Improve if and let* form Python generation (#792)

Fixes #793

Author: chrisrink10

CHANGELOG.md

@@ -18,6 +18,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
  * Allow vars to be callable to adhere to Clojure conventions (#767)
  * Adjust input path compatibility in `basilisp.core/load` input path to be relative to the namespace or the root path (#782)
  * No longer warn on unused bindings when their name begins with `_` (#756)
+ * Improve the Python generation for `if` and `let*` forms to avoid unnecessary extra assignments (#793)
 
 ### Fixed
  * Fix issue with `(count nil)` throwing an exception (#759)

src/basilisp/lang/compiler/generator.py

@@ -2138,10 +2138,18 @@ def _if_to_py_ast(ctx: GeneratorContext, node: If) -> GeneratedPyAST:
     then_ast = __if_body_to_py_ast(ctx, node.then, result_name)
     else_ast = __if_body_to_py_ast(ctx, node.else_, result_name)
 
-    test_name = genname(_IF_TEST_PREFIX)
-    test_assign = ast.Assign(
-        targets=[ast.Name(id=test_name, ctx=ast.Store())], value=test_ast.node
-    )
+    # Suppress the duplicate assignment statement if the `if` statement test is already
+    # an ast.Name instance.
+    if_test_deps: List[ast.AST] = []
+    if isinstance(test_ast.node, ast.Name):
+        test_name = test_ast.node.id
+    else:
+        test_name = genname(_IF_TEST_PREFIX)
+        if_test_deps.append(
+            ast.Assign(
+                targets=[ast.Name(id=test_name, ctx=ast.Store())], value=test_ast.node
+            )
+        )
 
     ifstmt = ast.If(
         test=ast.BoolOp(
@@ -2168,7 +2176,7 @@ def _if_to_py_ast(ctx: GeneratorContext, node: If) -> GeneratedPyAST:
         node=ast.Name(id=result_name, ctx=ast.Load())
         if result_name is not None
         else _noop_node(),
-        dependencies=list(chain(test_ast.dependencies, [test_assign, ifstmt])),
+        dependencies=list(chain(test_ast.dependencies, if_test_deps, [ifstmt])),
     )
 
 
@@ -2297,19 +2305,23 @@ def _let_to_py_ast(ctx: GeneratorContext, node: Let) -> GeneratedPyAST:
                 sym.symbol(binding.name), binding_name, LocalType.LET
             )
 
-        let_result_name = genname(_LET_RESULT_PREFIX)
         body_ast = _synthetic_do_to_py_ast(ctx, node.body)
         let_body_ast.extend(map(statementize, body_ast.dependencies))
 
         if node.env.pos == NodeSyntacticPosition.EXPR:
-            let_body_ast.append(
-                ast.Assign(
-                    targets=[ast.Name(id=let_result_name, ctx=ast.Store())],
-                    value=body_ast.node,
+            if isinstance(body_ast.node, ast.Name):
+                let_result_node = body_ast.node
+            else:
+                let_result_name = genname(_LET_RESULT_PREFIX)
+                let_result_node = ast.Name(id=let_result_name, ctx=ast.Load())
+                let_body_ast.append(
+                    ast.Assign(
+                        targets=[ast.Name(id=let_result_name, ctx=ast.Store())],
+                        value=body_ast.node,
+                    )
                 )
-            )
             return GeneratedPyAST(
-                node=ast.Name(id=let_result_name, ctx=ast.Load()),
+                node=let_result_node,
                 dependencies=let_body_ast,
             )
         else:

tests/basilisp/compiler_test.py

@@ -2900,6 +2900,9 @@ def test_if(self, lcompile: CompileFn):
         """
         assert "YELLING" == lcompile(code)
 
+    def test_if_with_name(self, lcompile: CompileFn):
+        assert lcompile("(let* [a true] (if a :a :b))") == kw.keyword("a")
+
     def test_truthiness(self, lcompile: CompileFn):
         # Valid false values
         assert kw.keyword("b") == lcompile("(if false :a :b)")
@@ -3244,6 +3247,9 @@ def test_let_may_have_empty_body(self, lcompile: CompileFn):
         assert None is lcompile("(let* [])")
         assert None is lcompile("(let* [a :kw])")
 
+    def test_let_return_bound_name(self, lcompile: CompileFn):
+        assert lcompile("(let* [a :a b a] b)") == kw.keyword("a")
+
     def test_let_bindings_get_tag_ast(self, lcompile: CompileFn, ns: runtime.Namespace):
         lcompile('(let [^python/str s "a string"] s)')
         hints = typing.get_type_hints(ns.module)