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