Do not include undefined Elixir variables in the expanded ~PY sigil

lib/pythonx.ex

@@ -281,9 +281,17 @@ defmodule Pythonx do
   defmacro sigil_PY({:<<>>, _meta, [code]}, []) when is_binary(code) do
     %{referenced: referenced, defined: defined} = Pythonx.AST.scan_globals(code)
 
+    caller = __CALLER__
+
     globals_entries =
-      for name <- referenced do
-        {name, {String.to_atom(name), [], nil}}
+      for name <- referenced,
+          name_atom = String.to_atom(name),
+          # We only reference variables that are actually defined.
+          # This way, if an undefined variable is referenced in the
+          # Python code, it results in an informative Python error,
+          # rather than Elixir compile error.
+          Macro.Env.has_var?(caller, {name_atom, nil}) do
+        {name, {name_atom, [], nil}}
       end
 
     assignments =

test/pythonx_test.exs

@@ -374,6 +374,21 @@ defmodule PythonxTest do
       assert Keyword.keys(binding) == [:x]
     end
 
+    test "results in a Python error when a variable is undefined" do
+      assert_raise Pythonx.Error, ~r/NameError: name 'x' is not defined/, fn ->
+        Code.eval_string(
+          ~S'''
+          import Pythonx
+
+          ~PY"""
+          x + 1
+          """
+          ''',
+          []
+        )
+      end
+    end
+
     test "global redefinition" do
       {_result, binding} =
         Code.eval_string(
@@ -414,7 +429,7 @@ defmodule PythonxTest do
       assert repr(result) == "43"
     end
 
-    test "does not result in unused variables" do
+    test "does not result in unused variables diagnostics" do
       {_result, diagnostics} =
         Code.with_diagnostics(fn ->
           Code.eval_string(~s'''