fix: Context (tinygrad#1430)

* Fixed issue in Context

* Cleaned up fix

Now that DEBUG.value = 3 always works we can do so in __new__ as well.

Author: AlexTelon

test/test_helpers.py

@@ -99,5 +99,12 @@ def test():
     test()
     self.assertEqual(VARIABLE.value, 0)
 
+  def test_context_exit_reverts_updated_values(self):
+    D = ContextVar("D", 1)
+    D.value = 2
+    with Context(D=3):
+      ...
+    assert D.value == 2, f"Expected D to be 2, but was {D.value}. Indicates that Context.__exit__ did not restore to the correct value."
+
 if __name__ == '__main__':
   unittest.main()

tinygrad/helpers.py

@@ -30,10 +30,11 @@ class Context(contextlib.ContextDecorator):
   stack: ClassVar[List[dict[str, int]]] = [{}]
   def __init__(self, **kwargs): self.kwargs = kwargs
   def __enter__(self):
-    for k,v in self.kwargs.items(): ContextVar._cache[k].value = v
-    Context.stack.append(self.kwargs)
+    Context.stack[-1] = {k:o.value for k,o in ContextVar._cache.items()} # Store current state.
+    for k,v in self.kwargs.items(): ContextVar._cache[k].value = v # Update to new temporary state.
+    Context.stack.append(self.kwargs) # Store the temporary state so we know what to undo later.
   def __exit__(self, *args):
-    for k in Context.stack.pop(): ContextVar._cache[k].value = Context.stack[-1].get(k, Context.stack[0][k])
+    for k in Context.stack.pop(): ContextVar._cache[k].value = Context.stack[-1].get(k, ContextVar._cache[k].value)
 
 class ContextVar:
   _cache: ClassVar[Dict[str, ContextVar]] = {}
@@ -42,7 +43,7 @@ class ContextVar:
   def __new__(cls, key, default_value):
     if key in ContextVar._cache: return ContextVar._cache[key]
     instance = ContextVar._cache[key] = super().__new__(cls)
-    instance.value = Context.stack[0][key] = getenv(key, default_value)
+    instance.value = getenv(key, default_value)
     return instance
   def __bool__(self): return bool(self.value)
   def __ge__(self, x): return self.value >= x