diff --git a/nac3core/src/type_check/signature.rs b/nac3core/src/type_check/signature.rs
index 81895252..7b26e827 100644
--- a/nac3core/src/type_check/signature.rs
+++ b/nac3core/src/type_check/signature.rs
@@ -2,7 +2,7 @@
 use super::context::TopLevelContext;
 use super::primitives::*;
 use super::typedef::*;
-use rustpython_parser::ast::{ExpressionType, Statement, StatementType, StringGroup};
+use rustpython_parser::ast::{ExpressionType, Statement, StatementType, StringGroup, ComprehensionKind};
 use std::collections::HashMap;
 
 // TODO: fix condition checking, return error message instead of panic...
@@ -161,6 +161,85 @@ fn resolve_function<'a>(
     }
 }
 
+fn get_expression_unknowns<'a>(defined: &mut Vec<&'a str>, unknowns: &mut Vec<&'a str>, expr: &'a ExpressionType) {
+    match expr {
+        ExpressionType::BoolOp { values, .. } => {
+            for v in values.iter() {
+                get_expression_unknowns(defined, unknowns, &v.node)
+            }
+        }
+        ExpressionType::Binop { a, b, .. } => {
+            get_expression_unknowns(defined, unknowns, &a.node);
+            get_expression_unknowns(defined, unknowns, &b.node);
+        }
+        ExpressionType::Subscript { a, b } => {
+            get_expression_unknowns(defined, unknowns, &a.node);
+            get_expression_unknowns(defined, unknowns, &b.node);
+        }
+        ExpressionType::Unop { a, .. } => {
+            get_expression_unknowns(defined, unknowns, &a.node);
+        }
+        ExpressionType::Compare { vals, .. } => {
+            for v in vals.iter() {
+                get_expression_unknowns(defined, unknowns, &v.node)
+            }
+        }
+        ExpressionType::Attribute { value, .. } => {
+            get_expression_unknowns(defined, unknowns, &value.node);
+        }
+        ExpressionType::Call { function, args, .. } => {
+            get_expression_unknowns(defined, unknowns, &function.node);
+            for v in args.iter() {
+                get_expression_unknowns(defined, unknowns, &v.node)
+            }
+        }
+        ExpressionType::List { elements } => {
+            for v in elements.iter() {
+                get_expression_unknowns(defined, unknowns, &v.node)
+            }
+        }
+        ExpressionType::Tuple { elements } => {
+            for v in elements.iter() {
+                get_expression_unknowns(defined, unknowns, &v.node)
+            }
+        }
+        ExpressionType::Comprehension { kind, generators } => {
+            if generators.len() != 1 {
+                unimplemented!()
+            }
+            let g = &generators[0];
+            get_expression_unknowns(defined, unknowns, &g.iter.node);
+            let mut scoped = defined.clone();
+            get_expression_unknowns(defined, &mut scoped, &g.target.node);
+            for if_expr in g.ifs.iter() {
+                get_expression_unknowns(&mut scoped, unknowns, &if_expr.node);
+            }
+            match kind.as_ref() {
+                ComprehensionKind::List { element } => {
+                    get_expression_unknowns(&mut scoped, unknowns, &element.node);
+                }
+                _ => unimplemented!()
+            }
+        }
+        ExpressionType::Slice { elements } => {
+            for v in elements.iter() {
+                get_expression_unknowns(defined, unknowns, &v.node);
+            }
+        }
+        ExpressionType::Identifier { name } => {
+            if !defined.contains(&name.as_str()) && !unknowns.contains(&name.as_str()) {
+                unknowns.push(name);
+            }
+        }
+        ExpressionType::IfExpression { test, body, orelse } => {
+            get_expression_unknowns(defined, unknowns, &test.node);
+            get_expression_unknowns(defined, unknowns, &body.node);
+            get_expression_unknowns(defined, unknowns, &orelse.node);
+        }
+        _ => ()
+    };
+}
+
 pub fn resolve_signatures<'a>(ctx: &mut TopLevelContext<'a>, stmts: &'a [Statement]) {
     for stmt in stmts.iter() {
         match &stmt.node {