Doc/library/zlib.rst - platform/external/python/cpython3 - Git at Google


            
            :mod:`!zlib` --- Compression compatible with :program:`gzip`

            
            ============================================================


            .. module:: zlib

            
               :synopsis: Low-level interface to compression and decompression routines

            
                          compatible with gzip.


            --------------


            For applications that require data compression, the functions in this module

            
            allow compression and decompression, using the zlib library. The zlib library

            
            has its own home page at https://www.zlib.net.   There are known

            
            incompatibilities between the Python module and versions of the zlib library

            
            earlier than 1.1.3; 1.1.3 has a `security vulnerability <https://zlib.net/zlib_faq.html#faq33>`_, so we recommend using

            
            1.1.4 or later.


            zlib's functions have many options and often need to be used in a particular

            
            order.  This documentation doesn't attempt to cover all of the permutations;

            
            consult the zlib manual at http://www.zlib.net/manual.html for authoritative

            
            information.


            For reading and writing ``.gz`` files see the :mod:`gzip` module.


            The available exception and functions in this module are:


            .. exception:: error


               Exception raised on compression and decompression errors.


            .. function:: adler32(data[, value])


               Computes an Adler-32 checksum of *data*.  (An Adler-32 checksum is almost as

            
               reliable as a CRC32 but can be computed much more quickly.)  The result

            
               is an unsigned 32-bit integer.  If *value* is present, it is used as

            
               the starting value of the checksum; otherwise, a default value of 1

            
               is used.  Passing in *value* allows computing a running checksum over the

            
               concatenation of several inputs.  The algorithm is not cryptographically

            
               strong, and should not be used for authentication or digital signatures.  Since

            
               the algorithm is designed for use as a checksum algorithm, it is not suitable

            
               for use as a general hash algorithm.


               .. versionchanged:: 3.0

            
                  The result is always unsigned.


            .. function:: compress(data, /, level=-1, wbits=MAX_WBITS)


               Compresses the bytes in *data*, returning a bytes object containing compressed data.

            
               *level* is an integer from ``0`` to ``9`` or ``-1`` controlling the level of compression;

            
               ``1`` (Z_BEST_SPEED) is fastest and produces the least compression, ``9`` (Z_BEST_COMPRESSION)

            
               is slowest and produces the most.  ``0`` (Z_NO_COMPRESSION) is no compression.

            
               The default value is ``-1`` (Z_DEFAULT_COMPRESSION).  Z_DEFAULT_COMPRESSION represents a default

            
               compromise between speed and compression (currently equivalent to level 6).


               .. _compress-wbits:


               The *wbits* argument controls the size of the history buffer (or the

            
               "window size") used when compressing data, and whether a header and

            
               trailer is included in the output.  It can take several ranges of values,

            
               defaulting to ``15`` (MAX_WBITS):


               * +9 to +15: The base-two logarithm of the window size, which

            
                 therefore ranges between 512 and 32768.  Larger values produce

            
                 better compression at the expense of greater memory usage.  The

            
                 resulting output will include a zlib-specific header and trailer.


               * −9 to −15: Uses the absolute value of *wbits* as the

            
                 window size logarithm, while producing a raw output stream with no

            
                 header or trailing checksum.


               * +25 to +31 = 16 + (9 to 15): Uses the low 4 bits of the value as the

            
                 window size logarithm, while including a basic :program:`gzip` header

            
                 and trailing checksum in the output.


               Raises the :exc:`error` exception if any error occurs.


               .. versionchanged:: 3.6

            
                  *level* can now be used as a keyword parameter.


               .. versionchanged:: 3.11

            
                  The *wbits* parameter is now available to set window bits and

            
                  compression type.


            .. function:: compressobj(level=-1, method=DEFLATED, wbits=MAX_WBITS, memLevel=DEF_MEM_LEVEL, strategy=Z_DEFAULT_STRATEGY[, zdict])


               Returns a compression object, to be used for compressing data streams that won't

            
               fit into memory at once.


               *level* is the compression level -- an integer from ``0`` to ``9`` or ``-1``.

            
               A value of ``1`` (Z_BEST_SPEED) is fastest and produces the least compression,

            
               while a value of ``9`` (Z_BEST_COMPRESSION) is slowest and produces the most.

            
               ``0`` (Z_NO_COMPRESSION) is no compression.  The default value is ``-1`` (Z_DEFAULT_COMPRESSION).

            
               Z_DEFAULT_COMPRESSION represents a default compromise between speed and compression

            
               (currently equivalent to level 6).


