Seeking deeper understanding of python equality (==)

[Jonathan Kaczynski]

Hi,

I was recently trying to explain how python equality works and ran into a
gap in my knowledge. I haven't found any good pages going beneath a surface
level explanation of python equality comparison.

I'll post my investigations below. What I think I'm looking for is where in
the source code (https://github.com/python/cpython) does the equality
comparison occur. I have an idea but wanted to ask first.


Using the dis module, we see the comparison operator is a single bytecode,
which is expected.

❯ docker run -it --rm ubuntu:jammy
root at 919d94c98191:/# apt-get update
root at 919d94c98191:/# apt-get --yes install python3
root at 919d94c98191:/# cat >play.py <<EOF
import dis
import uuid

def test():
    x = uuid.uuid4()
    y = str(x)
    x == y
    return

dis.dis(test)
EOF
root at f33b02fef026:/# python3 play.py
... snip ...
  7          16 LOAD_FAST                0 (x)
             18 LOAD_FAST                1 (y)
             20 COMPARE_OP               2 (==)
             22 POP_TOP
... snip ...


Stepping through the code with gdb, we see it jump from the compare
operator to the dunder-eq method on the UUID object. What I want to be able
to do is explain the in-between steps. Also, if you change `x == y` to `y
== x`, you still see the same behavior, which I assume has to do with
dunder-eq being defined on the UUID class and thus given priority.

❯ docker run -it --rm ubuntu:jammy
root at 919d94c98191:/# apt-get update
root at 919d94c98191:/# apt-get --yes install dpkg-source-gitarchive
root at 919d94c98191:/# sed -i 's/^# deb-src/deb-src/' /etc/apt/sources.list
root at 919d94c98191:/# apt-get update
root at 919d94c98191:/# apt-get --yes install gdb python3.10-dbg
root at 919d94c98191:/# apt-get source python3.10-dbg
root at 919d94c98191:/# cat >play.py <<EOF
import uuid
x = uuid.uuid4()
y = str(x)
breakpoint()
x == y
EOF
root at 919d94c98191:/# gdb python3.10-dbg
(gdb) dir python3.10-3.10.4/Python
(gdb) run play.py
Starting program: /usr/bin/python3.10-dbg play.py

warning: Error disabling address space randomization: Operation not
permitted
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
> //play.py(5)<module>()
-> x == y
(Pdb) s
--Call--
> /usr/lib/python3.10/uuid.py(239)__eq__()
-> def __eq__(self, other):


Thank you,
Jonathan