Repository: ipython/ipython

Issue:
`_log_format_changed()` missing 3 required positional arguments
In `IPython.config.application`, the `_log_datefmt_changed` handler calls `_log_format_changed` but does not pass it the required arguments. My guess would be that this is the correct implementation:

```
def _log_datefmt_changed(self, name, old, new):
    self._log_format_changed(name, self.log_format, self.log_format)
```

However I am not really sure what the `name` parameter is for, so I might be wrong.

Discussion:

Patch:
diff --git a/IPython/config/application.py b/IPython/config/application.py
index ef97162b3..264d3793a 100644
--- a/IPython/config/application.py
+++ b/IPython/config/application.py
@@ -159,7 +159,7 @@ def _log_level_changed(self, name, old, new):
         help="The date format used by logging formatters for %(asctime)s"
     )
     def _log_datefmt_changed(self, name, old, new):
-        self._log_format_changed()
+        self._log_format_changed('log_format', self.log_format, self.log_format)
     
     log_format = Unicode("[%(name)s]%(highlevel)s %(message)s", config=True,
         help="The Logging format template",


Test patch:
diff --git a/IPython/config/tests/test_application.py b/IPython/config/tests/test_application.py
index a03d548c2..5da6a1306 100644
--- a/IPython/config/tests/test_application.py
+++ b/IPython/config/tests/test_application.py
@@ -80,6 +80,7 @@ def test_log(self):
         # trigger reconstruction of the log formatter
         app.log.handlers = [handler]
         app.log_format = "%(message)s"
+        app.log_datefmt = "%Y-%m-%d %H:%M"
         app.log.info("hello")
         nt.assert_in("hello", stream.getvalue())