I often use to the Python REPL to perform some simple calculations (typically some combinatorial or probabilities computation).
I believe it would be a nice improvement if the number displayed in the REPL would be formatted as if f"{result:_}" was given (with the underscore separators). For example:

 >>> 36 ** 7
 78_364_164_096

As I understand things:
* the REPL always shows the __repr__
* updating the __repr__ for int/float is a no-no for backward compatibility reasons

If these assumptions are correct (please correct me if this is wrong), then I guess this cannot be implemented, unless I missed something?

We can't change the repr of int/float.

However, you can use sys.displayhook to achieve what you want:

import sys

def displayhook(o):
    if o is None:
        return
    __builtins__._ = None
    if isinstance(o, (int, float)):
        print(format(o, '_'))
    else:
        print(repr(o))
    __builtins__._ = o

sys.displayhook = displayhook

Then:

>>> 12312312
12_312_312
>>> 123123e123
1.23123e+128
>>> None
>>> 'test'
'test'

Oops, the float example should be:

>>> 123123.9
123_123.9

And you can probably use sitecustomize.py to import this.

Since I don't see any action item here, I'm going to close this issue.

Thanks a lot Eric for your answers, I did not know about sys.displayhook.
Now I know I can easily implement this myself, but the larger question would be: "do we want to update the default displayhook for those simple use cases".