ENH: Make swapaxes always return a view

swapaxes now returns a view of the input array when the axes to swap are both the same, not the input array as it used to do. Fixes numpy#5260
mwcraig · Jan 21, 2015 · 1d3bcb4 · 1d3bcb4
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diff --git a/doc/release/1.10.0-notes.rst b/doc/release/1.10.0-notes.rst
@@ -50,12 +50,17 @@ the case of matrices.  Matrices are special cased for backward
 compatibility and still return 1-D arrays as before. If you need to
 preserve the matrix subtype, use the methods instead of the functions.
 
-*rollaxis* always returns a view
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+*rollaxis* and *swapaxes* always return a view
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Previously, a view was returned except when no change was made in the order
 of the axes, in which case the input array was returned.  A view is now
 returned in all cases.
 
+C API
+~~~~~
+The changes to *swapaxes* also apply to the *PyArray_SwapAxes* C function,
+which now returns a view in all cases.
+
 
 New Features
 ============

diff --git a/numpy/core/fromnumeric.py b/numpy/core/fromnumeric.py
@@ -464,8 +464,10 @@ def swapaxes(a, axis1, axis2):
     Returns
     -------
     a_swapped : ndarray
-        If `a` is an ndarray, then a view of `a` is returned; otherwise
-        a new array is created.
+        For Numpy >= 1.10, if `a` is an ndarray, then a view of `a` is
+        returned; otherwise a new array is created. For earlier Numpy
+        versions a view of `a` is returned only if the order of the
+        axes is changed, otherwise the input array is returned.
 
     Examples
     --------

diff --git a/numpy/core/src/multiarray/shape.c b/numpy/core/src/multiarray/shape.c
@@ -653,19 +653,8 @@ PyArray_SwapAxes(PyArrayObject *ap, int a1, int a2)
 {
     PyArray_Dims new_axes;
     npy_intp dims[NPY_MAXDIMS];
-    int n, i, val;
-    PyObject *ret;
-
-    if (a1 == a2) {
-        Py_INCREF(ap);
-        return (PyObject *)ap;
-    }
-
-    n = PyArray_NDIM(ap);
-    if (n <= 1) {
-        Py_INCREF(ap);
-        return (PyObject *)ap;
-    }
+    int n = PyArray_NDIM(ap);
+    int i;
 
     if (a1 < 0) {
         a1 += n;
@@ -683,25 +672,20 @@ PyArray_SwapAxes(PyArrayObject *ap, int a1, int a2)
                         "bad axis2 argument to swapaxes");
         return NULL;
     }
+
+    for (i = 0; i < n; ++i) {
+        dims[i] = i;
+    }
+    dims[a1] = a2;
+    dims[a2] = a1;
+
     new_axes.ptr = dims;
     new_axes.len = n;
 
-    for (i = 0; i < n; i++) {
-        if (i == a1) {
-            val = a2;
-        }
-        else if (i == a2) {
-            val = a1;
-        }
-        else {
-            val = i;
-        }
-        new_axes.ptr[i] = val;
-    }
-    ret = PyArray_Transpose(ap, &new_axes);
-    return ret;
+    return PyArray_Transpose(ap, &new_axes);
 }
 
+
 /*NUMPY_API
  * Return Transpose.
  */
@@ -969,7 +953,7 @@ PyArray_Ravel(PyArrayObject *arr, NPY_ORDER order)
 
         PyArray_CreateSortedStridePerm(PyArray_NDIM(arr),
                                 PyArray_STRIDES(arr), strideperm);
-        
+
         for (i = ndim-1; i >= 0; --i) {
             if (PyArray_DIM(arr, strideperm[i].perm) == 1) {
                 /* A size one dimension does not matter */