feat(frontend-python): implement len for tracers

This makes fhe.array extension work with tensors as well!

frontends/concrete-python/concrete/fhe/extensions/array.py

@@ -44,9 +44,7 @@ def array(values: Any) -> Union[np.ndarray, Tracer]:
         if not isinstance(value, Tracer):
             values[i] = Tracer.sanitize(value)
 
-        if not values[i].output.is_scalar:
-            message = "Encrypted arrays can only be created from scalars"
-            raise ValueError(message)
+        assert values[i].output.is_scalar
 
     dtype = combine_dtypes([value.output.dtype for value in values])
     is_encrypted = True

frontends/concrete-python/concrete/fhe/tracing/tracer.py

@@ -863,6 +863,13 @@ class Tracer:
 
         return Tracer._trace_numpy_operation(np.transpose, self)
 
+    def __len__(self):
+        shape = self.shape
+        if len(shape) == 0:
+            message = "object of type 'Tracer' where 'shape == ()' has no len()"
+            raise TypeError(message)
+        return shape[0]
+
 
 class Annotation(Tracer):
     """

frontends/concrete-python/tests/compilation/test_compiler.py

@@ -155,6 +155,18 @@ def test_compiler_bad_trace(helpers):
 
     assert str(excinfo.value) == "Function 'g' returned '[{} ()]', which is not supported"
 
+    # len on scalar
+    # -------------
+
+    def len_on_scalar(x):
+        return len(x)
+
+    with pytest.raises(TypeError) as excinfo:
+        compiler = Compiler(len_on_scalar, {"x": "encrypted"})
+        compiler.trace(inputset=[1, 2, 3], configuration=configuration)
+
+    assert str(excinfo.value) == "object of type 'Tracer' where 'shape == ()' has no len()"
+
 
 def test_compiler_bad_compile(helpers):
     """

frontends/concrete-python/tests/execution/test_others.py

@@ -526,11 +526,11 @@ def copy_modify(x):
             id="fusable_additional_2",
         ),
         pytest.param(
-            lambda x: x + x.shape[0] + x.ndim + x.size,
+            lambda x: x + x.shape[0] + x.ndim + x.size + len(x),
             {
                 "x": {"status": "encrypted", "range": [0, 15], "shape": (3, 2)},
             },
-            id="x + x.shape[0] + x.ndim + x.size",
+            id="x + x.shape[0] + x.ndim + x.size + len(x)",
         ),
         pytest.param(
             lambda x: (50 * np.sin(x.transpose())).astype(np.int64),

frontends/concrete-python/tests/extensions/test_array.py

@@ -1,37 +0,0 @@
-"""
-Tests of 'array' extension.
-"""
-
-import pytest
-
-from concrete import fhe
-
-
-@pytest.mark.parametrize(
-    "function,parameters,expected_error",
-    [
-        pytest.param(
-            lambda x, y: fhe.array([x, y]),
-            {
-                "x": {"range": [0, 10], "status": "encrypted", "shape": ()},
-                "y": {"range": [0, 10], "status": "encrypted", "shape": (2, 3)},
-            },
-            "Encrypted arrays can only be created from scalars",
-        ),
-    ],
-)
-def test_bad_array(function, parameters, expected_error, helpers):
-    """
-    Test array with bad parameters.
-    """
-
-    parameter_encryption_statuses = helpers.generate_encryption_statuses(parameters)
-    configuration = helpers.configuration()
-
-    compiler = fhe.Compiler(function, parameter_encryption_statuses)
-
-    with pytest.raises(ValueError) as excinfo:
-        inputset = helpers.generate_inputset(parameters)
-        compiler.compile(inputset, configuration)
-
-    assert str(excinfo.value) == expected_error