2, 3)
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322 b'\x00\x00\x00\x01\x00\x00\x00\x02\x00\x03\x00\x00'
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323
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324 This only works when native size and alignment are in effect; standard size and
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325 alignment does not enforce any alignment.
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326
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327
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328 .. seealso::
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329
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330 Module :mod:`array`
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331 Packed binary storage of homogeneous data.
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332
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333 Module :mod:`xdrlib`
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334 Packing and unpacking of XDR data.
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335
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336
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337 .. _struct-objects:
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338
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339 Classes
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340 -------
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341
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342 The :mod:`struct` module also defines the following type:
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343
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344
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345 .. class:: Struct(format)
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346
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347 Return a new Struct object which writes and reads binary data according to
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348 the format string *format*. Creating a Struct object once and calling its
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349 methods is more efficient than calling the :mod:`struct` functions with the
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350 same format since the format string only needs to be compiled once.
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351
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352
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353 Compiled Struct objects support the following methods and attributes:
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354
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355 .. method:: pack(v1, v2, ...)
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356
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357 Identical to the :func:`pack` function, using the compiled format.
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358 (``len(result)`` will equal :attr:`self.size`.)
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359
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360
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361 .. method:: pack_into(buffer, offset, v1, v2, ...)
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362
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363 Identical to the :func:`pack_into` function, using the compiled format.
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364
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365
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366 .. method:: unpack(buffer)
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367
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368 Identical to the :func:`unpack` function, using the compiled format.
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369 (``len(buffer)`` must equal :attr:`self.size`).
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370
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371
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372 .. method:: unpack_from(buffer, offset=0)
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373
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374 Identical to the :func:`unpack_from` function, using the compiled format.
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375 (``len(buffer[offset:])`` must be at least :attr:`self.size`).
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376
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377
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378 .. attribute:: format
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379
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380 The format string used to construct this Struct object.
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381
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382 .. attribute:: size
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383
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384 The calculated size of the struct (and hence of the bytes object produced
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385 by the :meth:`pack` method) corresponding to :attr:`format`.
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386