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| -rw-r--r-- | Doc/tutorial/introduction.rst | 31 |
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:: |