[Python] Deal Design by Contract to prove a Python script with decorators

I have a dockerized Deal that I can successfully run using

docker run -it -v `pwd`:/scratch -w /scratch --rm deal python3 -m deal prove deal_demo_cat.py


which produces

deal_demo_cat.py
cat
proved! post-condition, post-condition, post-condition


The code (copied from https://deal.readthedocs.io/basic/motivation.html) in deal_demo_cat.py is

#!/usr/bin/env python3
import deal

@deal.ensure(lambda left, right, result: result.startswith(left))
@deal.ensure(lambda left, right, result: result.endswith(right))
@deal.ensure(lambda left, right, result: len(result) == len(left) + len(right))
def cat(left: str, right: str) -> str:
return left + right


That works. However, when I write my function in add_one.py

#!/usr/bin/env python3
import deal

@deal.pre(lambda number: number<1E100) # Precondition must be true before the function is executed.
@deal.pre(lambda number: number>-1E100)
@deal.ensure(lambda number, result: result==number+1)
def add_one_to_arg(number: int | float) -> int | float:
"""
do the addition
"""
return number + 1


and then run

docker run -it -v `pwd`:/scratch -w /scratch --rm deal python3 -m deal prove add_one.py


I get no output to the terminal. My question is why I am not getting any output. Is something missing from my script?

For reproducibility, here's the Dockerfile used to containerize Deal:

FROM phusion/baseimage:jammy-1.0.2@sha256:1584de70d2f34df8e2e21d2f59aa7b5ee75f3fd5e26c4f13155137b2d5478745

# Use baseimage-docker's init system
CMD ["/sbin/my_init"]

# TODO: pin the apt package versions
# Update and install packages
RUN apt update && apt -y upgrade && apt -y install \
python3 \
python3-pip

WORKDIR /opt/

RUN python3 -m pip install --user 'deal[all]'

# RUN python3 -m pip install deal-solver

RUN python3 -m pip install pytest

# https://github.com/timothycrosley/hypothesis-auto
# https://timothycrosley.github.io/hypothesis-auto/
RUN python3 -m pip install hypothesis-auto

RUN python3 -m pip install mypy

# as of April 2025 typeguard 4.2.2 is available, but the `CallMemo` feature isn't available after 4.0
RUN python3 -m pip install typeguard==3.0.2


and then I use

docker build -f Dockerfile -t deal .

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