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1 """Simple class to read IFF chunks. |
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2 |
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3 An IFF chunk (used in formats such as AIFF, TIFF, RMFF (RealMedia File |
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4 Format)) has the following structure: |
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5 |
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6 +----------------+ |
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7 | ID (4 bytes) | |
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8 +----------------+ |
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9 | size (4 bytes) | |
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10 +----------------+ |
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11 | data | |
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12 | ... | |
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13 +----------------+ |
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14 |
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15 The ID is a 4-byte string which identifies the type of chunk. |
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16 |
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17 The size field (a 32-bit value, encoded using big-endian byte order) |
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18 gives the size of the whole chunk, including the 8-byte header. |
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19 |
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20 Usually an IFF-type file consists of one or more chunks. The proposed |
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21 usage of the Chunk class defined here is to instantiate an instance at |
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22 the start of each chunk and read from the instance until it reaches |
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23 the end, after which a new instance can be instantiated. At the end |
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24 of the file, creating a new instance will fail with a EOFError |
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25 exception. |
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26 |
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27 Usage: |
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28 while True: |
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29 try: |
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30 chunk = Chunk(file) |
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31 except EOFError: |
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32 break |
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33 chunktype = chunk.getname() |
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34 while True: |
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35 data = chunk.read(nbytes) |
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36 if not data: |
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37 pass |
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38 # do something with data |
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39 |
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40 The interface is file-like. The implemented methods are: |
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41 read, close, seek, tell, isatty. |
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42 Extra methods are: skip() (called by close, skips to the end of the chunk), |
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43 getname() (returns the name (ID) of the chunk) |
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44 |
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45 The __init__ method has one required argument, a file-like object |
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46 (including a chunk instance), and one optional argument, a flag which |
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47 specifies whether or not chunks are aligned on 2-byte boundaries. The |
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48 default is 1, i.e. aligned. |
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49 """ |
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50 |
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51 class Chunk: |
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52 def __init__(self, file, align=True, bigendian=True, inclheader=False): |
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53 import struct |
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54 self.closed = False |
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55 self.align = align # whether to align to word (2-byte) boundaries |
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56 if bigendian: |
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57 strflag = '>' |
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58 else: |
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59 strflag = '<' |
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60 self.file = file |
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61 self.chunkname = file.read(4) |
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62 if len(self.chunkname) < 4: |
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63 raise EOFError |
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64 try: |
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65 self.chunksize = struct.unpack(strflag+'L', file.read(4))[0] |
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66 except struct.error: |
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67 raise EOFError |
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68 if inclheader: |
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69 self.chunksize = self.chunksize - 8 # subtract header |
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70 self.size_read = 0 |
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71 try: |
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72 self.offset = self.file.tell() |
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73 except (AttributeError, IOError): |
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74 self.seekable = False |
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75 else: |
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76 self.seekable = True |
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77 |
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78 def getname(self): |
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79 """Return the name (ID) of the current chunk.""" |
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80 return self.chunkname |
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81 |
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82 def getsize(self): |
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83 """Return the size of the current chunk.""" |
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84 return self.chunksize |
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85 |
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86 def close(self): |
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87 if not self.closed: |
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88 self.skip() |
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89 self.closed = True |
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90 |
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91 def isatty(self): |
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92 if self.closed: |
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93 raise ValueError, "I/O operation on closed file" |
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94 return False |
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95 |
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96 def seek(self, pos, whence=0): |
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97 """Seek to specified position into the chunk. |
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98 Default position is 0 (start of chunk). |
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99 If the file is not seekable, this will result in an error. |
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100 """ |
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101 |
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102 if self.closed: |
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103 raise ValueError, "I/O operation on closed file" |
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104 if not self.seekable: |
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105 raise IOError, "cannot seek" |
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106 if whence == 1: |
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107 pos = pos + self.size_read |
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108 elif whence == 2: |
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109 pos = pos + self.chunksize |
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110 if pos < 0 or pos > self.chunksize: |
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111 raise RuntimeError |
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112 self.file.seek(self.offset + pos, 0) |
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113 self.size_read = pos |
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114 |
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115 def tell(self): |
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116 if self.closed: |
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117 raise ValueError, "I/O operation on closed file" |
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118 return self.size_read |
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119 |
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120 def read(self, size=-1): |
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121 """Read at most size bytes from the chunk. |
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122 If size is omitted or negative, read until the end |
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123 of the chunk. |
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124 """ |
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125 |
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126 if self.closed: |
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127 raise ValueError, "I/O operation on closed file" |
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128 if self.size_read >= self.chunksize: |
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129 return '' |
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130 if size < 0: |
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131 size = self.chunksize - self.size_read |
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132 if size > self.chunksize - self.size_read: |
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133 size = self.chunksize - self.size_read |
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134 data = self.file.read(size) |
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135 self.size_read = self.size_read + len(data) |
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136 if self.size_read == self.chunksize and \ |
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137 self.align and \ |
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138 (self.chunksize & 1): |
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139 dummy = self.file.read(1) |
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140 self.size_read = self.size_read + len(dummy) |
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141 return data |
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142 |
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143 def skip(self): |
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144 """Skip the rest of the chunk. |
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145 If you are not interested in the contents of the chunk, |
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146 this method should be called so that the file points to |
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147 the start of the next chunk. |
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148 """ |
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149 |
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150 if self.closed: |
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151 raise ValueError, "I/O operation on closed file" |
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152 if self.seekable: |
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153 try: |
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154 n = self.chunksize - self.size_read |
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155 # maybe fix alignment |
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156 if self.align and (self.chunksize & 1): |
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157 n = n + 1 |
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158 self.file.seek(n, 1) |
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159 self.size_read = self.size_read + n |
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160 return |
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161 except IOError: |
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162 pass |
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163 while self.size_read < self.chunksize: |
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164 n = min(8192, self.chunksize - self.size_read) |
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165 dummy = self.read(n) |
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166 if not dummy: |
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167 raise EOFError |