python元组赋值（如果元组可变）

Question

I am new to python. Tuples are said to be immutable but why is that we can do something like this. ie to concatenate and change the original value

a=(1,2,3)
>>> a
(1, 2, 3)
>>> b=(4,5,6)
>>> b
(4, 5, 6)
>>> a=a+b
>>> a
(1, 2, 3, 4, 5, 6)

in this case aren't we changing the values in the tuple a?

Answer 1

No, you're making a new tuple. Consider.

>>> a = (1, 2, 3)
>>> a1 = a

Now a1 and a are the same tuple. Not just similar looking ones; they're the same . Then

>>> b = (4, 5, 6)
>>> a = a + b

Now a is (1, 2, 3, 4, 5, 6) . Did we change the first tuple we made? Let's ask Python.

>>> a
(1, 2, 3, 4, 5, 6)
>>> a1
(1, 2, 3)

Nope, the original tuple stayed the same.

This distinction is important. You made a new tuple and happened to give it the same name. That means that, if you were writing a complicated program with a huge object hierarchy, the change you just made to a wouldn't break any of the other objects or code that depended on the previous value of a . It only broke your particular a variable that you control. On the other hand, if we had a list a = [1, 2, 3] and started appending to it, then any other object who happened to hold a reference to that list is now going to see the changes, which results in messy errors at a distance.