Check for negative values before doing substr (#32)

* 1629: Check for negative values before doing substr

* 1629: Fix lint and unit test

* 1629: Reuse substring method from ibis.backends.base.sql.string

ibis_bigquery/compiler.py

@@ -274,6 +274,16 @@ def _string_right(translator, expr):
     )
 
 
+def _string_substring(translator, expr):
+    op = expr.op()
+    arg, start, length = op.args
+    if length.op().value < 0:
+        raise ValueError('Length parameter should not be a negative value.')
+
+    base_substring = operation_registry[ops.Substring]
+    base_substring(translator, expr)
+
+
 def _array_literal_format(expr):
     return str(list(expr.op().value))
 
@@ -418,6 +428,7 @@ def _formatter(translator, expr):
         ops.StringJoin: _string_join,
         ops.StringAscii: _string_ascii,
         ops.StringFind: _string_find,
+        ops.Substring: _string_substring,
         ops.StrRight: _string_right,
         ops.Repeat: fixed_arity('REPEAT', 2),
         ops.RegexSearch: _regex_search,

tests/unit/test_compiler.py

@@ -258,6 +258,16 @@ def test_binary():
     assert result == expected
 
 
+def test_substring():
+    t = ibis.table([('value', 'string')], name='t')
+    expr = t["value"].substr(3, -1)
+    with pytest.raises(Exception) as exception_info:
+        ibis_bigquery.compile(expr)
+
+    expected = 'Length parameter should not be a negative value.'
+    assert str(exception_info.value) == expected
+
+
 def test_bucket():
     t = ibis.table([('value', 'double')], name='t')
     buckets = [0, 1, 3]