WIP: starting slicing

.pre-commit-config.yaml

@@ -70,7 +70,7 @@ repos:
   hooks:
   - id: mypy
     files: ^src
-    additional_dependencies: [numpy==1.22.4, pytest, uhi, types-dataclasses]
+    additional_dependencies: [numpy~=1.23.0, pytest, uhi, types-dataclasses]
 
 - repo: https://github.com/mgedmin/check-manifest
   rev: "0.48"

src/boost_histogram/_internal/axis.py

@@ -226,10 +226,16 @@ def extent(self) -> int:
         """
         return self._ax.extent  # type: ignore[no-any-return]
 
-    def __getitem__(self, i: AxCallOrInt) -> Union[int, str, Tuple[float, float]]:
+    def __getitem__(
+        self: T, i: Union[AxCallOrInt, slice]
+    ) -> Union[int, str, Tuple[float, float], T]:
         """
         Access a bin, using normal Python syntax for wraparound.
         """
+        if isinstance(i, slice):
+            raise NotImplementedError(
+                f"Slicing not supported on {self.__class__.__name__}"
+            )
         # UHI support
         if callable(i):
             i = i(self)
@@ -241,6 +247,7 @@ def __getitem__(self, i: AxCallOrInt) -> Union[int, str, Tuple[float, float]]:
                     f"Out of range access, {i} is more than {self._ax.size}"
                 )
         assert not callable(i)
+        assert not isinstance(i, slice)
         return self.bin(i)
 
     @property
@@ -612,6 +619,9 @@ def _repr_args_(self) -> List[str]:
         return ret
 
 
+TStrC = TypeVar("TStrC", bound="StrCategory")
+
+
 @set_module("boost_histogram.axis")
 @register({ca.category_str_growth, ca.category_str})
 class StrCategory(BaseCategory, family=boost_histogram):
@@ -660,6 +670,16 @@ def __init__(
 
         super().__init__(ax, metadata, __dict__)
 
+    def __getitem__(
+        self: TStrC, i: Union[AxCallOrInt, slice]
+    ) -> Union[int, str, Tuple[float, float], TStrC]:
+
+        if isinstance(i, slice):
+            new_cats = list(self)[i]
+            return self.__class__(new_cats, __dict__=self.__dict__)  # type: ignore[arg-type]
+        else:
+            return super().__getitem__(i)
+
     def index(self, value: Union[float, str]) -> int:
         """
         Return the fractional index(es) given a value (or values) on the axis.

tests/test_axis.py

@@ -839,6 +839,26 @@ def test_edges_centers_widths(self, ref, growth):
         assert_allclose(a.centers, [0.5, 1.5, 2.5])
         assert_allclose(a.widths, [1, 1, 1])
 
+    def test_slicing(self, growth):
+        Cat = bh.axis.StrCategory
+        ref = ["a", "b", "c", "d", "e"]
+
+        a = Cat(ref, growth=growth)
+        b = a[1:3]
+        assert list(a)[1:3] == list(b)
+        assert a.__dict__ == b.__dict__
+        assert a.traits.growth == b.traits.growth
+
+    def test_empty_slice(self, growth):
+        Cat = bh.axis.StrCategory
+        ref = ["a", "b", "c", "d", "e"]
+        a = Cat(ref, growth=growth)
+        if growth:
+            assert a[0:0] == Cat([], growth=True)
+        else:
+            with pytest.raises(RuntimeError):
+                a[0:0]
+
 
 class TestBoolean:
     def test_init(self):