Does itertools.product evaluate its arguments lazily?

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itertools.product generates its results lazily, but this is not true for the arguments. They are evaluated eagerly. Each iterable argument is first converted to a tuple:

The evaluation of the arguments (not the production of results) is very similar to the Python implementation shown in the docs:

...
pools = [tuple(pool) for pool in args] * repeat

Whereas, in the CPython implementation, pools is a tuple of tuples:

for (i=0; i < nargs ; ++i) {
     PyObject *item = PyTuple_GET_ITEM(args, i);
     PyObject *pool = PySequence_Tuple(item);   /* here */
     if (pool == NULL)
         goto error;
     PyTuple_SET_ITEM(pools, i, pool);
     indices[i] = 0;
 }

This is so since product sometimes needs to go over an iterable more than once, which is not possible if the arguments were left as iterators that can only be consumed once.

You practically cannot build a tuple from an itertools.count object. Consider slicing to a reasonable length with itertools.islice before passing to product.

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