Use ast module instead of compiler module for parsing files.

This CL is part of the work getting GYP to run under Python 3.
The compiler module we used to parse GYP files was removed in
Python 3, so this CL switches to use the ast module instead,
which exists in both Python 2 and Python 3.

This change is not sufficient to run under Python 3; there's
more work remaining. Things should still work under Python 2
just fine.

This work is derived from [email protected]'s
original work in https://codereview.chromium.org/1454433002/.

Bug: gyp:36

Change-Id: I9a9835560491c3d8cd5426623484dc4a46af1d86
Reviewed-on: https://chromium-review.googlesource.com/c/1360352
Reviewed-by: Mark Mentovai <[email protected]>

pylib/gyp/input.py

@@ -2,14 +2,8 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-from compiler.ast import Const
-from compiler.ast import Dict
-from compiler.ast import Discard
-from compiler.ast import List
-from compiler.ast import Module
-from compiler.ast import Node
-from compiler.ast import Stmt
-import compiler
+import ast
+
 import gyp.common
 import gyp.simple_copy
 import multiprocessing
@@ -183,43 +177,38 @@ def CheckedEval(file_contents):
   Note that this is slower than eval() is.
   """
 
-  ast = compiler.parse(file_contents)
-  assert isinstance(ast, Module)
-  c1 = ast.getChildren()
-  assert c1[0] is None
-  assert isinstance(c1[1], Stmt)
-  c2 = c1[1].getChildren()
-  assert isinstance(c2[0], Discard)
-  c3 = c2[0].getChildren()
-  assert len(c3) == 1
-  return CheckNode(c3[0], [])
+  syntax_tree = ast.parse(file_contents)
+  assert isinstance(syntax_tree, ast.Module)
+  c1 = syntax_tree.body
+  assert len(c1) == 1
+  c2 = c1[0]
+  assert isinstance(c2, ast.Expr)
+  return CheckNode(c2.value, [])
 
 
 def CheckNode(node, keypath):
-  if isinstance(node, Dict):
-    c = node.getChildren()
+  if isinstance(node, ast.Dict):
     dict = {}
-    for n in range(0, len(c), 2):
-      assert isinstance(c[n], Const)
-      key = c[n].getChildren()[0]
+    for key, value in zip(node.keys, node.values):
+      assert isinstance(key, ast.Str)
+      key = key.s
       if key in dict:
         raise GypError("Key '" + key + "' repeated at level " +
               repr(len(keypath) + 1) + " with key path '" +
               '.'.join(keypath) + "'")
       kp = list(keypath)  # Make a copy of the list for descending this node.
       kp.append(key)
-      dict[key] = CheckNode(c[n + 1], kp)
+      dict[key] = CheckNode(value, kp)
     return dict
-  elif isinstance(node, List):
-    c = node.getChildren()
+  elif isinstance(node, ast.List):
     children = []
-    for index, child in enumerate(c):
+    for index, child in enumerate(node.elts):
       kp = list(keypath)  # Copy list.
       kp.append(repr(index))
       children.append(CheckNode(child, kp))
     return children
-  elif isinstance(node, Const):
-    return node.getChildren()[0]
+  elif isinstance(node, ast.Str):
+    return node.s
   else:
     raise TypeError("Unknown AST node at key path '" + '.'.join(keypath) +
          "': " + repr(node))