Add parse_index function for parsing NDIndex from string

.gitignore

@@ -130,3 +130,15 @@ dmypy.json
 
 # Pyre type checker
 .pyre/
+
+# jetbrains ide stuff
+*.iml
+.idea/
+
+# vscode ide stuff
+*.code-workspace
+.history/
+.vscode/
+
+# project scratch dir
+/scratch/

ndindex/literal_eval_index.py

@@ -0,0 +1,67 @@
+import ast
+
+def literal_eval_index(node_or_string):
+    """
+    "Safely" (needs validation) evaluate an expression node or a string containing
+    a (limited subset) of valid numpy index or slice expressions.
+    """
+    if isinstance(node_or_string, str):
+        node_or_string = ast.parse('dummy[{}]'.format(node_or_string.lstrip(" \t")) , mode='eval')
+    if isinstance(node_or_string, ast.Expression):
+        node_or_string = node_or_string.body
+    if isinstance(node_or_string, ast.Subscript):
+        node_or_string = node_or_string.slice
+
+    def _raise_malformed_node(node):
+        raise ValueError(f'malformed node or string: {node!r}')
+
+    # from cpy37, should work until they remove ast.Num (not until cpy310)
+    def _convert_num(node):
+        if isinstance(node, ast.Constant):
+            if isinstance(node.value, (int, float, complex)):
+                return node.value
+        elif isinstance(node, ast.Num):
+            # ast.Num was removed from ast grammar in cpy38
+            return node.n
+        _raise_malformed_node(node)
+    def _convert_signed_num(node):
+        if isinstance(node, ast.UnaryOp) and isinstance(node.op, (ast.UAdd, ast.USub)):
+            operand = _convert_num(node.operand)
+            if isinstance(node.op, ast.UAdd):
+                return + operand
+            else:
+                return - operand
+        return _convert_num(node)
+
+    def _convert(node):
+        if isinstance(node, ast.Tuple):
+            return tuple(map(_convert, node.elts))
+        elif isinstance(node, ast.List):
+            return list(map(_convert, node.elts))
+        elif isinstance(node, ast.Slice):
+            return slice(
+                _convert(node.lower) if node.lower is not None else None,
+                _convert(node.upper) if node.upper is not None else None,
+                _convert(node.step) if node.step is not None else None,
+            )
+        elif isinstance(node, ast.Call) and isinstance(node.func, ast.Name) and node.func.id == 'slice' and node.keywords == []:
+            # support for parsing slices written out as 'slice(...)' objects
+            return slice(*map(_convert, node.args))
+        elif isinstance(node, ast.NameConstant) and node.value is None:
+            # support for literal None in slices, eg 'slice(None, ...)'
+            return None
+        elif isinstance(node, ast.Ellipsis):
+            # support for three dot '...' ellipsis syntax
+            return ...
+        elif isinstance(node, ast.Name) and node.id == 'Ellipsis':
+            # support for 'Ellipsis' ellipsis syntax
+            return ...
+        elif isinstance(node, ast.Index):
+            # ast.Index was removed from ast grammar in cpy39
+            return _convert(node.value)
+        elif isinstance(node, ast.ExtSlice):
+            # ast.ExtSlice was removed from ast grammar in cpy39
+            return tuple(map(_convert, node.dims))
+
+        return _convert_signed_num(node)
+    return _convert(node_or_string)

ndindex/tests/test_literal_eval_index.py

@@ -0,0 +1,69 @@
+import ast
+from hypothesis import given, example
+from hypothesis.strategies import one_of
+import pytest
+
+from ..literal_eval_index import literal_eval_index
+from .helpers import ellipses, ints, slices, tuples, _doesnt_raise
+
+Tuples = tuples(one_of(
+    ellipses(),
+    ints(),
+    slices(),
+)).filter(_doesnt_raise)
+
+ndindexStrs = one_of(
+    ellipses(),
+    ints(),
+    slices(),
+    Tuples,
+).map(lambda x: f'{x}')
+
+class _Dummy:
+    def __getitem__(self, x):
+        return x
+_dummy = _Dummy()
+
+@example('3')
+@example('-3')
+@example('...')
+@example('Ellipsis')
+@example('+3')
+@example('3:4')
+@example('3:-4')
+@example('3, 5, 14, 1')
+@example('3, -5, 14, -1')
+@example('3:15, 5, 14:99, 1')
+@example('3:15, -5, 14:-99, 1')
+@example(':15, -5, 14:-99:3, 1')
+@example('3:15, -5, :, [1,2,3]')
+@example('slice(None)')
+@example('slice(None, None)')
+@example('slice(None, None, None)')
+@example('slice(14)')
+@example('slice(12, 14)')
+@example('slice(12, 72, 14)')
+@example('slice(-12, -72, 14)')
+@example('3:15, -5, slice(12, -14), (1,2,3)')
+@example('..., -5, slice(12, -14), (1,2,3)')
+@example('3:15, -5, slice(12, -14), (1,2,3), Ellipsis')
+@given(ndindexStrs)
+def test_literal_eval_index_hypothesis(ixStr):
+    assert eval(f'_dummy[{ixStr}]') == literal_eval_index(ixStr)
+
+def test_literal_eval_index_malformed_raise():
+    with pytest.raises(ValueError):
+        # we don't allow the bitwise not unary op
+        ixStr = '~3'
+        literal_eval_index(ixStr)
+
+def test_literal_eval_index_ensure_coverage():
+    # ensure full coverage, regarless of cpy version and accompanying changes to the ast grammar
+    for node in (
+        ast.Constant(7),
+        ast.Num(7),
+        ast.Index(ast.Constant(7)),
+    ):
+        assert literal_eval_index(node) == 7
+
+    assert literal_eval_index(ast.ExtSlice((ast.Constant(7), ast.Constant(7), ast.Constant(7)))) == (7, 7, 7)

setup.py

@@ -20,8 +20,10 @@
         "sympy",
     ],
     tests_require=[
-        'pytest',
-        'hypothesis',
+        "pytest",
+        "pytest-flakes",
+        "pytest-tornasync",
+        "hypothesis",
     ],
     classifiers=[
         "Programming Language :: Python :: 3",