Notifying a trait with a DataFrame instance throws Value Error

traitlets.py の366行目にある link._update_target から始め、次に pandas を使って issue の DataFrame の例を再現します。比較結果がブール値でない場合にどのように処理されるかを確認してください。pandas DataFrame インスタンスを含む traits をリンクしても、示されている ValueError が発生しなくなれば完了です。

Hey there,

At first, thank you for this amazing library!

I noticed that there are problems when linking multiple objects using the link function, if the value of a trait is a dataframe.
To me it looks like the compare logic overridden by pandas is causing the problem.
Here is an example:

import traitlets
from traitlets import link, directional_link
import pandas as pd

class SomeClass(traitlets.HasTraits):
    df = traitlets.Instance(klass=pd.DataFrame, allow_none=True)

foo = SomeClass()
baz = SomeClass()
bar = SomeClass()


# Will not work
link((foo, "df"), (baz, "df"))
foo.df = pd.DataFrame() # Throws ValueError

Stacktrace:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
traitlets_dataframe.ipynb Cell 4' in <cell line: 5>()
      2 link((foo, "df"), (baz, "df"))
      4 # Throws ValueError
----> 5 foo.df = pd.DataFrame()

File .\lib\site-packages\traitlets\traitlets.py:712, in TraitType.__set__(self, obj, value)
    710     raise TraitError('The "%s" trait is read-only.' % self.name)
    711 else:
--> 712     self.set(obj, value)

File .\lib\site-packages\traitlets\traitlets.py:701, in TraitType.set(self, obj, value)
    697     silent = False
    698 if silent is not True:
    699     # we explicitly compare silent to True just in case the equality
    700     # comparison above returns something other than True/False
--> 701     obj._notify_trait(self.name, old_value, new_value)

File .\lib\site-packages\traitlets\traitlets.py:1371, in HasTraits._notify_trait(self, name, old_value, new_value)
   1370 def _notify_trait(self, name, old_value, new_value):
-> 1371     self.notify_change(
   1372         Bunch(
   1373             name=name,
   1374             old=old_value,
   1375             new=new_value,
   1376             owner=self,
   1377             type="change",
   1378         )
   1379     )

File .\lib\site-packages\traitlets\traitlets.py:1383, in HasTraits.notify_change(self, change)
   1381 def notify_change(self, change):
   1382     """Notify observers of a change event"""
-> 1383     return self._notify_observers(change)

File .\lib\site-packages\traitlets\traitlets.py:1428, in HasTraits._notify_observers(self, event)
   1425 elif isinstance(c, EventHandler) and c.name is not None:
   1426     c = getattr(self, c.name)
-> 1428 c(event)

File .\lib\site-packages\traitlets\traitlets.py:366, in link._update_target(self, change)
    364 with self._busy_updating():
    365     setattr(self.target[0], self.target[1], self._transform(change.new))
--> 366     if getattr(self.source[0], self.source[1]) != change.new:
    367         raise TraitError(
    368             "Broken link {}: the source value changed while updating "
    369             "the target.".format(self)
    370         )

File .\lib\site-packages\pandas\core\generic.py:1527, in NDFrame.__nonzero__(self)
   1525 @final
   1526 def __nonzero__(self):
-> 1527     raise ValueError(
   1528         f"The truth value of a {type(self).__name__} is ambiguous. "
   1529         "Use a.empty, a.bool(), a.item(), a.any() or a.all()."
   1530     )

ValueError: The truth value of a DataFrame is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().

As you can see, hhe problem is in traitlets.py: 366, because getattr(self.source[0], self.source[1]) != change.new does not return a bool value in the case of a DataFrame.

Would it be possible to make this function compatible with pandas, or possibly define a custom function for comparison?

Thank you in advance!