1 """Simple class to read IFF chunks. 2 3 An IFF chunk (used in formats such as AIFF, TIFF, RMFF (RealMedia File 4 Format)) has the following structure: 5 6 +----------------+ 7 | ID (4 bytes) | 8 +----------------+ 9 | size (4 bytes) | 10 +----------------+ 11 | data | 12 | ... | 13 +----------------+ 14 15 The ID is a 4-byte string which identifies the type of chunk. 16 17 The size field (a 32-bit value, encoded using big-endian byte order) 18 gives the size of the whole chunk, including the 8-byte header. 19 20 Usually an IFF-type file consists of one or more chunks. The proposed 21 usage of the Chunk class defined here is to instantiate an instance at 22 the start of each chunk and read from the instance until it reaches 23 the end, after which a new instance can be instantiated. At the end 24 of the file, creating a new instance will fail with an EOFError 25 exception. 26 27 Usage: 28 while True: 29 try: 30 chunk = Chunk(file) 31 except EOFError: 32 break 33 chunktype = chunk.getname() 34 while True: 35 data = chunk.read(nbytes) 36 if not data: 37 pass 38 # do something with data 39 40 The interface is file-like. The implemented methods are: 41 read, close, seek, tell, isatty. 42 Extra methods are: skip() (called by close, skips to the end of the chunk), 43 getname() (returns the name (ID) of the chunk) 44 45 The __init__ method has one required argument, a file-like object 46 (including a chunk instance), and one optional argument, a flag which 47 specifies whether or not chunks are aligned on 2-byte boundaries. The 48 default is 1, i.e. aligned. 49 """ 50 51 class Chunk: 52 def __init__(self, file, align=True, bigendian=True, inclheader=False): 53 import struct 54 self.closed = False 55 self.align = align # whether to align to word (2-byte) boundaries 56 if bigendian: 57 strflag = '>' 58 else: 59 strflag = '<' 60 self.file = file 61 self.chunkname = file.read(4) 62 if len(self.chunkname) < 4: 63 raise EOFError 64 try: 65 self.chunksize = struct.unpack(strflag+'L', file.read(4))[0] 66 except struct.error: 67 raise EOFError 68 if inclheader: 69 self.chunksize = self.chunksize - 8 # subtract header 70 self.size_read = 0 71 try: 72 self.offset = self.file.tell() 73 except (AttributeError, IOError): 74 self.seekable = False 75 else: 76 self.seekable = True 77 78 def getname(self): 79 """Return the name (ID) of the current chunk.""" 80 return self.chunkname 81 82 def getsize(self): 83 """Return the size of the current chunk.""" 84 return self.chunksize 85 86 def close(self): 87 if not self.closed: 88 try: 89 self.skip() 90 finally: 91 self.closed = True 92 93 def isatty(self): 94 if self.closed: 95 raise ValueError, "I/O operation on closed file" 96 return False 97 98 def seek(self, pos, whence=0): 99 """Seek to specified position into the chunk. 100 Default position is 0 (start of chunk). 101 If the file is not seekable, this will result in an error. 102 """ 103 104 if self.closed: 105 raise ValueError, "I/O operation on closed file" 106 if not self.seekable: 107 raise IOError, "cannot seek" 108 if whence == 1: 109 pos = pos + self.size_read 110 elif whence == 2: 111 pos = pos + self.chunksize 112 if pos < 0 or pos > self.chunksize: 113 raise RuntimeError 114 self.file.seek(self.offset + pos, 0) 115 self.size_read = pos 116 117 def tell(self): 118 if self.closed: 119 raise ValueError, "I/O operation on closed file" 120 return self.size_read 121 122 def read(self, size=-1): 123 """Read at most size bytes from the chunk. 124 If size is omitted or negative, read until the end 125 of the chunk. 126 """ 127 128 if self.closed: 129 raise ValueError, "I/O operation on closed file" 130 if self.size_read >= self.chunksize: 131 return '' 132 if size < 0: 133 size = self.chunksize - self.size_read 134 if size > self.chunksize - self.size_read: 135 size = self.chunksize - self.size_read 136 data = self.file.read(size) 137 self.size_read = self.size_read + len(data) 138 if self.size_read == self.chunksize and \ 139 self.align and \ 140 (self.chunksize & 1): 141 dummy = self.file.read(1) 142 self.size_read = self.size_read + len(dummy) 143 return data 144 145 def skip(self): 146 """Skip the rest of the chunk. 147 If you are not interested in the contents of the chunk, 148 this method should be called so that the file points to 149 the start of the next chunk. 150 """ 151 152 if self.closed: 153 raise ValueError, "I/O operation on closed file" 154 if self.seekable: 155 try: 156 n = self.chunksize - self.size_read 157 # maybe fix alignment 158 if self.align and (self.chunksize & 1): 159 n = n + 1 160 self.file.seek(n, 1) 161 self.size_read = self.size_read + n 162 return 163 except IOError: 164 pass 165 while self.size_read < self.chunksize: 166 n = min(8192, self.chunksize - self.size_read) 167 dummy = self.read(n) 168 if not dummy: 169 raise EOFError 170