               *method* is the compression algorithm. Currently, the only supported value is

            
               :const:`DEFLATED`.


               The *wbits* parameter controls the size of the history buffer (or the

            
               "window size"), and what header and trailer format will be used. It has

            
               the same meaning as `described for compress() <#compress-wbits>`__.


               The *memLevel* argument controls the amount of memory used for the

            
               internal compression state. Valid values range from ``1`` to ``9``.

            
               Higher values use more memory, but are faster and produce smaller output.


               *strategy* is used to tune the compression algorithm. Possible values are

            
               :const:`Z_DEFAULT_STRATEGY`, :const:`Z_FILTERED`, :const:`Z_HUFFMAN_ONLY`,

            
               :const:`Z_RLE` (zlib 1.2.0.1) and :const:`Z_FIXED` (zlib 1.2.2.2).


               *zdict* is a predefined compression dictionary. This is a sequence of bytes

            
               (such as a :class:`bytes` object) containing subsequences that are expected

            
               to occur frequently in the data that is to be compressed. Those subsequences

            
               that are expected to be most common should come at the end of the dictionary.


               .. versionchanged:: 3.3

            
                  Added the *zdict* parameter and keyword argument support.


            .. function:: crc32(data[, value])


               .. index::

            
                  single: Cyclic Redundancy Check

            
                  single: checksum; Cyclic Redundancy Check


               Computes a CRC (Cyclic Redundancy Check) checksum of *data*. The

            
               result is an unsigned 32-bit integer. If *value* is present, it is used

            
               as the starting value of the checksum; otherwise, a default value of 0

            
               is used.  Passing in *value* allows computing a running checksum over the

            
               concatenation of several inputs.  The algorithm is not cryptographically

            
               strong, and should not be used for authentication or digital signatures.  Since

            
               the algorithm is designed for use as a checksum algorithm, it is not suitable

            
               for use as a general hash algorithm.


               .. versionchanged:: 3.0

            
                  The result is always unsigned.


            .. function:: decompress(data, /, wbits=MAX_WBITS, bufsize=DEF_BUF_SIZE)


               Decompresses the bytes in *data*, returning a bytes object containing the

            
               uncompressed data.  The *wbits* parameter depends on

            
               the format of *data*, and is discussed further below.

            
               If *bufsize* is given, it is used as the initial size of the output

            
               buffer.  Raises the :exc:`error` exception if any error occurs.


               .. _decompress-wbits:


               The *wbits* parameter controls the size of the history buffer

            
               (or "window size"), and what header and trailer format is expected.

            
               It is similar to the parameter for :func:`compressobj`, but accepts

            
               more ranges of values:


               * +8 to +15: The base-two logarithm of the window size.  The input

            
                 must include a zlib header and trailer.


               * 0: Automatically determine the window size from the zlib header.

            
                 Only supported since zlib 1.2.3.5.


               * −8 to −15: Uses the absolute value of *wbits* as the window size

            
                 logarithm.  The input must be a raw stream with no header or trailer.


               * +24 to +31 = 16 + (8 to 15): Uses the low 4 bits of the value as

            
                 the window size logarithm.  The input must include a gzip header and

            
                 trailer.


               * +40 to +47 = 32 + (8 to 15): Uses the low 4 bits of the value as

            
                 the window size logarithm, and automatically accepts either

            
                 the zlib or gzip format.


               When decompressing a stream, the window size must not be smaller

            
               than the size originally used to compress the stream; using a too-small

            
               value may result in an :exc:`error` exception. The default *wbits* value

            
               corresponds to the largest window size and requires a zlib header and

            
               trailer to be included.


               *bufsize* is the initial size of the buffer used to hold decompressed data.  If

            
               more space is required, the buffer size will be increased as needed, so you

            
               don't have to get this value exactly right; tuning it will only save a few calls

            
               to :c:func:`malloc`.


               .. versionchanged:: 3.6

            
                  *wbits* and *bufsize* can be used as keyword arguments.


            .. function:: decompressobj(wbits=MAX_WBITS[, zdict])


               Returns a decompression object, to be used for decompressing data streams that

            
               won't fit into memory at once.


               The *wbits* parameter controls the size of the history buffer (or the

            
               "window size"), and what header and trailer format is expected.  It has

            
               the same meaning as `described for decompress() <#decompress-wbits>`__.


               The *zdict* parameter specifies a predefined compression dictionary. If

            
               provided, this must be the same dictionary as was used by the compressor that

            
               produced the data that is to be decompressed.


               .. note::


                  If *zdict* is a mutable object (such as a :class:`bytearray`), you must not

            
                  modify its contents between the call to :func:`decompressobj` and the first

            
                  call to the decompressor's ``decompress()`` method.


               .. versionchanged:: 3.3

            
                  Added the *zdict* parameter.


            Compression objects support the following methods:


            .. method:: Compress.compress(data)


               Compress *data*, returning a bytes object containing compressed data for at least

            
               part of the data in *data*.  This data should be concatenated to the output

            
               produced by any preceding calls to the :meth:`compress` method.  Some input may

            
               be kept in internal buffers for later processing.


            .. method:: Compress.flush([mode])


               All pending input is processed, and a bytes object containing the remaining compressed

            
               output is returned.  *mode* can be selected from the constants

            
               :const:`Z_NO_FLUSH`, :const:`Z_PARTIAL_FLUSH`, :const:`Z_SYNC_FLUSH`,

            
               :const:`Z_FULL_FLUSH`, :const:`Z_BLOCK` (zlib 1.2.3.4), or :const:`Z_FINISH`,

            
               defaulting to :const:`Z_FINISH`.  Except :const:`Z_FINISH`, all constants

            
               allow compressing further bytestrings of data, while :const:`Z_FINISH` finishes the

            
               compressed stream and prevents compressing any more data.  After calling :meth:`flush`

            
               with *mode* set to :const:`Z_FINISH`, the :meth:`compress` method cannot be called again;

            
               the only realistic action is to delete the object.


            .. method:: Compress.copy()


               Returns a copy of the compression object.  This can be used to efficiently

            
               compress a set of data that share a common initial prefix.


            .. versionchanged:: 3.8

            
               Added :func:`copy.copy` and :func:`copy.deepcopy` support to compression

            
               objects.


            Decompression objects support the following methods and attributes:


            .. attribute:: Decompress.unused_data


               A bytes object which contains any bytes past the end of the compressed data. That is,

            
               this remains ``b""`` until the last byte that contains compression data is

            
               available.  If the whole bytestring turned out to contain compressed data, this is

            
               ``b""``, an empty bytes object.


            .. attribute:: Decompress.unconsumed_tail


               A bytes object that contains any data that was not consumed by the last

            
               :meth:`decompress` call because it exceeded the limit for the uncompressed data

            
               buffer.  This data has not yet been seen by the zlib machinery, so you must feed

            
               it (possibly with further data concatenated to it) back to a subsequent

            
               :meth:`decompress` method call in order to get correct output.


            .. attribute:: Decompress.eof


               A boolean indicating whether the end of the compressed data stream has been

            
               reached.


               This makes it possible to distinguish between a properly formed compressed

            
               stream, and an incomplete or truncated one.


               .. versionadded:: 3.3


            .. method:: Decompress.decompress(data, max_length=0)


               Decompress *data*, returning a bytes object containing the uncompressed data

            
               corresponding to at least part of the data in *string*.  This data should be

            
               concatenated to the output produced by any preceding calls to the

            
               :meth:`decompress` method.  Some of the input data may be preserved in internal

            
               buffers for later processing.


               If the optional parameter *max_length* is non-zero then the return value will be

            
               no longer than *max_length*. This may mean that not all of the compressed input

            
               can be processed; and unconsumed data will be stored in the attribute

            
               :attr:`unconsumed_tail`. This bytestring must be passed to a subsequent call to

            
               :meth:`decompress` if decompression is to continue.  If *max_length* is zero

            
               then the whole input is decompressed, and :attr:`unconsumed_tail` is empty.


               .. versionchanged:: 3.6

            
                  *max_length* can be used as a keyword argument.


            .. method:: Decompress.flush([length])


               All pending input is processed, and a bytes object containing the remaining

            
               uncompressed output is returned.  After calling :meth:`flush`, the

            
               :meth:`decompress` method cannot be called again; the only realistic action is

            
               to delete the object.


               The optional parameter *length* sets the initial size of the output buffer.


            .. method:: Decompress.copy()


               Returns a copy of the decompression object.  This can be used to save the state

            
               of the decompressor midway through the data stream in order to speed up random

            
               seeks into the stream at a future point.


            .. versionchanged:: 3.8

            
               Added :func:`copy.copy` and :func:`copy.deepcopy` support to decompression

            
               objects.


            Information about the version of the zlib library in use is available through

            
            the following constants:


            .. data:: ZLIB_VERSION


               The version string of the zlib library that was used for building the module.

