Beginnings of a patch to use Python AST for compiling

ast.diff

diff --git a/fluent.runtime/fluent/runtime/codegen.py b/fluent.runtime/fluent/runtime/codegen.py
index f00f750..b76b147 100644
--- a/fluent.runtime/fluent/runtime/codegen.py
+++ b/fluent.runtime/fluent/runtime/codegen.py
@@ -3,6 +3,7 @@ Utilities for doing Python code generation
 """
 from __future__ import absolute_import, unicode_literals
 
+import ast
 import keyword
 import re
 import sys
@@ -52,6 +53,10 @@ UNKNOWN_TYPE = object
 
 
 class PythonAst(object):
+    """
+    Base class representing a simplified Python AST (not the real one).
+    Generates real `ast.*` nodes via `as_ast()` method.
+    """
     def simplify(self, changes):
         """
         Simplify the statement/expression, returning either a modified
@@ -64,6 +69,9 @@ class PythonAst(object):
         """
         return self
 
+    def as_ast(self):
+        raise NotImplementedError("{!r}.as_ast()".format(self.__class__))
+
 
 class Scope(PythonAst):
     def __init__(self, parent_scope=None):
@@ -205,7 +213,8 @@ def cleanup_name(name):
 
 
 class Module(Scope):
-    pass
+    def as_ast(self):
+        return ast.Module([s.as_ast() for s in self.statements])
 
 
 class Statement(PythonAst):
@@ -253,6 +262,20 @@ class Function(Block, Statement):
         self.args = args
         self.kwargs = kwargs
 
+    def as_ast(self):
+        return ast.FunctionDef(name=self.func_name,
+                               args=ast.arguments(args=[ast.arg(arg=arg_name, annotation=None,
+                                                                lineno=1, col_offset=1)
+                                                        for arg_name in self.args],
+                                                  vararg=None,
+                                                  kwonlyargs=[],
+                                                  kw_defaults=[],
+                                                  kwarg=None,
+                                                  defaults=[]),
+                               body=[statement.as_ast() for statement in self.statements],
+                               decorator_list=[],
+                               lineno=1, col_offset=1)
+
     def as_source_code(self):
         if not allowable_name(self.func_name):
             raise AssertionError("Expected '{0}' to be a valid Python identifier".format(self.func_name))
@@ -281,6 +304,9 @@ class Return(Statement):
     def __init__(self, value):
         self.value = value
 
+    def as_ast(self):
+        return ast.Return(self.value.as_ast(), lineno=1, col_offset=1)
+
     def as_source_code(self):
         return 'return {0}'.format(self.value.as_source_code())
 
@@ -371,6 +397,9 @@ class String(Expression):
     def __init__(self, string_value):
         self.string_value = string_value
 
+    def as_ast(self):
+        return ast.Str(self.string_value, lineno=1, col_offset=1)
+
     def as_source_code(self):
         retval = repr(self.string_value)
         # We eventually call 'exec' in a module that has 'from __future__ import
diff --git a/fluent.runtime/fluent/runtime/compiler.py b/fluent.runtime/fluent/runtime/compiler.py
index 85255df..49b0c28 100644
--- a/fluent.runtime/fluent/runtime/compiler.py
+++ b/fluent.runtime/fluent/runtime/compiler.py
@@ -126,7 +126,9 @@ def compile_messages(messages, locale, use_isolating=True, functions=None, debug
     # TODO - it would be nice to be able to get back to FTL source file lines,
     # if were knew what they were, and pass absolute filename that to 'compile'
     # builtin as the filepath. Instead of that just use 'exec' for now.
-    exec(module.as_source_code(), module_globals)
+
+    code_obj = compile(module.as_ast(), '<string>', 'exec')
+    exec(code_obj, module_globals)
     retval = {}
     for key, val in message_mapping.items():
         if key.startswith('-'):
diff --git a/fluent.runtime/tests/test_compiler.py b/fluent.runtime/tests/test_compiler.py
index 6690eac..3d96cdc 100644
--- a/fluent.runtime/tests/test_compiler.py
+++ b/fluent.runtime/tests/test_compiler.py
@@ -5,6 +5,8 @@ from collections import OrderedDict
 
 import babel
 
+from ast_decompiler import decompile
+
 from fluent.runtime.builtins import BUILTINS
 from fluent.runtime.compiler import messages_to_module
 from fluent.runtime.errors import FluentCyclicReferenceError, FluentError, FluentReferenceError
@@ -40,7 +42,7 @@ def compile_messages_to_python(source, locale, use_isolating=False, functions=No
         messages, locale,
         use_isolating=use_isolating,
         functions=_functions)
-    return module.as_source_code(), errors
+    return decompile(module.as_ast()), errors
 
 
 class TestCompiler(unittest.TestCase):