py/objslice: Add support for indices() method on slice objects.

Instances of the slice class are passed to __getitem__() on objects when
the user indexes them with a slice.  In practice the majority of the time
(other than passing it on untouched) is to work out what the slice means in
the context of an array dimension of a particular length.  Since Python 2.3
there has been a method on the slice class, indices(), that takes a
dimension length and returns the real start, stop and step, accounting for
missing or negative values in the slice spec.  This commit implements such
a indices() method on the slice class.

It is configurable at compile-time via MICROPY_PY_BUILTINS_SLICE_INDICES,
disabled by default, enabled on unix, stm32 and esp32 ports.

This commit also adds new tests for slice indices and for slicing unicode
strings.

1 files changed, 27 insertions, 0 deletions

diff --git a/tests/basics/slice_indices.py b/tests/basics/slice_indices.py
new file mode 100644
index 0000000000..b7f439ccca
--- /dev/null
+++ b/tests/basics/slice_indices.py
@@ -0,0 +1,27 @@
+# Test builtin slice indices resolution
+
+# A class that returns an item key
+class A:
+    def __getitem__(self, idx):
+        return idx
+
+# Make sure that we have slices and .indices()
+try:
+    A()[2:5].indices(10)
+except:
+    print("SKIP")
+    raise SystemExit
+
+print(A()[:].indices(10))
+print(A()[2:].indices(10))
+print(A()[:7].indices(10))
+print(A()[2:7].indices(10))
+print(A()[2:7:2].indices(10))
+print(A()[2:7:-2].indices(10))
+print(A()[7:2:2].indices(10))
+print(A()[7:2:-2].indices(10))
+
+print(A()[2:7:2].indices(5))
+print(A()[2:7:-2].indices(5))
+print(A()[7:2:2].indices(5))
+print(A()[7:2:-2].indices(5))