Fix missing default globals in eval/exec when embedding

Fixes #887.

include/pybind11/eval.h

@@ -27,12 +27,7 @@ enum eval_mode {
 };
 
 template <eval_mode mode = eval_expr>
-object eval(str expr, object global = object(), object local = object()) {
+object eval(str expr, object global = globals(), object local = object()) {
-    if (!global) {
-        global = reinterpret_borrow<object>(PyEval_GetGlobals());
-        if (!global)
-            global = dict();
-    }
     if (!local)
         local = global;
 
@@ -55,29 +50,24 @@ object eval(str expr, object global = object(), object local = object()) {
 }
 
 template <eval_mode mode = eval_expr, size_t N>
-object eval(const char (&s)[N], object global = object(), object local = object()) {
+object eval(const char (&s)[N], object global = globals(), object local = object()) {
     /* Support raw string literals by removing common leading whitespace */
     auto expr = (s[0] == '\n') ? str(module::import("textwrap").attr("dedent")(s))
                                : str(s);
     return eval<mode>(expr, global, local);
 }
 
-inline void exec(str expr, object global = object(), object local = object()) {
+inline void exec(str expr, object global = globals(), object local = object()) {
     eval<eval_statements>(expr, global, local);
 }
 
 template <size_t N>
-void exec(const char (&s)[N], object global = object(), object local = object()) {
+void exec(const char (&s)[N], object global = globals(), object local = object()) {
     eval<eval_statements>(s, global, local);
 }
 
 template <eval_mode mode = eval_statements>
-object eval_file(str fname, object global = object(), object local = object()) {
+object eval_file(str fname, object global = globals(), object local = object()) {
-    if (!global) {
-        global = reinterpret_borrow<object>(PyEval_GetGlobals());
-        if (!global)
-            global = dict();
-    }
     if (!local)
         local = global;
 

include/pybind11/pybind11.h

@@ -799,8 +799,12 @@ public:
 };
 
 /// \ingroup python_builtins
-/// Return a dictionary representing the global symbol table, i.e. ``__main__.__dict__``.
+/// Return a dictionary representing the global variables in the current execution frame,
-inline dict globals() { return module::import("__main__").attr("__dict__").cast<dict>(); }
+/// or ``__main__.__dict__`` if there is no frame (usually when the interpreter is embedded).
+inline dict globals() {
+    PyObject *p = PyEval_GetGlobals();
+    return reinterpret_borrow<dict>(p ? p : module::import("__main__").attr("__dict__").ptr());
+}
 
 NAMESPACE_BEGIN(detail)
 /// Generic support for creating new Python heap types

tests/test_embed/test_interpreter.cpp

@@ -153,3 +153,10 @@ TEST_CASE("Subinterpreter") {
     REQUIRE(py::hasattr(py::module::import("__main__"), "main_tag"));
     REQUIRE(py::hasattr(py::module::import("widget_module"), "extension_module_tag"));
 }
+
+TEST_CASE("Execution frame") {
+    // When the interpreter is embedded, there is no execution frame, but `py::exec`
+    // should still function by using reasonable globals: `__main__.__dict__`.
+    py::exec("var = dict(number=42)");
+    REQUIRE(py::globals()["var"]["number"].cast<int>() == 42);
+}