Fix #444582. Add shutil.which function for finding programs on the system path.
diff --git a/Doc/library/shutil.rst b/Doc/library/shutil.rst
index f88c6de..b581ce8 100644
--- a/Doc/library/shutil.rst
+++ b/Doc/library/shutil.rst
@@ -247,6 +247,30 @@
.. versionadded:: 3.3
+.. function:: which(cmd, mode=os.F_OK | os.X_OK, path=None)
+
+ Return the full path to an executable which would be run if the given
+ *cmd* was called. If no *cmd* would be called, return ``None``.
+
+ *mode* is a permission mask passed a to :func:`os.access`, by default
+ determining if the file exists and executable.
+
+ When no *path* is specified, the results of :func:`os.environ` are
+ used, returning either the "PATH" value or a fallback of :attr:`os.defpath`.
+
+ On Windows, the current directory is always prepended to the *path*
+ whether or not you use the default or provide your own, which
+ is the behavior the command shell uses when finding executables.
+ Additionaly, when finding the *cmd* in the *path*, the
+ ``PATHEXT`` environment variable is checked. For example, if you
+ call ``shutil.which("python")``, :func:`which` will search
+ ``PATHEXT`` to know that it should look for ``python.exe`` within
+ the *path* directories.
+
+ >>> print(shutil.which("python"))
+ 'c:\\python33\\python.exe'
+
+ .. versionadded:: 3.3
.. exception:: Error