Add __builtins__ to globals argument of `py::exec` and `py::eval` if not present (#2616)

* Add __builtins__ to globals argument of `py::exec` and `py::eval` if not present

* Refactor into inline ensure_builtins_in_globals function

include/pybind11/eval.h

@@ -14,6 +14,22 @@
 #include "pybind11.h"
 
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
+PYBIND11_NAMESPACE_BEGIN(detail)
+
+inline void ensure_builtins_in_globals(object &global) {
+    #if PY_VERSION_HEX < 0x03080000
+        // Running exec and eval on Python 2 and 3 adds `builtins` module under
+        // `__builtins__` key to globals if not yet present.
+        // Python 3.8 made PyRun_String behave similarly. Let's also do that for
+        // older versions, for consistency.
+        if (!global.contains("__builtins__"))
+            global["__builtins__"] = module_::import(PYBIND11_BUILTINS_MODULE);
+    #else
+        (void) global;
+    #endif
+}
+
+PYBIND11_NAMESPACE_END(detail)
 
 enum eval_mode {
     /// Evaluate a string containing an isolated expression
@@ -31,6 +47,8 @@ object eval(str expr, object global = globals(), object local = object()) {
     if (!local)
         local = global;
 
+    detail::ensure_builtins_in_globals(global);
+
     /* PyRun_String does not accept a PyObject / encoding specifier,
        this seems to be the only alternative */
     std::string buffer = "# -*- coding: utf-8 -*-\n" + (std::string) expr;
@@ -85,6 +103,8 @@ object eval_file(str fname, object global = globals(), object local = object())
     if (!local)
         local = global;
 
+    detail::ensure_builtins_in_globals(global);
+
     int start;
     switch (mode) {
         case eval_expr:             start = Py_eval_input;   break;

tests/test_eval.cpp

@@ -88,4 +88,12 @@ TEST_SUBMODULE(eval_, m) {
         }
         return false;
     });
+
+    // test_eval_empty_globals
+    m.def("eval_empty_globals", [](py::object global) {
+        if (global.is_none())
+            global = py::dict();
+        auto int_class = py::eval("isinstance(42, int)", global);
+        return global;
+    });
 }

tests/test_eval.py

@@ -25,3 +25,11 @@ def test_eval_file():
     assert m.test_eval_file(filename)
 
     assert m.test_eval_file_failure()
+
+
+def test_eval_empty_globals():
+    assert "__builtins__" in m.eval_empty_globals(None)
+
+    g = {}
+    assert "__builtins__" in m.eval_empty_globals(g)
+    assert "__builtins__" in g