cs.buffer 20200328

Facilities to do with buffers, particularly CornuCopyBuffer, an automatically refilling buffer to support parsing of data streams.

Latest release 20200328: CornuCopyBuffer.takev: bugfix adjustment of buf.offset, was not always done. CornuCopyBuffer.getitem: add slice support, note how expensive it is to use.

Facilities to do with buffers, particularly CornuCopyBuffer, an automatically refilling buffer to support parsing of data streams.

Function chunky(bfr_func)

Decorator for a function accepting a leading CornuCopyBuffer parameter. Returns a function accepting a leading data chunks parameter (bytes instances) and optional offset and 'copy_offsets` keywords parameters.

Example::

@chunky
def func(bfr, ...):

Class CopyingIterator

Wrapper for an iterator that copies every item retrieved to a callable.

Method CopyingIterator.__init__(self, I, copy_to)

Initialise with the iterator I and the callable copy_to.

Class CornuCopyBuffer

An automatically refilling buffer intended to support parsing of data streams.

Its purpose is to aid binary parsers which do not themselves need to handle sources specially; CornuCopyBuffers are trivially made from bytes, iterables of bytes and file-like objects. See cs.binary for convenient parsing classes which work against CornuCopyBuffers.

Attributes:

• buf: the first of any buffered leading chunks buffer of unparsed data from the input, available for direct inspection by parsers; normally however parsers will use .extend and .take.
• offset: the logical offset of the buffer; this excludes buffered data and unconsumed input data

The primary methods supporting parsing of data streams are .extend() and take(). Calling .extend(min_size) arranges that .buf contains at least min_size bytes. Calling .take(size) fetches exactly size bytes from .buf and the input source if necessary and returns them, adjusting .buf.

len(CornuCopyBuffer) returns the length of any buffered data.

bool(CornuCopyBuffer) tests whether len() > 0.

Indexing a CornuCopyBuffer accesses the buffered data only, returning an individual byte's value (an int).

A CornuCopyBuffer is also iterable, yielding data in whatever sizes come from its input_data source, preceeded by the current .buf if not empty.

A CornuCopyBuffer also supports the file methods .read, .tell and .seek supporting drop in use of the buffer in many file contexts. Backward seeks are not supported. .seek will take advantage of the input_data's .seek method if it has one, otherwise it will use reads.

Method CornuCopyBuffer.__init__(self, input_data, buf=None, offset=0, seekable=None, copy_offsets=None, copy_chunks=None)

Prepare the buffer.

Parameters:

• input_data: an iterable of data chunks (bytes-like instances); if your data source is a file see the .from_file factory; if your data source is a file descriptor see the .from_fd factory.
• buf: if not None, the initial state of the parse buffer
• offset: logical offset of the start of the buffer, default 0
• seekable: whether input_data has a working .seek method; the default is None meaning that it will be attempted on the first skip or seek
• copy_offsets: if not None, a callable for parsers to report pertinent offsets via the buffer's .report_offset method
• copy_chunks: if not None, every fetched data chunk is copied to this callable

The input_data is an iterable whose iterator may have some optional additional properties:

• seek: if present, this is a seek method after the fashion of file.seek; the buffer's seek, skip and skipto methods will take advantage of this if available.
• offset: the current byte offset of the iterator; this is used during the buffer initialisation to compute input_data_displacement, the difference between the buffer's logical offset and the input data's logical offset; if unavailable during initialisation this is presumed to be 0.
• end_offset: the end offset of the iterator if known.

Class FDIterator(_Iterator)

An iterator over the data of a file descriptor.

Note: the iterator works with an os.dup() of the file descriptor so that it can close it with impunity; this requires the caller to close their descriptor.

Method FDIterator.__init__(self, fd, offset=None, readsize=None, align=True)

Initialise the iterator.

Parameters:

• fd: file descriptor
• offset: the initial logical offset, kept up to date by iteration; the default is the current file position.
• readsize: a preferred read size; if omitted then DEFAULT_READSIZE will be stored
• align: whether to align reads by default: if true then the iterator will do a short read to bring the offset into alignment with readsize; the default is True

Class FileIterator(_Iterator,SeekableIteratorMixin)

An iterator over the data of a file object.

