Avoid PD false positives on some non-DataFrame expressions (#1538)

src/ast/helpers.rs

@@ -212,6 +212,23 @@ pub fn is_constant_non_singleton(expr: &Expr) -> bool {
     is_constant(expr) && !is_singleton(expr)
 }
 
+/// Return `true` if an `Expr` is not a reference to a variable (or something
+/// that could resolve to a variable, like a function call).
+pub fn is_non_variable(expr: &Expr) -> bool {
+    matches!(
+        expr.node,
+        ExprKind::Constant { .. }
+            | ExprKind::Tuple { .. }
+            | ExprKind::List { .. }
+            | ExprKind::Set { .. }
+            | ExprKind::Dict { .. }
+            | ExprKind::SetComp { .. }
+            | ExprKind::ListComp { .. }
+            | ExprKind::DictComp { .. }
+            | ExprKind::GeneratorExp { .. }
+    )
+}
+
 /// Return the `Keyword` with the given name, if it's present in the list of
 /// `Keyword` arguments.
 pub fn find_keyword<'a>(keywords: &'a [Keyword], keyword_name: &str) -> Option<&'a Keyword> {