bpo-42958: Improve description of shallow= in filecmp.cmp docs (GH-27166)

Co-authored-by: Łukasz Langa <lukasz@langa.pl>
Co-authored-by: Alexander Vandenbulcke <alexander.vandenbulcke95@gmail.com>

Doc/library/filecmp.rst

@@ -22,8 +22,11 @@ The :mod:`filecmp` module defines the following functions:
    Compare the files named *f1* and *f2*, returning ``True`` if they seem equal,
    ``False`` otherwise.
 
-   If *shallow* is true, files with identical :func:`os.stat` signatures are
-   taken to be equal.  Otherwise, the contents of the files are compared.
+   If *shallow* is true and the :func:`os.stat` signatures (file type, size, and
+   modification time) of both files are identical, the files are taken to be
+   equal.
+
+   Otherwise, the files are treated as different if their sizes or contents differ.
 
    Note that no external programs are called from this function, giving it
    portability and efficiency.

Lib/filecmp.py

@@ -36,8 +36,9 @@ def cmp(f1, f2, shallow=True):
 
     f2 -- Second file name
 
-    shallow -- Just check stat signature (do not read the files).
-               defaults to True.
+    shallow -- treat files as identical if their stat signatures (type, size,
+               mtime) are identical. Otherwise, files are considered different
+               if their sizes or contents differ.  [default: True]
 
     Return value:
 

Misc/NEWS.d/next/Documentation/2021-07-15-11-19-03.bpo-42958.gC5IHM.rst

@@ -0,0 +1,2 @@
+Updated the docstring and docs of :func:`filecmp.cmp` to be more accurate
+and less confusing especially in respect to *shallow* arg.