On 9/29/19, Anthony Flury via Python-list <python-list@python.org> wrote: > > Using python 3.6 building a tuple like this : > > my_tuple = tuple([x*x for x in range(1,1000)])
The list comprehension is implemented internally as a function that builds and returns the list. This function creates an empty list and loops over the range iterator to evaluate the expression and append the result. Appending to a list uses an over-allocation strategy to efficiently grow the list. Finally, the list is passed to the tuple constructor, which can efficiently and quickly create a tuple from the list because it's simply copying a PyObject * array in C. > my_tuple = tuple(x*x for x in range(1,1000)) In this case a generator is created instead of a list. This is passed to the tuple constructor, which iterates the generator. There's no __length_hint__() for a generator, so it starts with a length 10 tuple. The tuple grows with an over-allocation rule, and at the end it's resized to the actual length. I expect the generator-based expression to be a bit more expensive. Iterating a generator requires resuming evaluation of the code object up to a yield, which suspends evaluation. For the list comprehension, the loop that builds the list executes continuously, without an interruption to yield a value in each pass. -- https://mail.python.org/mailman/listinfo/python-list
