File Explorer

1"""Simple class to read IFF chunks.2 3An IFF chunk (used in formats such as AIFF, TIFF, RMFF (RealMedia File4Format)) has the following structure:5 6+----------------+7| ID (4 bytes)   |8+----------------+9| size (4 bytes) |10+----------------+11| data           |12| ...            |13+----------------+14 15The ID is a 4-byte string which identifies the type of chunk.16 17The size field (a 32-bit value, encoded using big-endian byte order)18gives the size of the whole chunk, including the 8-byte header.19 20Usually an IFF-type file consists of one or more chunks.  The proposed21usage of the Chunk class defined here is to instantiate an instance at22the start of each chunk and read from the instance until it reaches23the end, after which a new instance can be instantiated.  At the end24of the file, creating a new instance will fail with an EOFError25exception.26 27Usage:28while True:29    try:30        chunk = Chunk(file)31    except EOFError:32        break33    chunktype = chunk.getname()34    while True:35        data = chunk.read(nbytes)36        if not data:37            pass38        # do something with data39 40The interface is file-like.  The implemented methods are:41read, close, seek, tell, isatty.42Extra methods are: skip() (called by close, skips to the end of the chunk),43getname() (returns the name (ID) of the chunk)44 45The __init__ method has one required argument, a file-like object46(including a chunk instance), and one optional argument, a flag which47specifies whether or not chunks are aligned on 2-byte boundaries.  The48default is 1, i.e. aligned.49"""50 51class Chunk:52    def __init__(self, file, align=True, bigendian=True, inclheader=False):53        import struct54        self.closed = False55        self.align = align      # whether to align to word (2-byte) boundaries56        if bigendian:57            strflag = '>'58        else:59            strflag = '<'60        self.file = file61        self.chunkname = file.read(4)62        if len(self.chunkname) < 4:63            raise EOFError64        try:65            self.chunksize = struct.unpack_from(strflag+'L', file.read(4))[0]66        except struct.error:67            raise EOFError from None68        if inclheader:69            self.chunksize = self.chunksize - 8 # subtract header70        self.size_read = 071        try:72            self.offset = self.file.tell()73        except (AttributeError, OSError):74            self.seekable = False75        else:76            self.seekable = True77 78    def getname(self):79        """Return the name (ID) of the current chunk."""80        return self.chunkname81 82    def getsize(self):83        """Return the size of the current chunk."""84        return self.chunksize85 86    def close(self):87        if not self.closed:88            try:89                self.skip()90            finally:91                self.closed = True92 93    def isatty(self):94        if self.closed:95            raise ValueError("I/O operation on closed file")96        return False97 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 OSError("cannot seek")108        if whence == 1:109            pos = pos + self.size_read110        elif whence == 2:111            pos = pos + self.chunksize112        if pos < 0 or pos > self.chunksize:113            raise RuntimeError114        self.file.seek(self.offset + pos, 0)115        self.size_read = pos116 117    def tell(self):118        if self.closed:119            raise ValueError("I/O operation on closed file")120        return self.size_read121 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 end125        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 b''132        if size < 0:133            size = self.chunksize - self.size_read134        if size > self.chunksize - self.size_read:135            size = self.chunksize - self.size_read136        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 data144 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 to149        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_read157                # maybe fix alignment158                if self.align and (self.chunksize & 1):159                    n = n + 1160                self.file.seek(n, 1)161                self.size_read = self.size_read + n162                return163            except OSError:164                pass165        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 EOFError170