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 a 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 self.skip() 89 self.closed = True 90 91 def isatty(self): 92 if self.closed: 93 raise ValueError, "I/O operation on closed file" 94 return False 95 96 def seek(self, pos, whence=0): 97 """Seek to specified position into the chunk. 98 Default position is 0 (start of chunk). 99 If the file is not seekable, this will result in an error. 100 """ 101 102 if self.closed: 103 raise ValueError, "I/O operation on closed file" 104 if not self.seekable: 105 raise IOError, "cannot seek" 106 if whence == 1: 107 pos = pos + self.size_read 108 elif whence == 2: 109 pos = pos + self.chunksize 110 if pos < 0 or pos > self.chunksize: 111 raise RuntimeError 112 self.file.seek(self.offset + pos, 0) 113 self.size_read = pos 114 115 def tell(self): 116 if self.closed: 117 raise ValueError, "I/O operation on closed file" 118 return self.size_read 119 120 def read(self, size=-1): 121 """Read at most size bytes from the chunk. 122 If size is omitted or negative, read until the end 123 of the chunk. 124 """ 125 126 if self.closed: 127 raise ValueError, "I/O operation on closed file" 128 if self.size_read >= self.chunksize: 129 return '' 130 if size < 0: 131 size = self.chunksize - self.size_read 132 if size > self.chunksize - self.size_read: 133 size = self.chunksize - self.size_read 134 data = self.file.read(size) 135 self.size_read = self.size_read + len(data) 136 if self.size_read == self.chunksize and \ 137 self.align and \ 138 (self.chunksize & 1): 139 dummy = self.file.read(1) 140 self.size_read = self.size_read + len(dummy) 141 return data 142 143 def skip(self): 144 """Skip the rest of the chunk. 145 If you are not interested in the contents of the chunk, 146 this method should be called so that the file points to 147 the start of the next chunk. 148 """ 149 150 if self.closed: 151 raise ValueError, "I/O operation on closed file" 152 if self.seekable: 153 try: 154 n = self.chunksize - self.size_read 155 # maybe fix alignment 156 if self.align and (self.chunksize & 1): 157 n = n + 1 158 self.file.seek(n, 1) 159 self.size_read = self.size_read + n 160 return 161 except IOError: 162 pass 163 while self.size_read < self.chunksize: 164 n = min(8192, self.chunksize - self.size_read) 165 dummy = self.read(n) 166 if not dummy: 167 raise EOFError 168
