fix: Set __file__ constant when using eval_file (#1300)

include/pybind11/eval.h

@@ -136,6 +136,14 @@ object eval_file(str fname, object global = globals(), object local = object())
         pybind11_fail("File \"" + fname_str + "\" could not be opened!");
     }
 
+// Python2 API requires this to be encoded to filesystemencoding.
+// Therefore, we don't bother supporting it.
+#if PY_VERSION_HEX >= 0x03000000
+    if (!global.contains("__file__")) {
+        global["__file__"] = std::move(fname);
+    }
+#endif
+
 #if PY_VERSION_HEX < 0x03000000 && defined(PYPY_VERSION)
     PyObject *result = PyRun_File(f, fname_str.c_str(), start, global.ptr(),
                                   local.ptr());

tests/test_cmake_build/test.py

@@ -3,5 +3,10 @@
 
 import test_cmake_build
 
+from .. import env
+
+if env.PY3:
+    assert isinstance(__file__, str)  # Test this is properly set
+
 assert test_cmake_build.add(1, 2) == 3
 print("{} imports, runs, and adds: 1 + 2 = 3".format(sys.argv[1]))