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Can tuples be replaced with lists all the time?


On 3/1/2014 4:20 PM, Mark Lawrence wrote:
> On 23/02/2014 17:48, Roy Smith wrote:

>> It also appears that tuples are more memory efficient.  I just ran some
>> quick tests on my OSX box.  Creating a list of 10 million [1, 2, 3, 4,
>> 5] lists gave me a 1445 MB process.   The name number of (1, 2, 3, 4, 5)
>> tuples was 748 MB.  I'm sure this is implementation dependent, but it
>> seems plausible to assume similar results will be had on other
>> implementations.

The numbers sound right.

> In CPython a list is overallocated so there's usually spare slots
> available if you want to add something to it.  In contrast you know when
> you create the tuple just how big it is so no overallocation is needed.

Besides which, in CPython, a tuple is one block of memory, with a 
PyObject header followed by the object references, while a list uses 2 
blocks of memory. The first has a PyObject header, followed by a 
reference to the list block, its allocated length (minimum 8 I believe), 
and its current in-use length. The over-allocated list block, which must 
also start on a 4 or 8 byte alignment boundary, has the object 
references plus extra space.


-- 
Terry Jan Reedy