Call the functions, do usual tests, just play around with the application, deploy it to staging, and Deal will check contracts in runtime. On contract violation, deal raises an exception. In general, you shouldn’t ever catch these exceptions because contracts must be never violated. Contract violation means a bug.

runtime exception example

Contracts on production

If you run Python with -O option, all contracts will be disabled. Under the hood, it’s controlled by the __debug__ variable:

The built-in variable __debug__ is True under normal circumstances, False when optimization is requested (command line option -O). Source: Python documentation

If needed, you can also explicitly enable or disable contracts in runtime:

# disable all contracts

# enable all contracts

# restore the default behavior
# (enabled if `__debug__` is True, disabled otherwise)

It’s easy to mess up with the contracts’ state when you change it manually. To help you a bit, deal will emit a RuntimeWarning if you accidentally enable contracts in production or disable them in tests. If you’ve got this warning and you know what you’re doing, pass warn=False to skip this check.

Permamently disable contracts

When contracts are disabled, functions are still get wrapped in case you want to enable contracts again, after all functions already initialized. That means, even if you disable contracts, there is still a small overhead in runtime that might be critical in for some applications. To avoid it and tell deal to disable contracts permanently, call deal.disable(permament=True). There is what you should know:

  1. If you permamently disable the contracts, you cannot enable them back anymore. Trying to do so will raise RuntimeError.

  2. This flag is checked only when functions are decorated, so you need to call it before importing any decorated functions.

  3. Functions that were decorated before you permamently disabled contracts will behave in the same way as if you just called deal.disable(), with a quick check of the state in runtime on each call.


If no error message or custom exception specified for a contract, deal will show contract source code and passed parameters as the exception message. By default, deal highlights syntax for this source code. If your terminal doesn’t support colors (which is possible on CI), you can specify NO_COLOR environment variable to disable syntax highlighting:

export NO_COLOR=1

See for more details.