Merge lp:~cjwatson/storm/is-and-is-not-operators into lp:storm

[Colin Watson]

Add storm.expr.Is and storm.expr.IsNot operators.

Launchpad has had custom `IsTrue` and `IsFalse` operators for a while, because there are a few places where `expr` or `expr == True` (or the equivalents for False) don't quite do what we need: for example, if you're working with a nullable boolean column, then SQL NULL semantics mean that `expr = TRUE` or `expr = FALSE` return NULL when `expr` is NULL, which can be undesirable at times.

In addition, `==` and `!=` comparisons with True/False/None are typically picked up as errors by modern linters, since in normal Python code they should use `is` or `is not` or simple boolean tests instead. Having to override this is an annoyance.

Compare https://docs.sqlalchemy.org/en/20/core/operators.html#identity-comparisons, although SQLAlchemy is more permissive; it seems to be happy to compile anything on the right-hand-side of `IS` or `IS NOT` and leave it up to the database. Since we already have to take some care with how booleans are rendered, and there's no guarantee that the spelling of `IS TRUE` etc. is going to match the way that boolean values themselves are spelled, I thought it better to limit the set of right-hand-side expressions that we will compile.

I've checked that current versions of all the databases currently supported by Storm (PostgreSQL, MySQL, and SQLite) support all the resulting expressions, although in the case of SQLite it does need at least version 3.23.0 (released in 2018).

[Andrey Fedoseev (andrey-fedoseev)]

review: Approve

=== modified file 'NEWS'
--- NEWS	2022-03-16 16:59:21 +0000
+++ NEWS	2022-10-18 17:38:52 +0000
@@ -6,6 +6,7 @@
 
 - Clarify exception when creating a property with both allow_none=False and
   default=None.
+- Add storm.expr.Is and storm.expr.IsNot operators.
 
 Bug fixes
 ---------

=== modified file 'storm/expr.py'
--- storm/expr.py	2020-12-07 22:38:36 +0000
+++ storm/expr.py	2022-10-18 17:38:52 +0000
@@ -1047,6 +1047,74 @@
     return compile(expr.exprs, state, join=expr.oper.lower())
 
 
+class Is(BinaryOper):
+    """The SQL C{IS ...} operators, e.g. C{IS NULL}.
+
+    C{Is(expr, None)} is synonymous with C{expr == None}, but is less likely
+    to trip up linters.
+
+    Unlike C{expr} or C{expr == True}, C{Is(expr, True)} returns C{FALSE}
+    when C{expr} is C{NULL}.
+
+    Unlike C{Not(expr)} or C{expr == False}, C{Is(expr, False)} returns
+    C{FALSE} when C{expr} is C{NULL}.
+    """
+    __slots__ = ()
+    oper = " IS "
+
[e-mail](Is)
+def compile_is(compile, is_, state):
+    tokens = [compile(is_.expr1, state), "IS"]
+    if is_.expr2 is None:
+        tokens.append("NULL")
+    elif is_.expr2 is True:
+        tokens.append("TRUE")
+    elif is_.expr2 is False:
+        tokens.append("FALSE")
+    else:
+        raise CompileError("expr2 must be None, True, or False")
+    return " ".join(tokens)
+
+@compile_python.when(Is)
+def compile_is(compile, is_, state):
+    return "%s is %s" % (compile(is_.expr1, state), compile(is_.expr2, state))
+
+
+class IsNot(BinaryOper):
+    """The SQL C{IS NOT ...} operators, e.g. C{IS NOT NULL}.
+
+    C{IsNot(expr, None)} is synonymous with C{expr != None}, but is less
+    likely to trip up linters.
+
+    Unlike C{Not(expr)} or C{expr != True}, C{IsNot(expr, True)} returns
+    C{TRUE} when C{expr} is C{NULL}.
+
+    Unlike C{expr} or C{expr != False}, C{IsNot(expr, False)} returns
+    C{TRUE} when C{expr} is C{NULL}.
+    """
+    __slots__ = ()
+    oper = " IS NOT "
+
[e-mail](IsNot)
+def compile_is_not(compile, is_not, state):
+    tokens = [compile(is_not.expr1, state), "IS NOT"]
+    if is_not.expr2 is None:
+        tokens.append("NULL")
+    elif is_not.expr2 is True:
+        tokens.append("TRUE")
+    elif is_not.expr2 is False:
+        tokens.append("FALSE")
+    else:
+        raise CompileError("expr2 must be None, True, or False")
+    return " ".join(tokens)
+
+@compile_python.when(IsNot)
+def compile_is_not(compile, is_not, state):
+    return "%s is not %s" % (
+        compile(is_not.expr1, state), compile(is_not.expr2, state)
+    )
+
+
 class Eq(BinaryOper):
     __slots__ = ()
     oper = " = "
@@ -1553,6 +1621,7 @@
 compile.set_precedence(20, SQL)
 compile.set_precedence(30, Or)
 compile.set_precedence(40, And)
+compile.set_precedence(45, Is, IsNot)
 compile.set_precedence(50, Eq, Ne, Gt, Ge, Lt, Le, Like, In)
 compile.set_precedence(60, LShift, RShift)
 compile.set_precedence(70, Add, Sub)
@@ -1560,6 +1629,7 @@
 
 compile_python.set_precedence(10, Or)
 compile_python.set_precedence(20, And)
+compile_python.set_precedence(25, Is, IsNot)
 compile_python.set_precedence(30, Eq, Ne, Gt, Ge, Lt, Le, Like, In)
 compile_python.set_precedence(40, LShift, RShift)
 compile_python.set_precedence(50, Add, Sub)

