4 ms·

It's worth noting that tuples aren't actually immutable in Python. Tuples have immutable length, but the values in those slots can be changed at runtime.

colanderman 10y ago
No, they definitely can't: >>> (1,2,3)[2]=5 Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: 'tuple' object does not support item assignment I wish they could. (Maybe Python 3 is different?)
- gakada 10y ago
  Tuples are immutable so that they can be used as keys in hash tables.
  - kazinator 10y ago
    But a sibling comment to yours by shoyer argues that mutable objects can be elements of a tuple. A tuple whose elements are mutable is effectively mutable. Can (1, 2, [3]) be used in a hash table, and what if the [3] is mutated to [3, 4]?
    - philipov 10y ago
      When it tries to hash the tuple, it finds the list inside of it, realizes the tuple is mutable, and throws an error.
- shoyer 10y ago
  You can't replace a value in a tuple, but you can modify it, e.g., >>> x = (1, 2, [3]) >>> x (1, 2, [3]) >>> x[2].append(4) >>> x (1, 2, [3, 4])
  - nemothekid 10y ago
    You haven't changed anything about the tuple. The holds a reference to a list and will always hold a reference to that list. What you did was modify the underlying list, the tuple itself is immutable.
  - kilink 10y ago
    ...Or can you? import ctypes def tuple_setitem(t, index, item): obj = ctypes.py_object(t) item = ctypes.py_object(item) ref_count = ctypes.c_long.from_address(id(t)) original_count = ref_count.value if original_count != 1: ref_count.value = 1 ctypes.pythonapi.Py_IncRef(item) ctypes.pythonapi.PyTuple_SetItem(obj, ctypes.c_ssize_t(index), item) ref_count.value = original_count >>> x = (1, 2, 3,) >>> tuple_setitem(x, 1, "foo") >>> x (1, "foo", 3) Disclaimer: for entertainment purposes only, do not try this at home!
  - pdonis 10y ago
    > You can't replace a value in a tuple, but you can modify it If it has mutable items in it, yes. But in that case, the tuple can't be used as a dictionary key; Python checks, not just that the tuple itself is immutable, but that every object in it is immutable.
  - deleted 10y ago
    [deleted]

colanderman 10y ago

No, they definitely can't: >>> (1,2,3)[2]=5 Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: 'tuple' object does not support item assignment I wish they could. (Maybe Python 3 is different?)

gakada 10y ago
Tuples are immutable so that they can be used as keys in hash tables.
- kazinator 10y ago
  But a sibling comment to yours by shoyer argues that mutable objects can be elements of a tuple. A tuple whose elements are mutable is effectively mutable. Can (1, 2, [3]) be used in a hash table, and what if the [3] is mutated to [3, 4]?
  - philipov 10y ago
    When it tries to hash the tuple, it finds the list inside of it, realizes the tuple is mutable, and throws an error.
shoyer 10y ago
You can't replace a value in a tuple, but you can modify it, e.g., >>> x = (1, 2, [3]) >>> x (1, 2, [3]) >>> x[2].append(4) >>> x (1, 2, [3, 4])
- nemothekid 10y ago
  You haven't changed anything about the tuple. The holds a reference to a list and will always hold a reference to that list. What you did was modify the underlying list, the tuple itself is immutable.
- kilink 10y ago
  ...Or can you? import ctypes def tuple_setitem(t, index, item): obj = ctypes.py_object(t) item = ctypes.py_object(item) ref_count = ctypes.c_long.from_address(id(t)) original_count = ref_count.value if original_count != 1: ref_count.value = 1 ctypes.pythonapi.Py_IncRef(item) ctypes.pythonapi.PyTuple_SetItem(obj, ctypes.c_ssize_t(index), item) ref_count.value = original_count >>> x = (1, 2, 3,) >>> tuple_setitem(x, 1, "foo") >>> x (1, "foo", 3) Disclaimer: for entertainment purposes only, do not try this at home!
- pdonis 10y ago
  > You can't replace a value in a tuple, but you can modify it If it has mutable items in it, yes. But in that case, the tuple can't be used as a dictionary key; Python checks, not just that the tuple itself is immutable, but that every object in it is immutable.
- deleted 10y ago
  [deleted]