Issue #26244: Clarify default zlib compression level in documentation

Based on patch by Aviv Palivoda.

Doc/library/zlib.rst

@@ -60,10 +60,11 @@ 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 ``0`` to ``9``. A value
-   of ``1`` is fastest and produces the least compression, while a value of
+   *level* is the compression level -- an integer from ``0`` to ``9`` or ``-1``.
+   A value of ``1`` is fastest and produces the least compression, while a value of
    ``9`` is slowest and produces the most. ``0`` is no compression. The default
-   value is ``6``.
+   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``.

Misc/ACKS

@@ -1071,6 +1071,7 @@ Joonas Paalasmaa
 Martin Packman
 Shriphani Palakodety
 Julien Palard
+Aviv Palivoda
 Ondrej Palkovsky
 Mike Pall
 Todd R. Palmer

Modules/clinic/zlibmodule.c.h

@@ -89,8 +89,9 @@ PyDoc_STRVAR(zlib_compressobj__doc__,
 "Return a compressor object.\n"
 "\n"
 "  level\n"
-"    The compression level (an integer in the range 0-9; default is 6).\n"
-"    Higher compression levels are slower, but produce smaller results.\n"
+"    The compression level (an integer in the range 0-9 or -1; default is\n"
+"    currently equivalent to 6).  Higher compression levels are slower,\n"
+"    but produce smaller results.\n"
 "  method\n"
 "    The compression algorithm.  If given, this must be DEFLATED.\n"
 "  wbits\n"
@@ -438,4 +439,4 @@ exit:
 #ifndef ZLIB_COMPRESS_COPY_METHODDEF
     #define ZLIB_COMPRESS_COPY_METHODDEF
 #endif /* !defined(ZLIB_COMPRESS_COPY_METHODDEF) */
-/*[clinic end generated code: output=7734aec079550bc8 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=cf81e1deae3af0ce input=a9049054013a1b77]*/

Modules/zlibmodule.c

@@ -390,8 +390,9 @@ zlib_decompress_impl(PyModuleDef *module, Py_buffer *data, int wbits,
 zlib.compressobj
 
     level: int(c_default="Z_DEFAULT_COMPRESSION") = Z_DEFAULT_COMPRESSION
-        The compression level (an integer in the range 0-9; default is 6).
-        Higher compression levels are slower, but produce smaller results.
+        The compression level (an integer in the range 0-9 or -1; default is
+        currently equivalent to 6).  Higher compression levels are slower,
+        but produce smaller results.
     method: int(c_default="DEFLATED") = DEFLATED
         The compression algorithm.  If given, this must be DEFLATED.
     wbits: int(c_default="MAX_WBITS") = MAX_WBITS
@@ -413,7 +414,7 @@ Return a compressor object.
 static PyObject *
 zlib_compressobj_impl(PyModuleDef *module, int level, int method, int wbits,
                       int memLevel, int strategy, Py_buffer *zdict)
-/*[clinic end generated code: output=2949bbb9a5723ccd input=b034847f8821f6af]*/
+/*[clinic end generated code: output=2949bbb9a5723ccd input=de2ffab6e910cd8b]*/
 {
     compobject *self = NULL;
     int err;