=== modified file 'storm/tests/expr.py'
--- storm/tests/expr.py	2020-12-07 22:38:36 +0000
+++ storm/tests/expr.py	2022-10-18 17:38:52 +0000
@@ -247,6 +247,16 @@
         like_expr = expr.contains_string(u"abc!!_%", case_sensitive=False)
         self.assertIs(False, like_expr.case_sensitive)
 
+    def test_is(self):
+        expr = Is(elem1, elem2)
+        self.assertEqual(expr.expr1, elem1)
+        self.assertEqual(expr.expr2, elem2)
+
+    def test_is_not(self):
+        expr = IsNot(elem1, elem2)
+        self.assertEqual(expr.expr1, elem1)
+        self.assertEqual(expr.expr2, elem2)
+
     def test_eq(self):
         expr = Eq(elem1, elem2)
         self.assertEqual(expr.expr1, elem1)
@@ -1101,6 +1111,56 @@
         self.assertEqual(statement, "?")
         self.assertVariablesEqual(state.parameters, [Variable("value")])
 
+    def test_is_null(self):
+        expr = Is(Func1(), None)
+        state = State()
+        statement = compile(expr, state)
+        self.assertEqual(statement, "func1() IS NULL")
+        self.assertEqual(state.parameters, [])
+
+    def test_is_true(self):
+        expr = Is(Func1(), True)
+        state = State()
+        statement = compile(expr, state)
+        self.assertEqual(statement, "func1() IS TRUE")
+        self.assertEqual(state.parameters, [])
+
+    def test_is_false(self):
+        expr = Is(Func1(), False)
+        state = State()
+        statement = compile(expr, state)
+        self.assertEqual(statement, "func1() IS FALSE")
+        self.assertEqual(state.parameters, [])
+
+    def test_is_invalid(self):
+        expr = Is(Func1(), "x")
+        self.assertRaises(CompileError, compile, expr)
+
+    def test_is_not_null(self):
+        expr = IsNot(Func1(), None)
+        state = State()
+        statement = compile(expr, state)
+        self.assertEqual(statement, "func1() IS NOT NULL")
+        self.assertEqual(state.parameters, [])
+
+    def test_is_not_true(self):
+        expr = IsNot(Func1(), True)
+        state = State()
+        statement = compile(expr, state)
+        self.assertEqual(statement, "func1() IS NOT TRUE")
+        self.assertEqual(state.parameters, [])
+
+    def test_is_not_false(self):
+        expr = IsNot(Func1(), False)
+        state = State()
+        statement = compile(expr, state)
+        self.assertEqual(statement, "func1() IS NOT FALSE")
+        self.assertEqual(state.parameters, [])
+
+    def test_is_not_invalid(self):
+        expr = IsNot(Func1(), "x")
+        self.assertRaises(CompileError, compile, expr)
+
     def test_eq(self):
         expr = Eq(Func1(), Func2())
         state = State()
@@ -2273,6 +2333,48 @@
         self.assertEqual(py_expr, "_0")
         self.assertEqual(state.parameters, ["value"])
 
+    def test_is_null(self):
+        expr = Is(Variable(True), None)
+        state = State()
+        statement = compile_python(expr, state)
+        self.assertEqual(statement, "_0 is None")
+        self.assertEqual(state.parameters, [True])
+
+    def test_is_true(self):
+        expr = Is(Variable(True), True)
+        state = State()
+        statement = compile_python(expr, state)
+        self.assertEqual(statement, "_0 is _1")
+        self.assertEqual(state.parameters, [True, True])
+
+    def test_is_false(self):
+        expr = Is(Variable(True), False)
+        state = State()
+        statement = compile_python(expr, state)
+        self.assertEqual(statement, "_0 is _1")
+        self.assertEqual(state.parameters, [True, False])
+
+    def test_is_not_null(self):
+        expr = IsNot(Variable(True), None)
+        state = State()
+        statement = compile_python(expr, state)
+        self.assertEqual(statement, "_0 is not None")
+        self.assertEqual(state.parameters, [True])
+
+    def test_is_not_true(self):
+        expr = IsNot(Variable(True), True)
+        state = State()
+        statement = compile_python(expr, state)
+        self.assertEqual(statement, "_0 is not _1")
+        self.assertEqual(state.parameters, [True, True])
+
+    def test_is_not_false(self):
+        expr = IsNot(Variable(True), False)
+        state = State()
+        statement = compile_python(expr, state)
+        self.assertEqual(statement, "_0 is not _1")
+        self.assertEqual(state.parameters, [True, False])
+
     def test_eq(self):
         expr = Eq(Variable(1), Variable(2))
         state = State()