BUG: Fix linux clipboard QApplication() creation

closes #14372

A Qt application cannot instantiate multiple
`QApplication` instances, so we create a new `QApplication` only when
the global `QApplication.instance()` is None.

Author: Pankaj Pandey <[email protected]>

Closes #14815 from pankajp/patch-2 and squashes the following commits:

40d70f9 [Pankaj Pandey] BUG: Fix linux clipboard QApplication() creation

Author: pankajp

doc/source/whatsnew/v0.20.0.txt

@@ -828,7 +828,7 @@ Bug Fixes
 
 - Bug in ``pd.read_msgpack()`` in which ``Series`` categoricals were being improperly processed (:issue:`14901`)
 - Bug in ``Series.ffill()`` with mixed dtypes containing tz-aware datetimes. (:issue:`14956`)
-
+- Bug in interactions with ``Qt`` when a ``QtApplication`` already exists (:issue:`14372`)
 - Bug in ``DataFrame.isin`` comparing datetimelike to empty frame (:issue:`15473`)
 
 - Bug in ``Series.where()`` and ``DataFrame.where()`` where array-like conditionals were being rejected (:issue:`15414`)

pandas/util/clipboard/clipboards.py

@@ -50,7 +50,8 @@ def init_qt_clipboard():
     # $DISPLAY should exist
     from PyQt4.QtGui import QApplication
 
-    app = QApplication([])
+    # use the global instance if it exists
+    app = QApplication.instance() or QApplication([])
 
     def copy_qt(text):
         cb = app.clipboard()