Python Assertion Helpers inspired by Shouldly
- 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 https://github.com/DirectXMan12/should_be.git#egg=ShouldBe
The slightly-less-easy way
$ git clone https://github.com/DirectXMan12/should_be.git $ cd should_be $ ./setup.py build $ sudo ./setup.py install
Writing your own assertions is fairly easy. There are two core parts of
BaseMixin and should_follow.
All assertions should be placed in classes that inherit from
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
object, there are a few additional considerations to make (see
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
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
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
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
mix() method is called (this will also check for classes that are registered
to subclasses of the ABCs).
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