Python Internals: A Python Lightning Talk

This code is for the What happens when you type print(‘Hello ____’)? A look at Python internals!.

Install Python from source

We'll be installing Python in debug mode which will allow us to see Python's bytecode that is generated when we use the Python interpreter:

wget http://www.python.org/ftp/python/3.9.2/Python-3.9.2.tgz
tar xzvf Python-3.9.2.tgz
cd Python-3.9.2
../configure --with-pydebug
make EXTRA_CFLAGS="-DPy_REF_DEBUG"   # Build Python in debug mode to get the bytecode output
make install

From the terminal try python to see the version is set correctly:

$ python 
Python 3.9.2 (default, Mar  8 2021, 14:54:05)
[Clang 12.0.0 (clang-1200.0.32.27)] on darwin
Type "help", "copyright", "credits" or "license" for more information.

How can I see the bytecode?

Now we'll see the __ltrace__ = None to see the Python bytecode in the terminal:

>>> __ltrace__ = None
>>> print('Hello <Name>')
0: 101, 0
push <built-in function print>
2: 100, 0
push 'Hello <Name>'
4: 131, 1
Hello PyLadies
ext_pop 'Hello PyLadies'
ext_pop <built-in function print>
push None
6: 70
pop None
8: 100, 1
push None
10: 83
pop None

What is all this output? The bytecode! Let's start by looking at the Python lexer.