Don't put redundant names in _stdlib (#1221)

This prevents pointless imports of hy.core, like importing `map` in Python 3.

hy/compiler.py

@@ -55,6 +55,11 @@ import keyword
 
 from collections import defaultdict
 
+if PY3:
+    import builtins
+else:
+    import __builtin__ as builtins
+
 
 _compile_time_ns = {}
 
@@ -75,6 +80,10 @@ def load_stdlib():
     for module in hy.core.STDLIB:
         mod = importlib.import_module(module)
         for e in mod.EXPORTS:
+            if getattr(mod, e) is not getattr(builtins, e, ''):
+                # Don't bother putting a name in _stdlib if it
+                # points to a builtin with the same name. This
+                # prevents pointless imports.
                 _stdlib[e] = module
 
 

tests/compilers/test_ast.py

@@ -263,6 +263,21 @@ def test_ast_require():
     cant_compile("(require [tests.resources.tlib [* *]])")
 
 
+def test_ast_no_pointless_imports():
+    def contains_import_from(code):
+        return any([isinstance(node, ast.ImportFrom)
+                   for node in hy_compile(tokenize(code), "__main__").body])
+    # `reduce` is a builtin in Python 2, but not Python 3.
+    # The version of `map` that returns an iterator is a builtin in
+    # Python 3, but not Python 2.
+    if PY3:
+        assert contains_import_from("reduce")
+        assert not contains_import_from("map")
+    else:
+        assert not contains_import_from("reduce")
+        assert contains_import_from("map")
+
+
 def test_ast_good_get():
     "Make sure AST can compile valid get"
     can_compile("(get x y)")