I've observed the same behavior with bpython-curses but not only with underscore and capital P. In addition this happens also with A, E, G, K, N, and Y (all capital).

Versions:
Python 3.8.3
bcrypt 3.2.0
blessings 1.7
bpython 0.20
certifi 2020.11.8
cffi 1.14.3
chardet 3.0.4
cryptography 3.2.1
curtsies 0.3.4
greenlet 0.4.17
idna 2.10
paramiko 2.7.2
pip 20.2.4
pycparser 2.20
Pygments 2.7.2
PyNaCl 1.4.0
requests 2.24.0
setuptools 41.2.0
six 1.15.0
urllib3 1.25.11
wcwidth 0.2.5
windows-curses 2.2.0

@bjhend I've upgraded to the latest version of the package and I confirm that this bug has been fixed.
Thanks to the developers

I've observed the same behavior with bpython-curses but not only with underscore and capital P. In addition this happens also with A, E, G, K, N, and Y (all capital).

Of course, it is all the letters in "KEY_NPAGE" and "KEY_PPAGE", have you seen the fiix?

elif key in ("KEY_NPAGE",)

the comma (added as a fix) transforms what was wrongly a string (a list of chars) into a correct tuple, in this case a one-element tuple

Versions:

bpython 0.20

@bjhend
Please upgrade to bpython 0.20.1

Upgrading solved it for me as well, thanks.

Yes, I've looked at the fix (tried it manually first) but didn't compare the letters. Sometimes bugs have funny effects.

Thanks for the hint. As I said, after upgrading to bpython 0.20.1 as well as after applying the fix manually beforehand, it worked or me.