# 12.2 `gzip` -- Support for gzip files

The data compression provided by the `zlib` module is compatible with that used by the GNU compression program gzip. Accordingly, the `gzip` module provides the `GzipFile` class to read and write gzip-format files, automatically compressing or decompressing the data so it looks like an ordinary file object. Note that additional file formats which can be decompressed by the gzip and gunzip programs, such as those produced by compress and pack, are not supported by this module.

The module defines the following items:

class `GzipFile`( | [filename[, mode[, compresslevel[, fileobj]]]])

  Constructor for the `GzipFile` class, which simulates most of the methods of a file object, with the exception of the `readinto()` and `truncate()` methods. At least one of fileobj and filename must be given a non-trivial value.

  The new class instance is based on fileobj, which can be a regular file, a `StringIO` object, or any other object which simulates a file. It defaults to `None`, in which case filename is opened to provide a file object.

  When fileobj is not `None`, the filename argument is only used to be included in the gzip file header, which may includes the original filename of the uncompressed file. It defaults to the filename of fileobj, if discernible; otherwise, it defaults to the empty string, and in this case the original filename is not included in the header.

  The mode argument can be any of `'r'`, `'rb'`, `'a'`, `'ab'`, `'w'`, or `'wb'`, depending on whether the file will be read or written. The default is the mode of fileobj if discernible; otherwise, the default is `'rb'`. If not given, the 'b' flag will be added to the mode to ensure the file is opened in binary mode for cross-platform portability.

  The compresslevel argument is an integer from `1` to `9` controlling the level of compression; `1` is fastest and produces the least compression, and `9` is slowest and produces the most compression. The default is `9`.

  Calling a `GzipFile` object's `close()` method does not close fileobj, since you might wish to append more material after the compressed data. This also allows you to pass a `StringIO` object opened for writing as fileobj, and retrieve the resulting memory buffer using the `StringIO` object's `getvalue()` method.

`open`( | filename[, mode[, compresslevel]])

  This is a shorthand for `GzipFile(filename,` `mode,` `compresslevel)`. The filename argument is required; mode defaults to `'rb'` and compresslevel defaults to `9`.

See Also:

Module `zlib`:
  The basic data compression module needed to support the gzip file format.