144 lines
4.4 KiB
Python
144 lines
4.4 KiB
Python
"""Simple class to read IFF chunks.
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An IFF chunk (used in formats such as AIFF, TIFF, RMFF (RealMedia File
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Format)) has the following structure:
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+----------------+
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| ID (4 bytes) |
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+----------------+
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| size (4 bytes) |
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+----------------+
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| data |
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| ... |
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+----------------+
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The ID is a 4-byte string which identifies the type of chunk.
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The size field (a 32-bit value, encoded using big-endian byte order)
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gives the size of the whole chunk, including the 8-byte header.
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Usually a IFF-type file consists of one or more chunks. The proposed
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usage of the Chunk class defined here is to instantiate an instance at
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the start of each chunk and read from the instance until it reaches
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the end, after which a new instance can be instantiated. At the end
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of the file, creating a new instance will fail with a EOFError
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exception.
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Usage:
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while 1:
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try:
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chunk = Chunk(file)
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except EOFError:
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break
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chunktype = chunk.getname()
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while 1:
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data = chunk.read(nbytes)
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if not data:
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pass
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# do something with data
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The interface is file-like. The implemented methods are:
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read, close, seek, tell, isatty.
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Extra methods are: skip() (called by close, skips to the end of the chunk),
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getname() (returns the name (ID) of the chunk)
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The __init__ method has one required argument, a file-like object
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(including a chunk instance), and one optional argument, a flag which
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specifies whether or not chunks are aligned on 2-byte boundaries. The
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default is 1, i.e. aligned.
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"""
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class Chunk:
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def __init__(self, file, align = 1):
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import struct
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self.closed = 0
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self.align = align # whether to align to word (2-byte) boundaries
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self.file = file
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self.chunkname = file.read(4)
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if len(self.chunkname) < 4:
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raise EOFError
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try:
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self.chunksize = struct.unpack('>l', file.read(4))[0]
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except struct.error:
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raise EOFError
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self.chunksize = self.chunksize - 8 # subtract header
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self.size_read = 0
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self.offset = self.file.tell()
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def getname(self):
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"""Return the name (ID) of the current chunk."""
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return self.chunkname
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def close(self):
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if not self.closed:
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self.skip()
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self.closed = 1
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def isatty(self):
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if self.closed:
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raise ValueError, "I/O operation on closed file"
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return 0
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def seek(self, pos, mode = 0):
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"""Seek to specified position into the chunk.
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Default position is 0 (start of chunk).
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If the file is not seekable, this will result in an error.
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"""
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if self.closed:
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raise ValueError, "I/O operation on closed file"
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if mode == 1:
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pos = pos + self.size_read
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elif mode == 2:
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pos = pos + self.chunk_size
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if pos < 0 or pos > self.chunksize:
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raise RuntimeError
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self.file.seek(self.offset + pos, 0)
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self.size_read = pos
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def tell(self):
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if self.closed:
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raise ValueError, "I/O operation on closed file"
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return self.size_read
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def read(self, n = -1):
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"""Read at most n bytes from the chunk.
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If n is omitted or negative, read until the end
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of the chunk.
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"""
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if self.closed:
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raise ValueError, "I/O operation on closed file"
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if self.size_read >= self.chunksize:
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return ''
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if n < 0:
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n = self.chunksize - self.size_read
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if n > self.chunksize - self.size_read:
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n = self.chunksize - self.size_read
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data = self.file.read(n)
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self.size_read = self.size_read + len(data)
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if self.size_read == self.chunksize and \
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self.align and \
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(self.chunksize & 1):
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dummy = self.file.read(1)
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self.size_read = self.size_read + len(dummy)
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return data
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def skip(self):
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"""Skip the rest of the chunk.
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If you are not interested in the contents of the chunk,
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this method should be called so that the file points to
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the start of the next chunk.
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"""
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if self.closed:
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raise ValueError, "I/O operation on closed file"
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try:
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self.file.seek(self.chunksize - self.size_read, 1)
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except RuntimeError:
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while self.size_read < self.chunksize:
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dummy = self.read(8192)
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if not dummy:
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raise EOFError
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