Why are assertEquals() parameters in the order (expected, actual)?

Question

Why do so many assertEquals() or similar function take the expected value as first parameter and the actual one as second ?
This seems counter-intuitive to me,

Answer 1

An ulterior purpose of assertEqual() is to demo code for human readers. Because the simplest function call is result = function(parameters), one gets used to thinking of the return value on the left and the call on the right.

So a test that documents a function would show a literal on the left and a call on the right.

assertEqual(15, sum((1,2,3,4,5)))

That is, (expected, actual). Similarly with an expression.

assertEqual(4, 2 + 2)

If you like lining things up (PEP8 notwithstanding), the expected parameter helps being shorter on the left:

assertEqual(42, 2 * 3 * 7)
assertEqual(42, (1 << 1) + (1 << 3) + (1 << 5))
assertEqual(42, int('110', int('110', 2)))

_{Thanks Andrew Weimholt and Ganesh Parameswaran for these formulae.}