Issue #15677: Document that zlib and gzip accept a compression level of 0 to mean 'no compression'.

Patch by Brian Brazil.

Doc/library/gzip.rst

@@ -38,7 +38,7 @@ The module defines the following items:
    ``'w'``, or ``'wb'`` for binary mode, or ``'rt'``, ``'at'``, or ``'wt'`` for
    text mode. The default is ``'rb'``.
 
-   The *compresslevel* argument is an integer from 1 to 9, as for the
+   The *compresslevel* argument is an integer from 0 to 9, as for the
    :class:`GzipFile` constructor.
 
    For binary mode, this function is equivalent to the :class:`GzipFile`
@@ -80,9 +80,10 @@ The module defines the following items:
    in text mode, use :func:`.open` (or wrap your :class:`GzipFile` with an
    :class:`io.TextIOWrapper`).
 
-   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``.
+   The *compresslevel* argument is an integer from ``0`` to ``9`` controlling
+   the level of compression; ``1`` is fastest and produces the least
+   compression, and ``9`` is slowest and produces the most compression. ``0``
+   is no compression. The default is ``9``.
 
    The *mtime* argument is an optional numeric timestamp to be written to
    the stream when compressing.  All :program:`gzip` compressed streams are

Doc/library/zlib.rst

@@ -52,10 +52,10 @@ The available exception and functions in this module are:
 .. function:: compress(data[, level])
 
    Compresses the bytes in *data*, returning a bytes object containing compressed data.
-   *level* is an integer from ``1`` to ``9`` controlling the level of compression;
+   *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.  The default value is ``6``.  Raises the :exc:`error`
-   exception if any error occurs.
+   produces the most.  ``0`` is no compression.  The default value is ``6``.
+   Raises the :exc:`error` exception if any error occurs.
 
 
 .. function:: compressobj(level=-1, method=DEFLATED, wbits=15, memlevel=8, strategy=Z_DEFAULT_STRATEGY[, zdict])
@@ -63,9 +63,10 @@ The available exception and functions in this module are:
    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 ``1`` to ``9``. A value
+   *level* is the compression level -- an integer from ``0`` to ``9``. A value
    of ``1`` is fastest and produces the least compression, while a value of
-   ``9`` is slowest and produces the most. The default value is ``6``.
+   ``9`` is slowest and produces the most. ``0`` is no compression. The default
+   value is ``6``.
 
    *method* is the compression algorithm. Currently, the only supported value is
    ``DEFLATED``.

Lib/gzip.py

@@ -160,9 +160,10 @@ class GzipFile(io.BufferedIOBase):
         A mode of 'r' is equivalent to one of 'rb', and similarly for 'w' and
         'wb', and 'a' and 'ab'.
 
-        The compresslevel argument is an integer from 1 to 9 controlling the
+        The compresslevel argument is an integer from 0 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.
+        and 9 is slowest and produces the most compression. 0 is no compression
+        at all. The default is 9.
 
         The mtime argument is an optional numeric timestamp to be written
         to the stream when compressing.  All gzip compressed streams
@@ -622,7 +623,7 @@ class GzipFile(io.BufferedIOBase):
 
 def compress(data, compresslevel=9):
     """Compress data in one shot and return the compressed string.
-    Optional argument is the compression level, in range of 1-9.
+    Optional argument is the compression level, in range of 0-9.
     """
     buf = io.BytesIO()
     with GzipFile(fileobj=buf, mode='wb', compresslevel=compresslevel) as f:

Misc/NEWS

@@ -264,6 +264,9 @@ Tools/Demos
 Documentation
 -------------
 
+- Issue #15677: Document that zlib and gzip accept a compression level of 0 to
+  mean 'no compression'. Patch by Brian Brazil.
+
 - Issue #8040: added a version switcher to the documentation.  Patch by
   Yury Selivanov.
 

Modules/zlibmodule.c

@@ -139,7 +139,7 @@ newcompobject(PyTypeObject *type)
 PyDoc_STRVAR(compress__doc__,
 "compress(string[, level]) -- Returned compressed string.\n"
 "\n"
-"Optional arg level is the compression level, in 1-9.");
+"Optional arg level is the compression level, in 0-9.");
 
 static PyObject *
 PyZlib_compress(PyObject *self, PyObject *args)
@@ -1227,7 +1227,7 @@ PyDoc_STRVAR(zlib_module_documentation,
 "zlib library, which is based on GNU zip.\n"
 "\n"
 "adler32(string[, start]) -- Compute an Adler-32 checksum.\n"
-"compress(string[, level]) -- Compress string, with compression level in 1-9.\n"
+"compress(string[, level]) -- Compress string, with compression level in 0-9.\n"
 "compressobj([level[, ...]]) -- Return a compressor object.\n"
 "crc32(string[, start]) -- Compute a CRC-32 checksum.\n"
 "decompress(string,[wbits],[bufsize]) -- Decompresses a compressed string.\n"