            
               This may be different from the zlib library actually used at runtime, which

            
               is available as :const:`ZLIB_RUNTIME_VERSION`.


            .. data:: ZLIB_RUNTIME_VERSION


               The version string of the zlib library actually loaded by the interpreter.


               .. versionadded:: 3.3


            .. seealso::


               Module :mod:`gzip`

            
                  Reading and writing :program:`gzip`\ -format files.


               http://www.zlib.net

            
                  The zlib library home page.


               http://www.zlib.net/manual.html

            
                  The zlib manual explains  the semantics and usage of the library's many

            
                  functions.
	:mod:`!zlib` --- Compression compatible with :program:`gzip`
	============================================================

	.. module:: zlib
	:synopsis: Low-level interface to compression and decompression routines
	compatible with gzip.

	--------------

	For applications that require data compression, the functions in this module
	allow compression and decompression, using the zlib library. The zlib library
	has its own home page at https://www.zlib.net. There are known
	incompatibilities between the Python module and versions of the zlib library
	earlier than 1.1.3; 1.1.3 has a `security vulnerability <https://zlib.net/zlib_faq.html#faq33>`_, so we recommend using
	1.1.4 or later.

	zlib's functions have many options and often need to be used in a particular
	order. This documentation doesn't attempt to cover all of the permutations;
	consult the zlib manual at http://www.zlib.net/manual.html for authoritative
	information.

	For reading and writing ``.gz`` files see the :mod:`gzip` module.

	The available exception and functions in this module are:


	.. exception:: error

	Exception raised on compression and decompression errors.


	.. function:: adler32(data[, value])

	Computes an Adler-32 checksum of data. (An Adler-32 checksum is almost as
	reliable as a CRC32 but can be computed much more quickly.) The result
	is an unsigned 32-bit integer. If value is present, it is used as
	the starting value of the checksum; otherwise, a default value of 1
	is used. Passing in value allows computing a running checksum over the
	concatenation of several inputs. The algorithm is not cryptographically
	strong, and should not be used for authentication or digital signatures. Since
	the algorithm is designed for use as a checksum algorithm, it is not suitable
	for use as a general hash algorithm.

	.. versionchanged:: 3.0
	The result is always unsigned.

	.. function:: compress(data, /, level=-1, wbits=MAX_WBITS)

	Compresses the bytes in data, returning a bytes object containing compressed data.
	level is an integer from ``0`` to ``9`` or ``-1`` controlling the level of compression;
	``1`` (Z_BEST_SPEED) is fastest and produces the least compression, ``9`` (Z_BEST_COMPRESSION)
	is slowest and produces the most. ``0`` (Z_NO_COMPRESSION) is no compression.
	The default value is ``-1`` (Z_DEFAULT_COMPRESSION). Z_DEFAULT_COMPRESSION represents a default
	compromise between speed and compression (currently equivalent to level 6).

	.. _compress-wbits:

	The wbits argument controls the size of the history buffer (or the
	"window size") used when compressing data, and whether a header and
	trailer is included in the output. It can take several ranges of values,
	defaulting to ``15`` (MAX_WBITS):

	* +9 to +15: The base-two logarithm of the window size, which
	therefore ranges between 512 and 32768. Larger values produce
	better compression at the expense of greater memory usage. The
	resulting output will include a zlib-specific header and trailer.

	* −9 to −15: Uses the absolute value of wbits as the
	window size logarithm, while producing a raw output stream with no
	header or trailing checksum.

	* +25 to +31 = 16 + (9 to 15): Uses the low 4 bits of the value as the
	window size logarithm, while including a basic :program:`gzip` header
	and trailing checksum in the output.

	Raises the :exc:`error` exception if any error occurs.

	.. versionchanged:: 3.6
	level can now be used as a keyword parameter.

	.. versionchanged:: 3.11
	The wbits parameter is now available to set window bits and
	compression type.

	.. function:: compressobj(level=-1, method=DEFLATED, wbits=MAX_WBITS, memLevel=DEF_MEM_LEVEL, strategy=Z_DEFAULT_STRATEGY[, zdict])

	Returns a compression object, to be used for compressing data streams that won't
	fit into memory at once.

	level is the compression level -- an integer from ``0`` to ``9`` or ``-1``.
	A value of ``1`` (Z_BEST_SPEED) is fastest and produces the least compression,
	while a value of ``9`` (Z_BEST_COMPRESSION) is slowest and produces the most.
	``0`` (Z_NO_COMPRESSION) is no compression. The default value is ``-1`` (Z_DEFAULT_COMPRESSION).
	Z_DEFAULT_COMPRESSION represents a default compromise between speed and compression
	(currently equivalent to level 6).

	method is the compression algorithm. Currently, the only supported value is
	:const:`DEFLATED`.

