API: add .get() operation on a Map type

docs/source/release.rst

@@ -34,6 +34,7 @@ New Features
 * ``ibis.pandas.from_dataframe`` convenience function (:issue:`1155`)
 * Remove the restriction on ``ROW_NUMBER()`` requiring it to have an
   ``ORDER BY`` clause (:issue:`1371`)
+* Add ``.get()`` operation on a Map type (:issue:`1376`)
 
 Bug Fixes
 ~~~~~~~~~

ibis/expr/api.py

@@ -1992,13 +1992,27 @@ def _array_slice(array, index):
 
 _add_methods(ArrayValue, _array_column_methods)
 
+
 # ---------------------------------------------------------------------
 # Map API
 
+def get(expr, key, default):
+    """
+    Return the mapped value for this key, or the default
+    if the key does not exist
+
+    Parameters
+    ----------
+    key : any
+    default : any
+    """
+    return _ops.MapValueOrDefaultForKey(expr, key, default).to_expr()
+
 
 _map_column_methods = dict(
     length=_unary_op('length', _ops.MapLength),
     __getitem__=_binop_expr('__getitem__', _ops.MapValueForKey),
+    get=get,
     keys=_unary_op('keys', _ops.MapKeys),
     values=_unary_op('values', _ops.MapValues),
     __add__=_binop_expr('__add__', _ops.MapConcat),

ibis/expr/operations.py

@@ -2746,6 +2746,27 @@ def output_type(self):
         return rules.shape_like(self.args[0], map_type.value_type)
 
 
+class MapValueOrDefaultForKey(ValueOp):
+
+    input_type = [
+        rules.map(dt.any, dt.any),
+        rules.one_of((dt.string, dt.int_), name='key'),
+        rules.value(name='default')
+    ]
+
+    def output_type(self):
+        map_type = self.args[0].type()
+        value_type = map_type.value_type
+        default_type = self.args[2].type()
+
+        if not (default_type is dt.null or
+                value_type.equals(default_type)):
+            raise ValueError("default type: {}  must be the same "
+                             "as the map value_type {}".format(
+                                 default_type, value_type))
+        return rules.shape_like(self.args[0], map_type.value_type)
+
+
 class MapKeys(ValueOp):
 
     input_type = [rules.map(dt.any, dt.any)]

ibis/expr/tests/test_value_exprs.py

@@ -150,13 +150,23 @@ def test_simple_map_operations():
     expr = ibis.literal(value)
     expr2 = ibis.literal(value2)
     assert isinstance(expr, ir.MapValue)
-    assert isinstance(expr['b'].op(), ops.MapValueForKey)
     assert isinstance(expr.length().op(), ops.MapLength)
-    assert isinstance(expr.keys().op(), ops.MapKeys)
-    assert isinstance(expr.values().op(), ops.MapValues)
     assert isinstance((expr + expr2).op(), ops.MapConcat)
     assert isinstance((expr2 + expr).op(), ops.MapConcat)
 
+    default = ibis.literal([0.0])
+    assert isinstance(expr.get('d', default).op(), ops.MapValueOrDefaultForKey)
+
+    # test for an invalid default type, nulls are ok
+    with pytest.raises(ValueError):
+        expr.get('d', ibis.literal('foo'))
+    assert isinstance(expr.get('d', ibis.literal(None)).op(),
+                      ops.MapValueOrDefaultForKey)
+
+    assert isinstance(expr['b'].op(), ops.MapValueForKey)
+    assert isinstance(expr.keys().op(), ops.MapKeys)
+    assert isinstance(expr.values().op(), ops.MapValues)
+
 
 @pytest.mark.parametrize(
     ['value', 'expected_type'],