12.1. zlib — Compression compatible with gzip

Para aplicativos que requerem compactação de dados, as funções neste módulo permitem compactação e descompactação, usando a biblioteca zlib. A biblioteca zlib tem seu próprio site em http://www.zlib.net. Existem incompatibilidades conhecidas entre o módulo Python e as versões da biblioteca zlib anteriores à 1.1.3; O 1.1.3 possui uma vulnerabilidade de segurança, por isso recomendamos o uso do 1.1.4 ou posterior.

As funções do zlib têm muitas opções e geralmente precisam ser usadas em uma ordem específica. Esta documentação não tenta cobrir todas as permutações; consulte o manual do zlib em http://www.zlib.net/manual.html para obter informações oficiais.

Para leitura e escrita de arquivos .gz, consulte o módulo gzip.

A exceção e as funções disponíveis neste módulo são:

exception zlib.error

Exceção levantada em erros de compactação e descompactação.

zlib.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.) If value is present, it is used as the starting value of the checksum; otherwise, a fixed default value is used. This 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.

This function always returns an integer object.

Nota

To generate the same numeric value across all Python versions and platforms use adler32(data) & 0xffffffff. If you are only using the checksum in packed binary format this is not necessary as the return value is the correct 32bit binary representation regardless of sign.

Alterado na versão 2.6: The return value is in the range [-2**31, 2**31-1] regardless of platform. In older versions the value is signed on some platforms and unsigned on others.

Alterado na versão 3.0: The return value is unsigned and in the range [0, 2**32-1] regardless of platform.

zlib.compress(string[, level])

Compresses the data in string, returning a string contained compressed data. level is an integer from 0 to 9 controlling the level of compression; 1 is fastest and produces the least compression, 9 is slowest and produces the most. 0 is no compression. The default value is 6. Raises the error exception if any error occurs.

zlib.compressobj([level[, method[, wbits[, memlevel[, strategy]]]]])

Returns a compression object, to be used for compressing data streams that won’t fit into memory at once. level is an integer from 0 to 9 or -1, controlling the level of compression; 1 is fastest and produces the least compression, 9 is slowest and produces the most. 0 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 DEFLATED.

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. The default is 15.

• +9 a +15: o logaritmo de base dois do tamanho da janela, que varia entre 512 e 32768. Valores maiores produzem melhor compactação às custas de maior uso de memória. A saída resultante incluirá um cabeçalho e uma sequência específicos para zlib.

• -9 a -15: Usa o valor absoluto de wbits como o logaritmo do tamanho da janela, enquanto produz um fluxo de saída bruto sem cabeçalho ou soma de verificação à direita.

• +25 to +31 = 16 + (9 to 15): Usa os 4 bits baixos do valor como logaritmo do tamanho da janela, incluindo um cabeçalho básico gzip e a soma de verificação à direita na saída.

memlevel controls the amount of memory used for internal compression state. Valid values range from 1 to 9. Higher values using more memory, but are faster and produce smaller output. The default is 8.

strategy is used to tune the compression algorithm. Possible values are Z_DEFAULT_STRATEGY, Z_FILTERED, and Z_HUFFMAN_ONLY. The default is Z_DEFAULT_STRATEGY.

zlib.crc32(data[, value])

Computes a CRC (Cyclic Redundancy Check) checksum of data. If value is present, it is used as the starting value of the checksum; otherwise, a fixed default value is used. This 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.

This function always returns an integer object.

Nota

To generate the same numeric value across all Python versions and platforms use crc32(data) & 0xffffffff. If you are only using the checksum in packed binary format this is not necessary as the return value is the correct 32bit binary representation regardless of sign.

Alterado na versão 2.6: The return value is in the range [-2**31, 2**31-1] regardless of platform. In older versions the value would be signed on some platforms and unsigned on others.

Alterado na versão 3.0: The return value is unsigned and in the range [0, 2**32-1] regardless of platform.

zlib.decompress(string[, wbits[, bufsize]])

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

O parâmetro wbits controla o tamanho do buffer do histórico (ou “tamanho da janela”) e qual formato de cabeçalho e sequência é esperado. É semelhante ao parâmetro para compressobj(), mas aceita mais intervalos de valores:

• +8 to +15: O logaritmo de base dois do tamanho da janela. A entrada deve incluir um cabeçalho e uma sequência de zlib.

• 0: Determina automaticamente o tamanho da janela no cabeçalho zlib. Suportado apenas desde o zlib 1.2.3.5.

• −8 to −15: Usa o valor absoluto de wbits como o logaritmo do tamanho da janela. A entrada deve ser um fluxo bruto sem cabeçalho ou sequência.

• +24 para +31 = 16 + (8 para 15): Usa os 4 bits baixos do valor como logaritmo do tamanho da janela. A entrada deve incluir um cabeçalho e sequência de gzip.

• +40 a +47 = 32 + (8 a 15): Usa os 4 bits baixos do valor como logaritmo do tamanho da janela e aceita automaticamente o formato zlib ou gzip.

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 error exception. The default wbits value is 15, which 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 malloc(). The default size is 16384.

zlib.decompressobj([wbits])

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().

Compression objects support the following methods:

Compress.compress(string)

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

Compress.flush([mode])

All pending input is processed, and a string containing the remaining compressed output is returned. mode can be selected from the constants Z_SYNC_FLUSH, Z_FULL_FLUSH, or Z_FINISH, defaulting to Z_FINISH. Z_SYNC_FLUSH and Z_FULL_FLUSH allow compressing further strings of data, while Z_FINISH finishes the compressed stream and prevents compressing any more data. After calling flush() with mode set to Z_FINISH, the compress() method cannot be called again; the only realistic action is to delete the object.

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.

Novo na versão 2.5.

Decompression objects support the following methods, and two attributes:

Decompress.unused_data

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

The only way to determine where a string of compressed data ends is by actually decompressing it. This means that when compressed data is contained part of a larger file, you can only find the end of it by reading data and feeding it followed by some non-empty string into a decompression object’s decompress() method until the unused_data attribute is no longer the empty string.

Decompress.unconsumed_tail

A string that contains any data that was not consumed by the last 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 decompress() method call in order to get correct output.

Decompress.decompress(string[, max_length])

Decompress string, returning a string 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 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 unconsumed_tail. This string must be passed to a subsequent call to decompress() if decompression is to continue. If max_length is not supplied then the whole input is decompressed, and unconsumed_tail is an empty string.

Decompress.flush([length])

All pending input is processed, and a string containing the remaining uncompressed output is returned. After calling flush(), the 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.

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.

Novo na versão 2.5.

Ver também

Module gzip

Reading and writing 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.