I use Python 3 and PyQt5. Here’s my test PyQt5 program, focus on the last 2 lines:

from PyQt5.QtCore import *
from PyQt5.QtWidgets import *
import sys

class window(QWidget):
def __init__(self,parent=None):


I know exec is a language keyword in Python. But code on Official PyQt5 Documentation (specifically the Object Destruction on Exit part). I see that example shows use of app.exec() which confuses me.

When I tested it on my machine. I found there is no any visible difference from my end. Both with and without _ produces the same output in no time difference.

My question is:

  • Is there anything wrong going when I use app.exec()? like clashing with Python’s internal exec? I suspect because both exec‘s are executing something.
  • If not, can I use both interchangeably?

That’s because until Python 3, exec was a reserved keyword, so the PyQt devs added underscore to it. From Python 3 onwards, exec is no longer a reserved keyword (because it is a builtin function; same situation as print), so it made sense in PyQt5 to provide a version without an underscore to be consistent with C++ docs, but keep a version with underscore for backwards compatibility. So for PyQt5 with Python 3, the two exec functions are the same. For older PyQt, only exec_() is available.

On the question of whether to prefer one over the other: using exec_ means you have one less thing to worry about if you ever decide to add support for PyQt4 and/or Python >= 2.6, and want to maintain a single code-base.

As of PyQt 6, app.exec_() is no longer supported, only app.exec() is.

Hence, when building new apps I only use the latter.