Fix #1237: iterator-handling bug in partition

Also, `partition` with n < 0 now raises an error.

NEWS

@@ -13,6 +13,7 @@ Changes from 0.12.1
    [ Bug Fixes ]
    * Shadowed comparison operators now use `and` instead of `&`
      for chained comparisons
+   * partition no longer prematurely exhausts input iterators
 
 Changes from 0.12.0
 

hy/core/language.hy

@@ -371,7 +371,9 @@
    more to skip elements, or less for a sliding window with overlap."
   (setv
    step (or step n)
-   slices (genexpr (itertools.islice coll start None step) [start (range n)]))
+   coll-clones (tee coll n)
+   slices (genexpr (islice (get coll-clones start) start None step)
+                   [start (range n)]))
   (if (is fillvalue -sentinel)
     (apply zip slices)
     (apply zip-longest slices {"fillvalue" fillvalue})))

tests/native_tests/core.hy

@@ -455,9 +455,11 @@
   ;; length 1 is valid
   (assert-equal (list (partition ten 1))
                 [(, 0) (, 1) (, 2) (, 3) (, 4) (, 5) (, 6) (, 7) (, 8) (, 9)])
-  ;; tuples of length < 1 don't crash
+  ;; length 0 returns an empty sequence
   (assert-equal (list (partition ten 0)) [])
-  (assert-equal (list (partition ten -1)) [])
+  ;; negative length raises ValueError
+  (try (do (partition ten -1) (assert False))
+       (except [ValueError]))
   ;; keep remainder with a fillvalue
   (assert-equal (list (partition ten 3 :fillvalue "x"))
                 [(, 0 1 2) (, 3 4 5) (, 6 7 8) (, 9 "x" "x")])
@@ -466,7 +468,13 @@
                 [(, 0 1) (, 3 4) (, 6 7)])
   ;; overlap with step < n
   (assert-equal (list (partition (range 5) 2 1))
-                [(, 0 1) (, 1 2) (, 2 3) (, 3 4)]))
+                [(, 0 1) (, 1 2) (, 2 3) (, 3 4)])
+  ;; tee the input as necessary
+  ;; https://github.com/hylang/hy/issues/1237
+  (assert-equal (list (take 4 (partition (cycle [1 2 3]) 3)))
+                [(, 1 2 3) (, 1 2 3) (, 1 2 3) (, 1 2 3)])
+  (assert-equal (list (partition (iter (range 10))))
+                [(, 0 1) (, 2 3) (, 4 5) (, 6 7) (, 8 9)]))
 
 (defn test-pos []
   "NATIVE: testing the pos? function"