improve internal attribute handling in env; make let constructs finalize their environments so no new bindings can be added; fix bug in lispylet, should create an env instance even if no bindings given

Technologicat · Technologicat · commit c5ba974809d4 · 2018-07-26T17:54:55.000+03:00
diff --git a/unpythonic/assignonce.py b/unpythonic/assignonce.py
@@ -25,7 +25,7 @@ class assignonce(_envcls):
 
     """
     def __setattr__(self, name, value):
-        if name == "_env" or name not in self:
+        if name in self._reserved_names or name not in self:
             return super().__setattr__(name, value)
         else:
             raise AttributeError("name '{:s}' is already defined".format(name))
diff --git a/unpythonic/env.py b/unpythonic/env.py
@@ -43,21 +43,26 @@ class env:
     instance itself will remain alive due to Python's scoping rules.
     """
     # do not allow bindings that would break functionality.
-    reserved_names = ("_env", "set", "clear")
+    _reserved_names = ("set", "clear", "finalize", "_env", "_allow_more_bindings",
+                       "_direct_write", "_reserved_names")
+    _direct_write = ("_env", "_allow_more_bindings")
 
     def __init__(self, **bindings):
         self._env = {}
+        self._allow_more_bindings = True  # "let" disables this once env setup done
         for name, value in bindings.items():
             setattr(self, name, value)
 
     # item access by name
     # https://docs.python.org/3/reference/datamodel.html#object.__setattr__
     # https://docs.python.org/3/reference/datamodel.html#object.__getattr__
     def __setattr__(self, name, value):
-        if name == "_env":  # hook to allow creating _env directly in self
+        if name in self._direct_write:  # hook to allow creating internal variables directly in self
             return super().__setattr__(name, value)
-        if name in self.reserved_names:
-            raise AttributeError("cannot overwrite reserved name '{:s}'; complete list: {}".format(name, self.reserved_names))
+        if name in self._reserved_names:
+            raise AttributeError("cannot overwrite reserved name '{:s}'; complete list: {}".format(name, self._reserved_names))
+        if not self._allow_more_bindings and name not in self:
+            raise AttributeError("name '{:s}' is not defined; adding new bindings to a finalized environment is not allowed".format(name))
 #        value = self._wrap(name, value)  # for "e.x << value" rebind syntax.
         self._env[name] = value  # make all other attrs else live inside _env
 
@@ -130,6 +135,16 @@ def clear(self):
         """Clear the environment, i.e. forget all bindings."""
         self._env = {}
 
+    def finalize(self):
+        """Finalize environment.
+
+        This stops the instance from accepting any more new bindings.
+
+        Existing bindings can still be overwritten even in a finalized
+        environment.
+        """
+        self._allow_more_bindings = False
+
     # For rebind syntax: "e.foo << newval" --> "e.foo.__lshift__(newval)",
     # so foo.__lshift__() must be set up to rebind e.foo.
     #
diff --git a/unpythonic/let.py b/unpythonic/let.py
@@ -189,6 +189,7 @@ def _let(mode, body, **bindings):
         for k in env:
             env[k] = env[k](env)
     # decorators need just the final env; else run body now
+    env.finalize()
     return env if body is None else body(env)
 
 # decorator factory: almost as fun as macros?
@@ -337,6 +338,15 @@ def result(*, env):
     else:
         assert False
 
+    try:
+        @blet(x=1)
+        def error1(*, env):
+            env.y = 2  # cannot add new bindings to a let environment
+    except AttributeError as err:
+        pass
+    else:
+        assert False
+
     print("All tests PASSED")
 
 if __name__ == '__main__':
diff --git a/unpythonic/lispylet.py b/unpythonic/lispylet.py
@@ -152,18 +152,17 @@ def bletrec(bindings):
 def _let(bindings, body, *, env=None, mode="let"):
     assert mode in ("let", "letrec")
 
+    env = env or _envcls()
+
     if not bindings:
+        env.finalize()
         # decorators need just the final env; else run body now
         return env if body is None else body(env)
 
-    env = env or _envcls()
     (k, v), *more = bindings
-
     if mode == "letrec" and callable(v):
         v = v(env)
-
     setattr(env, k, v)
-
     return _let(more, body, env=env, mode=mode)
 
 def _dlet(bindings, mode="let"):  # let and letrec decorator factory
@@ -253,6 +252,15 @@ def result(*, env):
     else:
         assert False
 
+    try:
+        @blet((('x', 1),))
+        def error1(*, env):
+            env.y = 2  # cannot add new bindings to a let environment
+    except AttributeError as err:
+        pass
+    else:
+        assert False
+
     print("All tests PASSED")
 
 if __name__ == '__main__':
