fix notebook memory leak

- fixes #1216

tqdm/notebook.py

@@ -14,6 +14,7 @@
 # import compatibility functions and utilities
 import re
 import sys
+from weakref import proxy
 
 # to inherit from the tqdm class
 from .std import tqdm as std_tqdm
@@ -75,17 +76,19 @@
 class TqdmHBox(HBox):
     """`ipywidgets.HBox` with a pretty representation"""
     def _repr_json_(self, pretty=None):
-        if not hasattr(self, "pbar"):
+        pbar = getattr(self, 'pbar', None)
+        if pbar is None:
             return {}
-        d = self.pbar.format_dict
+        d = pbar.format_dict
         if pretty is not None:
             d["ascii"] = not pretty
         return d
 
     def __repr__(self, pretty=False):
-        if not hasattr(self, "pbar"):
+        pbar = getattr(self, 'pbar', None)
+        if pbar is None:
             return super(TqdmHBox, self).__repr__()
-        return self.pbar.format_meter(**self._repr_json_(pretty))
+        return pbar.format_meter(**self._repr_json_(pretty))
 
     def _repr_pretty_(self, pp, *_, **__):
         pp.text(self.__repr__(True))
@@ -237,7 +240,7 @@ def __init__(self, *args, **kwargs):
         unit_scale = 1 if self.unit_scale is True else self.unit_scale or 1
         total = self.total * unit_scale if self.total else self.total
         self.container = self.status_printer(self.fp, total, self.desc, self.ncols)
-        self.container.pbar = self
+        self.container.pbar = proxy(self)
         self.displayed = False
         if display_here and self.delay <= 0:
             display(self.container)