Incorrect Exception in dsolve() : First implementation

[MechCoder]

This pull request is related to the issue http://code.google.com/p/sympy/issues/detail?id=1996

As mentioned in the above link , dsolve() returns wrong exceptions when it is not solvable , and the hint is either invalid or incorrect. I've changed it so that it raises the correct Exceptions.

from sympy import *
from sympy.abc import x
f = Function('f')
dsolve(f(x).diff(x)**2, f(x), 'separable')  #This now gives the correct ValueError
>>>  ValueError: ODE Derivative(f(x), x)**2 does not match hint separable

Also , as mentioned in the link , as needed

dsolve(f(x).diff(x)**2, f(x), 'fdsjf')  #This should give ValueError for incorrect hint
>>>  ValueError: Hint not recognized: fdsjf

Do look at the code for changes required.

[Krastanov]

A rather strange error from Travis, however it does not seem to be caused by this pull request:

[..snip..]
sympy/geometry/line.py[35] ................................... [OK]
sympy/geometry/entity.py[5] ..... [OK]
================ tests finished: 2243 passed, in 168.84 seconds ================
Exception RuntimeError: 'maximum recursion depth exceeded' in <generator object _preorder_traversal at 0x3808fa0> ignored
Fatal Python error: Cannot recover from stack overflow.
Done. Build script exited with 1

[MechCoder]

Yes , I noticed that in the build statement . Could you please tell what you think needs to be done , because it had passed all the tests on my laptop and I'm quite sure of the required output.

[asmeurer]

Add tests (use raises).

[MechCoder]

Sorry for being a noob but is the error because I haven't added tests? Could you please be a bit more clearer?

[Krastanov]

The travis build error is not due to the lack of tests. The requirement for tests is a convention and it is not tested itself (that would be a bit too meta :))

For the moment just add the tests, this will cause a new travis build run and if the error persist we will discuss it again. The error itself seems unrelated to your code.

[Krastanov]

More generally, there is a coverage test tool which you can use to check how much of your code is tested.

[MechCoder]

I've added tests using raises as mentioned.

[asmeurer]

This looks good if tests pass.

[MechCoder]

Could you merge it? .It seems to have passed the Travis

[glarrain]

I fear the added tests do not verify the code's behavior is correct. First of all, the messages are not checked, so the message to the user could be incorrect, with all tests passing. In fact, in line 1433

raises(ValueError, lambda: dsolve(f(x).diff(x)**2, f(x), 'seperable'))

it says 'seperable', not 'separable', thus the exception message will be "Hint not recognized: seperable" instead of the expected (I guess so) "ODE " + str(eq) + " does not match hint separable".

[asmeurer]

Oh, you're right. That should be fixed.

[asmeurer]

This was so trivial that I just pushed a fix in.

[asmeurer]

SymPy Bot Summary: ❌ Could not fetch the branch Manoj-Kumar-S/1996_issue.
@manoj-kumar-s: Please make sure that Manoj-Kumar-S/1996_issue has been pushed to GitHub and run the sympy-bot tests again.