Are tuples really immutable in Python? [duplicate]

问题

One question that I faced today, which actually tested the immutability of the tuples in Python:

Interviewer: Are tuples immutable in Python?

Me: Yes
Interviewer: So what does print(t1) here print?
t1 = (4, 5)
t1 = t1 + (91, 10)
print(t1)
Me: (4, 5, 91, 10)

Interviewer: How does immutability of tuple then define this behavior?

Me: It's got nothing to do with immutability, t1 label is referring to a new tuple.
Interviewer:
>>> t = (1, 2, [3, 4]) 
>>> t[2] += [5, 6]
What will happen next?
Me: TypeError is raised

Interviewer: What will be the value of t be after that?

Me: (1, 2, [3, 4]) or (1, 2, [3, 4, 5, 6]) maybe, not sure
Interviewer: What made you think the value will change to (1, 2, [3, 4, 5, 6]) and what will happen if I write
>>> t[2] = t[2] + [5, 6]
will t2 still be (1, 2, [3, 4, 5, 6]) after this operation?

回答1:

Even though the tuples are immutable, the object inside it can be mutuable

Since in >>> t = (1, 2, [3, 4]) the list is mutable so you can change the list value with Augmented assignment. += but then the exception is raised.

here the t[2] list is modified as you can see

t =(1,2,[3,4])
>>> id(t[2])
38073768
>>> t[2] += [5,6]
Traceback (most recent call last):
  File "<input>", line 1, in <module>
TypeError: 'tuple' object does not support item assignment
>>> t[2]
[3, 4, 5, 6]
>>> id(t[2])
38073768

As you can see the t[2] id never gets changed.

As for the second case: t[2] = t[2] + [5,6] - it creates a new list and then assign it to t[2]

>>> li = [1,2]
>>> id(li)
38036304
>>> li = li + [3,4]
>>> id(li)
38074368
>>> li += [5,6]
>>> id(li)
38074368

As you can see List = list + [] is a new list with a different id. and as the tuple is immutable t[2] doesn't get assigned to a new object in the second case

来源：https://stackoverflow.com/questions/32799300/are-tuples-really-immutable-in-python

标签

python

python-2.7

python-3.x

tuples

immutability