`gzip`

The Python gzip module provides a user-friendly interface for reading and writing files compressed in the gzip format, as well as compressing and decompressing data in memory. It wraps the lower-level zlib library to produce files compatible with the GNU gzip and gunzip programs.

Here is a quick in-memory round trip:

>>> import gzip

>>> blob = gzip.compress(b"Compress me!")
>>> blob[:3]
b'\x1f\x8b\x08'
>>> gzip.decompress(blob)
b'Compress me!'

The first two bytes are the gzip magic number, and the third identifies the deflate compression method.

Key Features

Reads and writes .gz files with an interface modeled after the built-in open()
Supports binary and text modes through gzip.open()
Compresses and decompresses bytes objects in memory with gzip.compress() and gzip.decompress()
Allows tuning the compression level from 0 (no compression) to 9 (best compression)
Handles multi-member gzip streams when decompressing
Integrates with context managers through the with statement for automatic cleanup

Frequently Used Classes and Functions

Object	Type	Description
`gzip.open()`	Function	Opens a gzip-compressed file in binary or text mode
`gzip.GzipFile`	Class	Provides a file-like object for reading and writing gzip streams
`gzip.compress()`	Function	Compresses a bytes object and returns the gzipped data
`gzip.decompress()`	Function	Decompresses gzipped bytes and returns the original data
`gzip.BadGzipFile`	Exception	Signals that a file is not a valid gzip stream

Examples

Writing a text file transparently compressed with gzip:

import gzip

with gzip.open("notes.txt.gz", "wt", encoding="utf-8") as f:
    f.write("Plain text goes in\n")
    f.write("and gets compressed.\n")

Reading the same file back into a string:

import gzip

with gzip.open("notes.txt.gz", "rt", encoding="utf-8") as f:
    print(f.read())

Run it on the command line:

$ python read_log.py
Plain text goes in
and gets compressed.

Choosing a faster compression level when working in memory:

>>> import gzip

>>> payload = b"Real Python reference article." * 100
>>> len(gzip.compress(payload, compresslevel=1))
83
>>> len(gzip.compress(payload, compresslevel=9))
74

Common Use Cases

The most common tasks for gzip include:

Reading and writing .gz log files without decompressing them first
Shrinking large text datasets such as CSV or JSON dumps before storing or transmitting them
Streaming compressed HTTP responses or file downloads
Producing archives that interoperate with the Unix gzip and gunzip tools
Compressing payloads in memory before sending them over a network

Real-World Example

Consider a scenario where you want to compress an existing plain-text log file into a .gz archive without loading the whole file into memory. You can pair gzip.open() with shutil.copyfileobj() to stream the data through:

import gzip
import shutil

with open("access.log", "rb") as file_in:
    with gzip.open("access.log.gz", "wb") as file_out:
        shutil.copyfileobj(file_in, file_out)

After running the script, access.log.gz contains the same contents as access.log, but in a form that any gzip-aware tool can read. Because both files are opened as context managers, their handles close automatically when the with blocks exit.

Tutorial

Python's zipfile: Manipulate Your ZIP Files Efficiently

In this guided tutorial, you'll learn how to manipulate ZIP files using Python's zipfile module from the standard library. Through hands-on examples, you'll learn how to read, write, compress, and extract files from your ZIP files quickly.

intermediate python stdlib

For additional information on related topics, take a look at the following resources: