feat(ux): add duckdb as the default backend

ibis/backends/dask/core.py

@@ -169,7 +169,7 @@ def execute_with_scope(
     # computing anything *and* before associating leaf nodes with data. This
     # allows clients to provide their own data for each leaf.
     if clients is None:
-        clients = expr._find_backends()
+        clients, _ = expr._find_backends()
 
     if aggcontext is None:
         aggcontext = agg_ctx.Summarize()

ibis/backends/pandas/core.py

@@ -205,7 +205,7 @@ def execute_with_scope(
     # computing anything *and* before associating leaf nodes with data. This
     # allows clients to provide their own data for each leaf.
     if clients is None:
-        clients = expr._find_backends()
+        clients, _ = expr._find_backends()
 
     if aggcontext is None:
         aggcontext = agg_ctx.Summarize()

ibis/backends/tests/test_client.py

@@ -1,4 +1,5 @@
 import platform
+import re
 
 import pandas as pd
 import pandas.testing as tm
@@ -7,6 +8,7 @@
 from pytest import mark, param
 
 import ibis
+import ibis.common.exceptions as com
 import ibis.expr.datatypes as dt
 from ibis.util import guid
 
@@ -661,3 +663,51 @@ def test_create_from_in_memory_table(con, t):
     finally:
         con.drop_table(tmp_name)
         assert tmp_name not in con.list_tables()
+
+
+def test_default_backend_no_duckdb(backend):
+    # backend is used to ensure that this test runs in CI in the setting
+    # where only the dependencies for a a given backend are installed
+
+    # if duckdb is available then this test won't fail and so we skip it
+    try:
+        import duckdb  # noqa: F401
+
+        pytest.skip(
+            "duckdb is installed; it will be used as the default backend"
+        )
+    except ImportError:
+        pass
+
+    df = pd.DataFrame({'a': [1, 2, 3]})
+    t = ibis.memtable(df)
+    expr = t.a.sum()
+
+    # run this twice to ensure that we hit the optimizations in
+    # `_default_backend`
+    for _ in range(2):
+        with pytest.raises(
+            com.IbisError,
+            match="Expression depends on no backends",
+        ):
+            expr.execute()
+
+
+@pytest.mark.duckdb
+def test_default_backend():
+    pytest.importorskip("duckdb")
+
+    df = pd.DataFrame({'a': [1, 2, 3]})
+    t = ibis.memtable(df)
+    expr = t.a.sum()
+    # run this twice to ensure that we hit the optimizations in
+    # `_default_backend`
+    for _ in range(2):
+        assert expr.execute() == df.a.sum()
+
+    sql = ibis.to_sql(expr)
+    rx = """\
+SELECT
+  SUM\\((\\w+)\\.a\\) AS sum
+FROM \\w+ AS \\1"""
+    assert re.match(rx, sql) is not None

ibis/config.py

@@ -86,6 +86,31 @@ def query_text_length_ge_zero(cls, query_text_length: int) -> int:
         return query_text_length
 
 
+_HAS_DUCKDB = True
+_DUCKDB_CON = None
+
+
+def _default_backend() -> Any:
+    global _HAS_DUCKDB, _DUCKDB_CON
+
+    if not _HAS_DUCKDB:
+        return None
+
+    if _DUCKDB_CON is not None:
+        return _DUCKDB_CON
+
+    try:
+        import duckdb as _  # noqa: F401
+    except ImportError:
+        _HAS_DUCKDB = False
+        return None
+
+    import ibis
+
+    _DUCKDB_CON = ibis.duckdb.connect(":memory:")
+    return _DUCKDB_CON
+
+
 class Options(BaseSettings):
     """Ibis configuration options."""
 
