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Simple, extensible test assertions.

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Simple, extensible test assertions.


Use `expect` to build test assertions.

from grain import expect
from yourmodule import your_func

n = your_func(10)

# Raises an `AssertionError` if `n` is not equal to `100`.

# Raises an `AssertionError` if `your_func(None)` does not raise a `TypeError`.
with expect.raises(TypeError):

Available assertion methods:

- expect(a).equal(b)
- expect(a).not_equal(b)
- expect(a).is_(b)
- expect(a).is_not(b)
- expect(a).true()
- expect(a).false()
- expect(a).in_(b)
- expect(a).not_in(b)
- expect(a).less(b)
- expect(a).less_equal(b)
- expect(a).greater(b)
- expect(a).greater_equal(b)
- expect(a).almost_equal(b)
- expect(a).not_almost_equal(b)
- expect.raises(exception_class)

Custom Assertions

`expect` is actually just a class. You can use `expect.extend()` to conveniently
create a copy that mixes in additional methods. This lets you add custom
assertion methods that work exactly like the built-in methods.

The following code will replace `expect` with a new copy that includes two
additional `even` and `odd` assertion methods.

from grain import expect

class CustomAssertions(object):
def even(self):
if self.actual % 2 == 0:
return'Expected {} to be even'.format(repr(self.actual)))

def odd(self):
if self.actual % 2 != 0:
return'Expected {} to be odd'.format(repr(self.actual)))

expect = expect.extend(CustomAssertions)


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