GH-116271 Docs: provide clarification for object assignments in the Tutorial section (#116283)

Co-authored-by: Ned Batchelder <ned@nedbatchelder.com>
Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>

1 files changed, 24 insertions, 7 deletions

diff --git a/Doc/tutorial/introduction.rst b/Doc/tutorial/introduction.rst
index 4536ab9..0f16dae 100644
--- a/Doc/tutorial/introduction.rst
+++ b/Doc/tutorial/introduction.rst
@@ -405,13 +405,6 @@ indexed and sliced::
    >>> squares[-3:]  # slicing returns a new list
    [9, 16, 25]
 
-All slice operations return a new list containing the requested elements.  This
-means that the following slice returns a
-:ref:`shallow copy <shallow_vs_deep_copy>` of the list::
-
-   >>> squares[:]
-   [1, 4, 9, 16, 25]
-
 Lists also support operations like concatenation::
 
    >>> squares + [36, 49, 64, 81, 100]
@@ -435,6 +428,30 @@ the :meth:`!list.append` *method* (we will see more about methods later)::
    >>> cubes
    [1, 8, 27, 64, 125, 216, 343]
 
+Simple assignment in Python never copies data. When you assign a list
+to a variable, the variable refers to the *existing list*.
+Any changes you make to the list through one variable will be seen
+through all other variables that refer to it.::
+
+   >>> rgb = ["Red", "Green", "Blue"]
+   >>> rgba = rgb
+   >>> id(rgb) == id(rgba)  # they reference the same object
+   True
+   >>> rgba.append("Alph")
+   >>> rgb
+   ["Red", "Green", "Blue", "Alph"]
+
+All slice operations return a new list containing the requested elements.  This
+means that the following slice returns a
+:ref:`shallow copy <shallow_vs_deep_copy>` of the list::
+
+   >>> correct_rgba = rgba[:]
+   >>> correct_rgba[-1] = "Alpha"
+   >>> correct_rgba
+   ["Red", "Green", "Blue", "Alpha"]
+   >>> rgba
+   ["Red", "Green", "Blue", "Alph"]
+
 Assignment to slices is also possible, and this can even change the size of the
 list or clear it entirely::