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Python Assertion Helpers inspired by Shouldly

Project description


  • forbiddenfruit
  • a version of python with which forbidden fruit will work (must implement the CTypes/CPython Python API)
  • Python 2.7 or 3.3 (it may work with other versions, such as other 3.x versions, but it has not been tested with these versions)




>>> import should_be.all
>>> class Cheese(object):
...     crackers = 3
>>> swiss = Cheese()
>>> swiss.crackers.should_be(4)
AssertionError: swiss.crackers should have been 4, but was 3


Because of the way the Python REPL shows stack traces, if the ‘should_be’ assertion is typed in a line on the REPL, ‘(unknown)’ will show instead of ‘swiss.crackers’. This is not an issue when the ‘should_be’ statement is in a file instead.


The easy way

$ sudo pip install

The slightly-less-easy way

$ git clone
$ cd should_be
$ ./ build
$ sudo ./ install


Writing your own assertions is fairly easy. There are two core parts of ShouldBe: BaseMixin and should_follow.

All assertions should be placed in classes that inherit from BaseMixin. BaseMixin provides the basic utilities for extending built-in objects with your assertions.

The class which holds your assertions should have a class variable called target_class. This is the class on which your assertions will be run. By default, this is set to object. If you wish to have your assertions run on object, there are a few additional considerations to make (see warning below).

Then, assertions should be defined as instance methods. Each method should call self.should_follow one or more times. Think of should_follow as assertTrue on steroids. It has the following signature: should_follow(self, assertion, msg=None, **kwargs). Obviously, assertion is an expression which, when False, causes should_follow to raise an AssertionError. So far, pretty normal. msg is where things get interesting. msg should be a new-style Python format string which contains only named substitutions. By default, should_follow will pass the txt and self keys to the format method, in addition to any keyword arguments passed to should_follow. self is, obviously, the current object. txt is the code that represents the current object. For instance, if we wrote (3).should_be(4), txt would be ‘(3)’. If we wrote cheese.variety.should_be('cheddar'), txt would be ‘cheese.variety’.

Once all of your assertions are written, you can simply write MyAssertionMixin.mix() to load your assertions. A setuptools hook is on the way for autoloading custom assertion mixins with import should_be.all.


When you extend object, you need to also create the proper mixins for NoneType, since None doesn’t have instance methods per-se (since self gets set to None, the Python interpreter complains). Thankfully, this is fairly easy. Simply create a class which inherits from NoneTypeMixin, and set the class variable source_class to the name of your object assertions class. You can then simply run MyNoneTypeMixin.mix(), and your methods will be automatically retrieved and converted from your object mixin class.


Assertions for ABCs (such as Sequence) will be automatically mixed in to ‘registered’ classes that do not inherit methods from the ABCs normally (such as list, etc) when the mix() method is called (this will also check for classes that are registered to subclasses of the ABCs).

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