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Expecter Gadget: better expectations (assertions) for Python.

branch: master
README.markdown

BASICS

Expecter Gadget helps you to write assertions. Never again will you forget which is expected and which is actual!

Basic expectations are easy:

>>> from expecter import expect
>>> expect('some' + 'thing') == 'something'
expect('something')
>>> expect(1) > 100
Traceback (most recent call last):
...
AssertionError: Expected something greater than 100 but got 1

Just read the expectations like a sentence. "expect(2) == 1 + 1" reads as "Expect 2 to equal 1 + 1". Obviously, the expectation is about 2, and it's being compared to 1 + 1. No ambiguity!

EXCEPTIONS

Expectations about exceptions use the "with" statement. Everything is good if the expected exception is raised:

>>> from __future__ import with_statement
>>> with expect.raises(KeyError):
...     {}[123]

If it's not raised, Expecter Gadget will raise an AssertionError:

>>> with expect.raises(KeyError):
...     pass
Traceback (most recent call last):
...
AssertionError: Expected an exception of type KeyError but got none

Exceptions that don't match the expected one will not be swallowed, so your test will error as you expect:

>>> from __future__ import with_statement
>>> with expect.raises(NameError):
...     {}[123]
Traceback (most recent call last):
...
KeyError: 123

CUSTOM EXPECTATIONS

You can add a custom expectation with the add_expectation method. You give it a predicate that should return true if the expectation succeeds and false if it fails. All expectation objects will grow a method with the name of your predicate method (so don't use a lambda). Appropriate exception messages will be generated when your predicate fails:

>>> import expecter
>>> def can_meow(thing):
...     return thing == 'kitty'
>>> expecter.add_expectation(can_meow)
>>> expect('kitty').can_meow()
>>> expect('puppy').can_meow()
Traceback (most recent call last):
...
AssertionError: Expected that 'puppy' can_meow, but it can't

API DOCUMENTATION

See http://expecter-gadget.readthedocs.org/en/latest/

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