This page is a collection of obscure facts about the Python programming language.

Table of Contents

the __debug__ constant

Python has a built-in boolean constant named __debug__.

It is set to True whenever you run a REPL shell or call a script with the python script.py i.e. by default.

It is set to False only when you pass the -O or the -OO flags to the python executable.

These flags are described in the Python manual page as:

-O Remove assert statements and any code conditional on the value of debug; augment the filename for compiled (bytecode) files by adding .opt-1 before the .pyc extension.

-OO Do -O and also discard docstrings; change the filename for compiled (bytecode) files by adding .opt-2 before the .pyc extension.

__debug__ in REPL shell

$ python3
Python 3.9.13 (main, May 24 2022, 21:28:31)
[Clang 13.1.6 (clang-1316.0.21.2)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> __debug__
True


$ python3 -O
Python 3.9.13 (main, May 24 2022, 21:28:31)
[Clang 13.1.6 (clang-1316.0.21.2)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> __debug__
False

__debug__ in scripts

$ cat script.py
print(__debug__)

$ python3 script.py
True
$ python3 -O script.py
False
$ python3 -OO script.py
False