Note: the iterator closes the file on __del__ or if its .close method is called.

Method FileIterator.__init__(self, fp, offset=None, readsize=None, align=False)

Initialise the iterator.

Parameters:

• fp: file object
• offset: the initial logical offset, kept up to date by iteration; the default is 0.
• readsize: a preferred read size; if omitted then DEFAULT_READSIZE will be stored
• align: whether to align reads by default: if true then the iterator will do a short read to bring the offset into alignment with readsize; the default is False

Class SeekableFDIterator(FDIterator,_Iterator,SeekableIteratorMixin)

An iterator over the data of a seekable file descriptor.

Note: the iterator works with an os.dup() of the file descriptor so that it can close it with impunity; this requires the caller to close their descriptor.

Class SeekableFileIterator(FileIterator,_Iterator,SeekableIteratorMixin)

An iterator over the data of a seekable file object.

Note: the iterator closes the file on del or if its .close method is called.

Method SeekableFileIterator.__init__(self, fp, offset=None, **kw)

Initialise the iterator.

Parameters:

• fp: file object
• offset: the initial logical offset, kept up to date by iteration; the default is the current file position.
• readsize: a preferred read size; if omitted then DEFAULT_READSIZE will be stored
• align: whether to align reads by default: if true then the iterator will do a short read to bring the offset into alignment with readsize; the default is False

Class SeekableIteratorMixin

Mixin supplying a logical with a seek method.

Class SeekableMMapIterator(_Iterator,SeekableIteratorMixin)

An iterator over the data of a mappable file descriptor.

Note: the iterator works with an mmap of an os.dup() of the file descriptor so that it can close it with impunity; this requires the caller to close their descriptor.

Method SeekableMMapIterator.__init__(self, fd, offset=None, readsize=None, align=True)

Initialise the iterator.

Parameters:

• offset: the initial logical offset, kept up to date by iteration; the default is the current file position.
• readsize: a preferred read size; if omitted then DEFAULT_READSIZE will be stored
• align: whether to align reads by default: if true then the iterator will do a short read to bring the offset into alignment with readsize; the default is True

Release Log

Release 20200328: CornuCopyBuffer.takev: bugfix adjustment of buf.offset, was not always done. CornuCopyBuffer.getitem: add slice support, note how expensive it is to use.

Release 20200229: New CornuCopyBuffer.byte0() method consuming the next byte and returning it as an int. CornuCopyBuffer.takev: bugfix for size=0, logic refactor. CornuCopyBuffer: new .selfcheck method.

Release 20200130: CornuCopyBuffer.skip: bugfix adjustment of skipto for already buffered data.

Release 20191230.1: Docstring updates. Semantic changes were in the previous release.

Release 20191230: CornuCopyBuffer: accept a size of Ellipsis in .take and .extend methods, indicating "all the remaining data". CornuCopyBuffer: refactor the buffering, replacing .buf with .bufs as an array of chunks; this enables support for the new .push method and reduces memory copying.

Release 20181231: Small bugfix.

Release 20181108: New at_eof() method. Python 2 tweak to support incidental import by python 2 even if unused.

Release 20180823: Better handling of seekable and unseekable input data. Tiny bugfix for from_bytes sanity check.

Release 20180810: Refactor SeekableFDIterator and SeekableFileIterator to subclass new SeekableIterator. New SeekableMMapIterator to process a memory mapped file descriptor, intended for large files. New CornuCopyBuffer.hint method to pass a length hint through to the input_data iterator if it has a hint method, causing it possibly to make a differently sized fetch. SeekableIterator: new del method calling self.close() - subclasses must provide a .close, which should be safe to call multiple times. CornuCopyBuffer: add support for .offset and .end_offset optional attributes on the input_data iterator. _BoundedBufferIterator: add .offset property plumbed to the underlying buffer offset. New CornuCopyBuffer.from_mmap to make a mmap backed buffer so that large data can be returned without penalty. Assorted fixes and doc improvements.

Release 20180805: Bugfixes for at_eof method and end_offset initialisation.

Release 20180726.1: Improve docstrings and release with better long_description.

Release 20180726: First PyPI release: CornuCopyBuffer and friends.