WIP: Support slices like '3:10:mean(2)'

tiled/_tests/test_array.py

@@ -166,3 +166,17 @@ def test_array_interface(context):
         # smoke test
         v.chunks
         v.dims
+
+
+def test_slices():
+    client = from_tree(cube_tree)
+    ac = client["tiny_cube"]
+    ac[:]
+    ac[:, :, :]
+    ac[..., 0, :1]
+    ac[1:4, -5:]
+    ac[-100:4, :-3]
+    ac[0, 3, 8]
+    ac[0, 3, :]
+    with pytest.raises(IndexError):
+        ac[100]

tiled/server/dependencies.py

@@ -1,3 +1,6 @@
+import builtins
+import collections
+import re
 from functools import lru_cache
 from typing import Optional
 
@@ -150,14 +153,52 @@ def slice_(
     "Specify and parse a block index parameter."
     import numpy
 
-    # IMPORTANT We are eval-ing a user-provider string here so we need to be
-    # very careful about locking down what can be in it. The regex above
-    # excludes any letters or operators, so it is not possible to execute
-    # functions or expensive arithmetic.
-    return tuple(
-        [
-            eval(f"numpy.s_[{dim!s}]", {"numpy": numpy})
-            for dim in (slice or "").split(",")
-            if dim
-        ]
-    )
+# and 'a:b:mean(c)' to represent downsampling, inspired by
+# https://uhi.readthedocs.io/en/latest/
+# Note: if you need to debug this, the interactive tool at https://regex101.com/ is your friend!
+DIM_REGEX = r"(?:(-?[0-9]+)|(?:([0-9]*|-[0-9]+):(?:([0-9]*|-[0-9]+))?(?::(mean|mean\([0-9]+\)|[0-9]*|-[0-9]+))?))"
+SLICE_REGEX = rf"^{DIM_REGEX}(,{DIM_REGEX})*$"
+DIM_PATTERN = re.compile(rf"^{DIM_REGEX}$")
+MEAN_PATTERN = re.compile(r"(mean|mean\(([0-9]+)\))")
+
+
+# This object is meant to be placed at slice.step and used by the consumer to
+# detect that it should agggregate, using
+# numpy.mean or skimage.transform.downscale_local_mean.
+Mean = collections.namedtuple("Mean", ["parameter"])
+
+
+def _int_or_none(s):
+    return int(s) if s else None
+
+
+def _mean_int_or_none(s):
+    if s is None:
+        return None
+    m = MEAN_PATTERN.match(s)
+    if m.group(0):
+        return Mean(m.group(1))
+    return _int_or_none(s)
+
+
+def slice_(
+    slice: str = Query("", regex=SLICE_REGEX),
+):
+    "Specify and parse a slice parameter."
+    slices = []
+    for dim in slice.split(","):
+        if dim:
+            match = DIM_PATTERN.match(dim)
+            # Group 1 matches an int, as in arr[i].
+            if match.group(1) is not None:
+                s = int(match.group(1))
+            else:
+                # Groups 2 through 4 match a slice, as in arr[i:j:k].
+                s = builtins.slice(
+                    _int_or_none(match.group(2)),
+                    _int_or_none(match.group(3)),
+                    _mean_int_or_none(match.group(4)),
+                )
+            slices.append(s)
+    print("slices", slices)
+    return tuple(slices)