ENH: Allow broadcast to be called with zero arguments

Follows on from gh-6905 which reduced the limit from 2 to 1. Let's go all the way to zero.

Just as for `broadcast(broadcast(a), b)` is interpreted as `broadcast(a, b)` , this change interprets
`broadcast(broadcast(), a)` as `broadcast(a)`.

3 files changed, 22 insertions, 15 deletions

diff --git a/numpy/core/src/multiarray/iterators.c b/numpy/core/src/multiarray/iterators.c
index 9fcdc91b2..75095f6f0 100644
--- a/numpy/core/src/multiarray/iterators.c
+++ b/numpy/core/src/multiarray/iterators.c
@@ -1262,10 +1262,14 @@ PyArray_MultiIterFromObjects(PyObject **mps, int n, int nadd, ...)
     int i, ntot, err=0;
 
     ntot = n + nadd;
-    if (ntot < 1 || ntot > NPY_MAXARGS) {
+    if (ntot < 0) {
         PyErr_Format(PyExc_ValueError,
-                     "Need at least 1 and at most %d "
-                     "array objects.", NPY_MAXARGS);
+                     "n and nadd arguments must be non-negative", NPY_MAXARGS);
+        return NULL;
+    }
+    if (ntot > NPY_MAXARGS) {
+        PyErr_Format(PyExc_ValueError,
+                     "At most %d array objects are supported.", NPY_MAXARGS);
         return NULL;
     }
     multi = PyArray_malloc(sizeof(PyArrayMultiIterObject));
@@ -1328,10 +1332,14 @@ PyArray_MultiIterNew(int n, ...)
 
     int i, err = 0;
 
-    if (n < 1 || n > NPY_MAXARGS) {
+    if (n < 0) {
+        PyErr_Format(PyExc_ValueError,
+                     "n argument must be non-negative", NPY_MAXARGS);
+        return NULL;
+    }
+    if (n > NPY_MAXARGS) {
         PyErr_Format(PyExc_ValueError,
-                     "Need at least 1 and at most %d "
-                     "array objects.", NPY_MAXARGS);
+                     "At most %d array objects are supported.", NPY_MAXARGS);
         return NULL;
     }
 
@@ -1409,13 +1417,12 @@ arraymultiter_new(PyTypeObject *NPY_UNUSED(subtype), PyObject *args, PyObject *k
             ++n;
         }
     }
-    if (n < 1 || n > NPY_MAXARGS) {
-        if (PyErr_Occurred()) {
-            return NULL;
-        }
+    if (PyErr_Occurred()) {
+        return NULL;
+    }
+    if (n > NPY_MAXARGS) {
         PyErr_Format(PyExc_ValueError,
-                     "Need at least 1 and at most %d "
-                     "array objects.", NPY_MAXARGS);
+                     "At most %d array objects are supported.", NPY_MAXARGS);
         return NULL;
     }
 
diff --git a/numpy/core/tests/test_numeric.py b/numpy/core/tests/test_numeric.py
index 1c53f9372..c799aaf6c 100644
--- a/numpy/core/tests/test_numeric.py
+++ b/numpy/core/tests/test_numeric.py
@@ -2736,6 +2736,8 @@ class TestBroadcast(object):
         arrs = [np.empty((6, 7)), np.empty((5, 6, 1)), np.empty((7,)),
                 np.empty((5, 1, 7))]
         mits = [np.broadcast(*arrs),
+                np.broadcast(np.broadcast(*arrs[:0]), np.broadcast(*arrs[0:])),
+                np.broadcast(np.broadcast(*arrs[:1]), np.broadcast(*arrs[1:])),
                 np.broadcast(np.broadcast(*arrs[:2]), np.broadcast(*arrs[2:])),
                 np.broadcast(arrs[0], np.broadcast(*arrs[1:-1]), arrs[-1])]
         for mit in mits:
@@ -2760,7 +2762,7 @@ class TestBroadcast(object):
         arr = np.empty((5,))
         for j in range(35):
             arrs = [arr] * j
-            if j < 1 or j > 32:
+            if j > 32:
                 assert_raises(ValueError, np.broadcast, *arrs)
             else:
                 mit = np.broadcast(*arrs)
diff --git a/numpy/lib/stride_tricks.py b/numpy/lib/stride_tricks.py
index 0dc36e41c..fd401c57c 100644
--- a/numpy/lib/stride_tricks.py
+++ b/numpy/lib/stride_tricks.py
@@ -186,8 +186,6 @@ def _broadcast_shape(*args):
     """Returns the shape of the arrays that would result from broadcasting the
     supplied arrays against each other.
     """
-    if not args:
-        return ()
     # use the old-iterator because np.nditer does not handle size 0 arrays
     # consistently
     b = np.broadcast(*args[:32])