@@ -106,10 +131,15 @@ class Options(BaseSettings):
         default=False,
         description="Render expressions as GraphViz PNGs when running in a Jupyter notebook.",  # noqa: E501
     )
+
     default_backend: Any = Field(
         default=None,
-        description="The default backend to use for execution.",
+        description=(
+            "The default backend to use for execution. "
+            "Defaults to DuckDB if not set."
+        ),
     )
+
     context_adjustment: ContextAdjustment = Field(
         default=ContextAdjustment(),
         description=ContextAdjustment.__doc__,

ibis/expr/types/core.py

@@ -7,13 +7,13 @@
 
 from public import public
 
-from ibis import config
 from ibis.common.exceptions import (
     ExpressionError,
     IbisError,
     IbisTypeError,
     TranslationError,
 )
+from ibis.config import _default_backend, options
 from ibis.expr.typing import TimeContext
 from ibis.util import UnnamedMarker, deprecated
 
@@ -33,7 +33,7 @@ def __init__(self, arg: ops.Node) -> None:
         self._arg = arg
 
     def __repr__(self) -> str:
-        if not config.options.interactive:
+        if not options.interactive:
             return self._repr()
 
         try:
@@ -103,7 +103,7 @@ def _key(self) -> tuple[Hashable, ...]:
         return type(self), self._safe_name, self.op()
 
     def _repr_png_(self) -> bytes | None:
-        if config.options.interactive or not config.options.graphviz_repr:
+        if options.interactive or not options.graphviz_repr:
             return None
         try:
             import ibis.expr.visualize as viz
@@ -189,14 +189,15 @@ def pipe(self, f, *args: Any, **kwargs: Any) -> Expr:
     def op(self) -> ops.Node:
         return self._arg
 
-    def _find_backends(self) -> list[BaseBackend]:
+    def _find_backends(self) -> tuple[list[BaseBackend], bool]:
         """Return the possible backends for an expression.
 
         Returns
         -------
         list[BaseBackend]
             A list of the backends found.
         """
+        import ibis.expr.operations as ops
         from ibis.backends.base import BaseBackend
 
         seen_backends: dict[
@@ -205,11 +206,13 @@ def _find_backends(self) -> list[BaseBackend]:
 
         stack = [self.op()]
         seen = set()
+        has_unbound = False
 
         while stack:
             node = stack.pop()
 
             if node not in seen:
+                has_unbound |= isinstance(node, ops.UnboundTable)
                 seen.add(node)
 
                 for arg in node.flat_args():
@@ -219,13 +222,36 @@ def _find_backends(self) -> list[BaseBackend]:
                     elif isinstance(arg, Expr):
                         stack.append(arg.op())
 
-        return list(seen_backends.values())
+        return list(seen_backends.values()), has_unbound
 
-    def _find_backend(self) -> BaseBackend:
-        backends = self._find_backends()
+    def _find_backend(self, *, use_default: bool = False) -> BaseBackend:
+        """Find the backend attached to an expression.
+
+        Parameters
+        ----------
+        use_default
+            If [`True`][True] and the default backend isn't set, initialize the
+            default backend and use that. This should only be set to `True` for
+            `.execute()`. For other contexts such as compilation, this option
+            doesn't make sense so the default value is [`False`][False].
+
+        Returns
+        -------
+        BaseBackend
+            A backend that is attached to the expression
+        """
+        backends, has_unbound = self._find_backends()
 
         if not backends:
-            default = config.options.default_backend
+            if has_unbound:
+                raise IbisError(
+                    "Expression contains unbound tables and therefore cannot "
+                    "be executed. Use ibis.<backend>.execute(expr) or "
+                    "assign a backend instance to "
+                    "`ibis.options.default_backend`."
+                )
+            if (default := options.default_backend) is None and use_default:
+                default = _default_backend()
             if default is None:
                 raise IbisError(
                     'Expression depends on no backends, and found no default'
@@ -262,7 +288,7 @@ def execute(
         params
             Mapping of scalar parameter expressions to value
         """
-        return self._find_backend().execute(
+        return self._find_backend(use_default=True).execute(
             self, limit=limit, timecontext=timecontext, params=params, **kwargs
         )
 

ibis/tests/expr/test_table.py

@@ -1551,3 +1551,14 @@ def test_memtable_filter():
     t = ibis.memtable([(1, 2), (3, 4), (5, 6)], columns=["x", "y"])
     expr = t.filter(t.x > 1)
     assert expr.columns == ["x", "y"]
+
+
+def test_default_backend_with_unbound_table():
+    t = ibis.table(dict(a="int"), name="t")
+    expr = t.a.sum()
+
+    with pytest.raises(
+        com.IbisError,
+        match="Expression contains unbound tables",
+    ):
+        assert expr.execute()