THRIFT-4623 Fix python complex nested struct includes

Fix the bug where the following:
```
struct C {
    1: i32 value
}

struct A {
    1: C nested = {}
}

struct B {
    1: A itm = {}
}
```
would generate the following invalid python code:
```
class C:
    thrift_spec = None

    def __init__(self, value=None):
        self.value = value

class A:
    thrift_spec = None

    def __init__(self, nested=C()):
        if nested is self.thrift_spec[1][4]:
            nested = C()
        self.nested = nested

class B:
    thrift_spec = None

    def __init__(self, itm=A()):
        if itm is self.thrift_spec[1][4]:
            itm = A()
        self.itm = itm
```
The problem with the above code is that `self.thrift_spec` was initialized and set after the module was loaded.
In python this would work when B is instantiated but since B immediately
invokes A() it happens before the thrift_spec is set and this would
result in `TypeError: 'NoneType' object is not subscriptable`.
By first initializing a temporary placeholder with a unique address to
compare to called `_THRIFT_DEFAULT` the generated code would look like:
```
_THRIFT_DEFAULT = object()

class C:
    def __init__(self, value=None):
        self.value = value

class A:
    def __init__(self, nested=_THRIFT_DEFAULT):
        if nested is _THRIFT_DEFAULT:
            nested = C()

class B:
    def __init__(self, itm=_THRIFT_DEFAULT):
        if itm is _THRIFT_DEFAULT:
            itm = A()
```
Here the instantiation of the default object is defered until after the
module has been fully loaded resulting in the correct value being
assigned.

Author: thomasbruggink

compiler/cpp/src/thrift/generate/t_py_generator.cc

@@ -452,6 +452,7 @@ void t_py_generator::init_generator() {
   } else {
     f_types_ << "all_structs = []" << '\n';
   }
+  f_types_ << "_THRIFT_DEFAULT = object()" << '\n';
 
   f_consts_ <<
     py_autogen_comment() << '\n' <<
@@ -907,10 +908,9 @@ void t_py_generator::generate_py_struct_definition(ostream& out,
 
     for (m_iter = members.begin(); m_iter != members.end(); ++m_iter) {
       // Initialize fields
-      t_type* type = (*m_iter)->get_type();
+      t_type* type = get_true_type((*m_iter)->get_type());
       if (!type->is_base_type() && !type->is_enum() && (*m_iter)->get_value() != nullptr) {
-        indent(out) << "if " << maybe_escape_identifier((*m_iter)->get_name()) << " is "
-                    << "self.thrift_spec[" << (*m_iter)->get_key() << "][4]:" << '\n';
+        indent(out) << "if " << maybe_escape_identifier((*m_iter)->get_name()) << " is _THRIFT_DEFAULT:" << '\n';
         indent_up();
         indent(out) << maybe_escape_identifier((*m_iter)->get_name()) << " = " << render_field_default_value(*m_iter)
                     << '\n';
@@ -1319,6 +1319,7 @@ void t_py_generator::generate_service(t_service* tservice) {
   }
 
   f_service_ << "all_structs = []" << '\n';
+  f_service_ << "_THRIFT_DEFAULT = object()" << '\n';
 
   // Generate the three main parts of the service
   generate_service_interface(tservice);
@@ -2784,7 +2785,12 @@ string t_py_generator::declare_argument(t_field* tfield) {
 
   result << " = ";
   if (tfield->get_value() != nullptr) {
-    result << render_field_default_value(tfield);
+    t_type* type = get_true_type(tfield->get_type());
+    if (!type->is_base_type() && !type->is_enum()) {
+      result << "_THRIFT_DEFAULT";
+    } else {
+      result << render_field_default_value(tfield);
+    }
   } else {
     result << "None";
   }

lib/py/Makefile.am

@@ -63,6 +63,7 @@ check-local: all py3-test
 	$(PYTHON) test/thrift_TNonblockingServer.py
 	$(PYTHON) test/thrift_TSerializer.py
 	THRIFT=${THRIFT} $(PYTHON) test/test_compiler/test_keyword_escape.py
+	THRIFT=${THRIFT} $(PYTHON) test/test_compiler/test_default_struct.py
 	$(PYTHON) test/test_recursion_depth.py
 
 

lib/py/test/test_compiler/Thrift4623.thrift

@@ -0,0 +1,31 @@
+//
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements. See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership. The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License. You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+namespace py thrift4623
+
+struct C {
+  1: i32 value
+}
+
+struct A {
+  1: C nested = {}
+}
+
+struct B {
+  1: A itm = {}
+}

lib/py/test/test_compiler/test_default_struct.py

@@ -0,0 +1,58 @@
+#!/usr/bin/env python
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements. See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership. The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License. You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied. See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+import importlib
+import os
+import subprocess
+import sys
+import tempfile
+
+from test_keyword_escape import find_thrift
+
+
+def test_default_struct_value():
+    thrift_file = os.path.join(os.path.dirname(__file__), 'Thrift4623.thrift')
+    thrift_bin = find_thrift()
+    if not thrift_bin:
+        print("WARNING: thrift compiler not found, skipping test")
+        return 0
+
+    with tempfile.TemporaryDirectory() as tmpdir:
+        result = subprocess.run(
+            [thrift_bin, '-gen', 'py', '-out', tmpdir, thrift_file],
+            capture_output=True, text=True)
+        if result.returncode != 0:
+            raise AssertionError("thrift compiler failed: " + result.stderr)
+
+        sys.path.insert(0, tmpdir)
+        try:
+            types = importlib.import_module('thrift4623.ttypes')
+            value = types.B()
+            assert isinstance(value.itm, types.A)
+            assert isinstance(value.itm.nested, types.C)
+            assert value.itm.nested.value is None
+        finally:
+            sys.path.pop(0)
+            for name in list(sys.modules):
+                if name == 'thrift4623' or name.startswith('thrift4623.'):
+                    del sys.modules[name]
+
+
+if __name__ == '__main__':
+    test_default_struct_value()
+    print('OK: default-valued struct fields are usable')