So, I have 3 ideas for how to fix the test in test_solvers.py that my simplify_minus_sign branch breaks. The problem is that the expected output is z = -(t+n*t)/n but simply typing that expression on the right hand side of an assertion causes it to be evaluated as (-t-n*t)/n so that the assertion fails. I think the core issue is that the results from solve_linear_system are passed through simplify.
Here are the three options that I see: 1. use simplify on the right hand side of the assertion: assert (test_result ) == simplify(-(t+n*t)/n) This is dangerous in that simplify is being used on the right and left hand sides of the assertion. A bug in simplify could alter both sides of the assertion and the test could still pass. This may be fine, since this test isn't supposed to test simplify. As long as simplify has its own long list of tests that catch all possible bugs, everything is fine. But this actually wasn't the case and a test failure outside of test_simplify.py revealed a bug in my initial patch. 2. use -1*expr.extract_multiplicatively(-1) on the right hand size of the assertion. This is basically doing the simplification work by hand. Some of the same arguements could apply as in #1, but maybe extract_multiplicatively isn't as complicated as simplify and there is less risk. 3. use a string in the assertion: str(test_result) == '-(t + n*t)/n' The issue here is that a change to the printer could cause the test to fail even if there is nothing really wrong with the output (whitespace differences for instance). Which of these 3 is best or should another option be pursued? Thanks, Ryan On Sat, Jun 27, 2009 at 4:41 PM, Ryan Krauss <[email protected]> wrote: > My assumption is that the output from solve_linear_system gets passed > through simplify. Otherwise my changes to simplify wouldn't have broken > this test. That also explains why res[z] == simplify(my_res) is True (I > think). > > So, while I agree that we should use sympy expressions in the tests > whereever possible, I don't know how to create a sympy expression for > > z = -(t+n*t)/n > > without using simplify. Using simplify on the expected result of the test > seems dangerous because someone like me modifying simplify could break > something and still pass tests. > > > On Sat, Jun 27, 2009 at 4:12 PM, Aaron S. Meurer <[email protected]>wrote: > >> Mul automatically distributes Number coefficients, including -1. The >> question isn't how to make my_res not do it, but why res[z] doesn't >> automatically do it. Did you do something like this? >>> print -(x+y)/z >> (-x - y)/z >> >>> print (x+y)/-z >> -(x + y)/z >> >> This looks like it is a bug in Mul that should be fixed. >> >> I don't think you should test strings unless there is a specific reason >> that you cannot test sympy expressions, such as testing a printer or an >> expression with dummy variables in it. Consider this: if in the future, >> automatic combining changes somehow, then a test sympy_func == sympy_expr >> will still pass, because both sides will automatically combined in the new >> way the same (assuming the new combining doesn't break sympy_func. On the >> other hand, if the test was structured as str(sympy_func) == "sympy_expr", >> then whoever is trying to write the new automatic combining has to go in and >> manually fix the test). I just spent three weeks changing the automatic >> combining of exponents in Mul so that they don't always combine, and I can't >> imagine how much harder my life would have been if I had to go in and >> rewrite a ton of tests because they were based on strings of expressions, >> not expressions themselves. >> >> Aaron Meurer >> On Jun 27, 2009, at 2:54 PM, Ryan Krauss wrote: >> >> Is it a good or a bad idea to re-write the test like this: >> In [45]: str(res[z]) == '-(t + n*t)/n' >> Out[45]: True >> >> >> On Sat, Jun 27, 2009 at 3:17 PM, Ryan Krauss <[email protected]> wrote: >> >>> My patch to simplify a minus 1 from the numerator and denominator of a >>> fraction causes some failures in test_solvers because the results now have a >>> minus 1 factored out. I tried changing one of the tests so that the result >>> would be the expected one and ran into an interesting problem. The whole >>> session is below, but here is the core issue: >>> >>> In [6]: res[z] >>> Out[6]: >>> -(t + n⋅t) >>> ────────── >>> n >>> >>> In [7]: res[z] == -(t+n*t)/n >>> Out[7]: False >>> >>> The problem is that the expression on the right is apparently altered as >>> it is executed so that >>> In [2]: my_res = -(t+n*t)/n >>> >>> In [3]: my_res >>> Out[3]: >>> -t - n⋅t >>> ──────── >>> n >>> >>> This relatex to lines 153 and 154 of solvers/tests/test_solvers.py: >>> assert solve_linear_system(M, x, y, z, t) == \ >>> {y: 0, z: -(t+t*n)/n, x: -(t+t*n)/n} >>> >>> I don't know how to write the test so that sympy doesn't factor in the >>> minus 1 in the numerator when it evalutes the assertion. >>> >>> A pretty version of my session can be found here: >>> http://paste.blixt.org/115586 >>> >>> What is the right way to fix the test in test_solvers line 153? >>> >>> Thanks, >>> >>> Ryan >>> >>> In [1]: x, y, z, t, n = symbols('xyztn') >>> >>> In [2]: my_res = -(t+n*t)/n >>> >>> In [3]: my_res >>> Out[3]: >>> -t - n⋅t >>> ──────── >>> n >>> >>> In [4]: M = Matrix([[0,0,n*(n+1),(n+1)**2,0], >>> ...: [n+1,n+1,-2*n-1,-(n+1),0], >>> ...: [-1, 0, 1, 0, 0]]) >>> >>> In [5]: res = solve_linear_system(M, x, y, z, t) >>> >>> In [6]: res[z] >>> Out[6]: >>> -(t + n⋅t) >>> ────────── >>> n >>> >>> In [7]: res[z] == -(t+n*t)/n >>> Out[7]: False >>> >>> In [8]: my_res = simplify(my_res) >>> >>> In [9]: my_res >>> Out[9]: >>> -(t + n⋅t) >>> ────────── >>> n >>> >>> In [10]: my_res = res[z] >>> >>> In [11]: my_res = -(t+n*t)/n >>> >>> In [12]: my_res == res[z] >>> Out[12]: False >>> >>> In [13]: my_res = simplify(my_res) >>> >>> In [14]: my_res >>> Out[14]: >>> -(t + n⋅t) >>> ────────── >>> n >>> >>> In [15]: my_res == res[z] >>> Out[15]: True >>> >>> >> >> >> >> >> >> >> >> > --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "sympy" group. 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