	The wbits parameter controls the size of the history buffer (or the
	"window size"), and what header and trailer format will be used. It has
	the same meaning as `described for compress() <#compress-wbits>`__.

	The memLevel argument controls the amount of memory used for the
	internal compression state. Valid values range from ``1`` to ``9``.
	Higher values use more memory, but are faster and produce smaller output.

	strategy is used to tune the compression algorithm. Possible values are
	:const:`Z_DEFAULT_STRATEGY`, :const:`Z_FILTERED`, :const:`Z_HUFFMAN_ONLY`,
	:const:`Z_RLE` (zlib 1.2.0.1) and :const:`Z_FIXED` (zlib 1.2.2.2).

	zdict is a predefined compression dictionary. This is a sequence of bytes
	(such as a :class:`bytes` object) containing subsequences that are expected
	to occur frequently in the data that is to be compressed. Those subsequences
	that are expected to be most common should come at the end of the dictionary.

	.. versionchanged:: 3.3
	Added the zdict parameter and keyword argument support.


	.. function:: crc32(data[, value])

	.. index::
	single: Cyclic Redundancy Check
	single: checksum; Cyclic Redundancy Check

	Computes a CRC (Cyclic Redundancy Check) checksum of data. The
	result is an unsigned 32-bit integer. If value is present, it is used
	as the starting value of the checksum; otherwise, a default value of 0
	is used. Passing in value allows computing a running checksum over the
	concatenation of several inputs. The algorithm is not cryptographically
	strong, and should not be used for authentication or digital signatures. Since
	the algorithm is designed for use as a checksum algorithm, it is not suitable
	for use as a general hash algorithm.

	.. versionchanged:: 3.0
	The result is always unsigned.

	.. function:: decompress(data, /, wbits=MAX_WBITS, bufsize=DEF_BUF_SIZE)

	Decompresses the bytes in data, returning a bytes object containing the
	uncompressed data. The wbits parameter depends on
	the format of data, and is discussed further below.
	If bufsize is given, it is used as the initial size of the output
	buffer. Raises the :exc:`error` exception if any error occurs.

	.. _decompress-wbits:

	The wbits parameter controls the size of the history buffer
	(or "window size"), and what header and trailer format is expected.
	It is similar to the parameter for :func:`compressobj`, but accepts
	more ranges of values:

	* +8 to +15: The base-two logarithm of the window size. The input
	must include a zlib header and trailer.

	* 0: Automatically determine the window size from the zlib header.
	Only supported since zlib 1.2.3.5.

	* −8 to −15: Uses the absolute value of wbits as the window size
	logarithm. The input must be a raw stream with no header or trailer.

	* +24 to +31 = 16 + (8 to 15): Uses the low 4 bits of the value as
	the window size logarithm. The input must include a gzip header and
	trailer.

	* +40 to +47 = 32 + (8 to 15): Uses the low 4 bits of the value as
	the window size logarithm, and automatically accepts either
	the zlib or gzip format.

	When decompressing a stream, the window size must not be smaller
	than the size originally used to compress the stream; using a too-small
	value may result in an :exc:`error` exception. The default wbits value
	corresponds to the largest window size and requires a zlib header and
	trailer to be included.

	bufsize is the initial size of the buffer used to hold decompressed data. If
	more space is required, the buffer size will be increased as needed, so you
	don't have to get this value exactly right; tuning it will only save a few calls
	to :c:func:`malloc`.

	.. versionchanged:: 3.6
	wbits and bufsize can be used as keyword arguments.

	.. function:: decompressobj(wbits=MAX_WBITS[, zdict])

	Returns a decompression object, to be used for decompressing data streams that
	won't fit into memory at once.

	The wbits parameter controls the size of the history buffer (or the
	"window size"), and what header and trailer format is expected. It has
	the same meaning as `described for decompress() <#decompress-wbits>`__.

	The zdict parameter specifies a predefined compression dictionary. If
	provided, this must be the same dictionary as was used by the compressor that
	produced the data that is to be decompressed.

	.. note::

	If zdict is a mutable object (such as a :class:`bytearray`), you must not
	modify its contents between the call to :func:`decompressobj` and the first
	call to the decompressor's ``decompress()`` method.

