12.1. zlib — Compression compatible avec gzip

Pour des applications nécessitant de compresser des données, les fonctions de ce module permettent la compression et la décompression via la bibliothèque zlib. La bibliothèque zlib a sa propre page web sur http://www.zlib.net. Il existe des incompatibilités connues entre le module Python et les versions de la bibliothèque zlib plus anciennes que la 1.1.3 ; 1.1.3 contient des failles de sécurité et nous recommandons d’utiliser plutôt la version 1.14 ou plus récente.

les fonctions zlib recèlent de nombreuses options et il est nécessaire de suivre un ordre précis. Cette documentation n’a pas pour but de couvrir la globalité des possibilités. Aussi, veuillez consulter le manuel zlib en ligne sur http://www.zlib.net/manual.html pour compléter davantage son utilisation.

Pour lire ou écrire des fichiers .gz veuillez consulter le module gzip.

Les exceptions et fonctions disponibles dans ce module sont :

exception zlib.error

Exception levée lors d’erreurs de compression et de décompression.

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.

Note

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.

Modifié dans la version 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.

Modifié dans la version 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.

  • De +9 à +15 : le logarithme binaire de la taille du tampon, par conséquent compris entre 512 et 32768. Des valeurs plus grandes produisent de meilleures compressions au dépens d’une utilisation mémoire plus grande. Le résultat final inclus des en-tête et des blocs spécifiques à zlib.
  • De -9 à -15 : utilise la valeur absolue de wbits comme logarithme binaire de la taille du tampon, et ne produit pas d’entêtes ni de bloc final.
  • De +25 à +31 = 16 + (9 à 15) : utilise les 4 bits de poids faible comme logarithme binaire de la taille du tampon, tout en incluant une entête gzip et une somme de contrôle finale.

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.

Note

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.

Modifié dans la version 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.

Modifié dans la version 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.

L’argument wbits contrôle la taille du tampon d’historique (« window size ») utilisé lors de la compression, et si un en-tête et un bloc final sont attendus. Similaire au paramètre de compressobj(), mais accepte une gamme plus large de valeurs :

  • De +8 à +15 : logarithme binaire pour la taille du tampon. L’entrée doit contenir un en-tête et un bloc zlib.
  • 0 : détermine automatiquement la taille du tampon à partir de l’en-tête zlib. Géré uniquement depuis zlib 1.2.3.5.
  • De -8 à -15 : utilise la valeur absolue de wbits comme logarithme binaire pour la taille du tampon. L’entrée doit être un flux brut, sans en-tête ni bloc final.
  • De +24 à +31 = 16 + (8 à 15) : utilise les 4 de poids faible comme logarithme binaire pour la taille du tampon. L’entrée doit contenir un en-tête gzip et son bloc final.
  • De +40 à +47 = 32 + (8 à 15) : utilise les 4 bits de poids faible comme logarithme binaire pour la taille du tampon, et accepte automatiquement les formats 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])

Renvoie un objet « décompresseur », à utiliser pour décompresser des flux de données qui ne rentrent pas entièrement en mémoire.

Le paramètre wbits contrôle la taille du tampon, et détermine quel format d’en-tête et de bloc sont prévus. Il a la même signification que décrit pour decompress().

Les objets de compression gèrent les méthodes suivantes :

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()

Renvoie une copie de l’objet « compresseur ». Utile pour compresser efficacement un ensemble de données qui partagent un préfixe initial commun.

Nouveau dans la version 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.

Le paramètre optionnel length définit la taille initiale du tampon de sortie.

Decompress.copy()

Renvoie une copie du décompresseur. Vous pouvez l’utiliser pour sauvegarder l’état de la décompression en cours, afin de pouvoir revenir rapidement à cet endroit plus tard.

Nouveau dans la version 2.5.

Voir aussi

Module gzip
Lire et écrire des fichiers au format gzip.
http://www.zlib.net
Page officielle de la bibliothèque zlib.
http://www.zlib.net/manual.html
La documentation de zlib explique le sens et l’utilisation des nombreuses fonctions fournies par la bibliothèque.