	.. versionchanged:: 3.3
	Added the zdict parameter.


	Compression objects support the following methods:


	.. method:: Compress.compress(data)

	Compress data, returning a bytes object containing compressed data for at least
	part of the data in data. This data should be concatenated to the output
	produced by any preceding calls to the :meth:`compress` method. Some input may
	be kept in internal buffers for later processing.


	.. method:: Compress.flush([mode])

	All pending input is processed, and a bytes object containing the remaining compressed
	output is returned. mode can be selected from the constants
	:const:`Z_NO_FLUSH`, :const:`Z_PARTIAL_FLUSH`, :const:`Z_SYNC_FLUSH`,
	:const:`Z_FULL_FLUSH`, :const:`Z_BLOCK` (zlib 1.2.3.4), or :const:`Z_FINISH`,
	defaulting to :const:`Z_FINISH`. Except :const:`Z_FINISH`, all constants
	allow compressing further bytestrings of data, while :const:`Z_FINISH` finishes the
	compressed stream and prevents compressing any more data. After calling :meth:`flush`
	with mode set to :const:`Z_FINISH`, the :meth:`compress` method cannot be called again;
	the only realistic action is to delete the object.


	.. method:: Compress.copy()

	Returns a copy of the compression object. This can be used to efficiently
	compress a set of data that share a common initial prefix.


	.. versionchanged:: 3.8
	Added :func:`copy.copy` and :func:`copy.deepcopy` support to compression
	objects.


	Decompression objects support the following methods and attributes:


	.. attribute:: Decompress.unused_data

	A bytes object which contains any bytes past the end of the compressed data. That is,
	this remains ``b""`` until the last byte that contains compression data is
	available. If the whole bytestring turned out to contain compressed data, this is
	``b""``, an empty bytes object.


	.. attribute:: Decompress.unconsumed_tail

	A bytes object that contains any data that was not consumed by the last
	:meth:`decompress` call because it exceeded the limit for the uncompressed data
	buffer. This data has not yet been seen by the zlib machinery, so you must feed
	it (possibly with further data concatenated to it) back to a subsequent
	:meth:`decompress` method call in order to get correct output.


	.. attribute:: Decompress.eof

	A boolean indicating whether the end of the compressed data stream has been
	reached.

	This makes it possible to distinguish between a properly formed compressed
	stream, and an incomplete or truncated one.

	.. versionadded:: 3.3


	.. method:: Decompress.decompress(data, max_length=0)

	Decompress data, returning a bytes object containing the uncompressed data
	corresponding to at least part of the data in string. This data should be
	concatenated to the output produced by any preceding calls to the
	:meth:`decompress` method. Some of the input data may be preserved in internal
	buffers for later processing.

	If the optional parameter max_length is non-zero then the return value will be
	no longer than max_length. This may mean that not all of the compressed input
	can be processed; and unconsumed data will be stored in the attribute
	:attr:`unconsumed_tail`. This bytestring must be passed to a subsequent call to
	:meth:`decompress` if decompression is to continue. If max_length is zero
	then the whole input is decompressed, and :attr:`unconsumed_tail` is empty.

	.. versionchanged:: 3.6
	max_length can be used as a keyword argument.


	.. method:: Decompress.flush([length])

	All pending input is processed, and a bytes object containing the remaining
	uncompressed output is returned. After calling :meth:`flush`, the
	:meth:`decompress` method cannot be called again; the only realistic action is
	to delete the object.

	The optional parameter length sets the initial size of the output buffer.


	.. method:: Decompress.copy()

	Returns a copy of the decompression object. This can be used to save the state
	of the decompressor midway through the data stream in order to speed up random
	seeks into the stream at a future point.


	.. versionchanged:: 3.8
	Added :func:`copy.copy` and :func:`copy.deepcopy` support to decompression
	objects.


	Information about the version of the zlib library in use is available through
	the following constants:


	.. data:: ZLIB_VERSION

	The version string of the zlib library that was used for building the module.
	This may be different from the zlib library actually used at runtime, which
	is available as :const:`ZLIB_RUNTIME_VERSION`.


	.. data:: ZLIB_RUNTIME_VERSION

	The version string of the zlib library actually loaded by the interpreter.

	.. versionadded:: 3.3


	.. seealso::

	Module :mod:`gzip`
	Reading and writing :program:`gzip`\ -format files.

	http://www.zlib.net
	The zlib library home page.

	http://www.zlib.net/manual.html
	The zlib manual explains the semantics and usage of the library's many
	functions.