On Fri, Apr 4, 2014 at 5:05 PM, Harold Erbin <[email protected]> wrote:
> Thank you for your comment Aaron.
> I have thought to this problem for the last month, but I did not come up
> with a solution. If I say that any non-Dimension Expr is equivalent to a
> dimensionless object, then it is equivalent to returning just the
> factor. But doing so I get the same problem with the Quantity class,
> where now the factor can be a Dummy. So here it is more subtle to
> handle, but as a first try I return just the unit.
> Now the solve command succeed, but the Dummy symbol where very important
> to get the unit right (here the unit is missing) and to get the factor
> correct (I have a 10^17 missing).
> The problem is that I don't see how to handle arbitrary Symbol/Expr in
> my code directly (even if it would be nice).
>
> I'm hesitating to remove this example to avoid blocking all this module
> just for this: then I could precise in the doc that one should not use
> solve for now, and try to find a better solution.
>
> What do you mean by delaying the evaluation?
>
I mean making it so that things don't evaluate when a Subs is performed.
This is e.g., how Derivative works (otherwise Derivative(f(x),
x).subs(f(x), Dummy('y')) would automatically go to 0, which is not what is
wanted.
Aaron Meurer
>
> Best,
>
> Harold
>
> Le 02/03/2014 01:25, Aaron Meurer a écrit :
> > solve just assumes that any Expr can be replaced with Dummy, I think.
> > I don't fully understand how your code works, but maybe you could let
> > a non-Dimension Expr object just be interpreted as dimensionless (you
> > may also need to delay some evaluation so that things don't collapse
> > when doing temporary dummy substitution).
> >
> > Aaron Meurer
> >
> > On Wed, Feb 26, 2014 at 4:29 AM, Harold E. <[email protected]>
> wrote:
> >> As a test for the unit system module, I have written some codes using
> the
> >> solve method, since Units (and other objects defined in the module) are
> >> supposed to behave as normal sympy objects.
> >> But when I call solve, it tries to create an intermediate Unit object
> where
> >> one of the argument is Dummy, so it returns an error since this argument
> >> should be a Unit or a Dimension object [2]. I tried to understand how
> solve
> >> is working, but I don't get it. Here is the full traceback
> >> File
> >>
> "/mnt/data/Documents/Projets/Programmation/sympy/doc/src/modules/physics/unitsystems/examples.rst",
> >> line 76, in examples.rst
> >> Failed example:
> >> Tr = solve(T**2/venus_a**3 - 4*pi**2 / mks["G"] / solar_mass, T)[1]
> >> Exception raised:
> >> Traceback (most recent call last):
> >> File "/usr/lib/python2.7/doctest.py", line 1289, in __run
> >> compileflags, 1) in test.globs
> >> File "<doctest examples.rst[25]>", line 1, in <module>
> >> Tr = solve(T**2/venus_a**3 - 4*pi**2 / mks["G"] / solar_mass,
> T)[1]
> >> File "sympy/solvers/solvers.py", line 893, in solve
> >> f[i] = nsimplify(fi, rational=True)
> >> File "sympy/simplify/simplify.py", line 3830, in nsimplify
> >> return _real_to_rational(expr, tolerance)
> >> File "sympy/simplify/simplify.py", line 3785, in _real_to_rational
> >> return p.subs(reps, simultaneous=True)
> >> File "sympy/core/basic.py", line 941, in subs
> >> rv = rv._subs(old, d, **kwargs)
> >> File "sympy/core/cache.py", line 93, in wrapper
> >> r = func(*args, **kw_args)
> >> File "sympy/core/basic.py", line 1063, in _subs
> >> rv = fallback(self, old, new)
> >> File "sympy/core/basic.py", line 1035, in fallback
> >> arg = arg._subs(old, new, **hints)
> >> File "sympy/core/cache.py", line 93, in wrapper
> >> r = func(*args, **kw_args)
> >> File "sympy/core/basic.py", line 1063, in _subs
> >> rv = fallback(self, old, new)
> >> File "sympy/core/basic.py", line 1035, in fallback
> >> arg = arg._subs(old, new, **hints)
> >> File "sympy/core/cache.py", line 93, in wrapper
> >> r = func(*args, **kw_args)
> >> File "sympy/core/basic.py", line 1063, in _subs
> >> rv = fallback(self, old, new)
> >> File "sympy/core/basic.py", line 1035, in fallback
> >> arg = arg._subs(old, new, **hints)
> >> File "sympy/core/cache.py", line 93, in wrapper
> >> r = func(*args, **kw_args)
> >> File "sympy/core/basic.py", line 1063, in _subs
> >> rv = fallback(self, old, new)
> >> File "sympy/core/basic.py", line 1040, in fallback
> >> rv = self.func(*args)
> >> File "sympy/physics/unitsystems/units.py", line 72, in __new__
> >> "instance; found %s" % type(dim))
> >> TypeError: 'dim' object should be Unit or Dimension instance; found
> >> <class 'sympy.core.symbol.Dummy'>
> >>
> >> When I wrote the test (commit 05ddf1aeac22eb1f01c9db1c7ab61d6ee8cc955b),
> >> this was working since I just copied-pasted the results I was seeing on
> >> IPython. One of the main thing which change since this moment is that
> Unit
> >> inherits from Expr instead of AtomicExpr. I tried to go back to the
> >> mentioned commit, but now I get a recursive call...
> >>
> >> This is the only remaining bug in the proposed pull request [3]. You can
> >> find the branch there [4].
> >>
> >> Do you have some ideas? if not, maybe we should give up (temporarily)
> the
> >> use of Units in equations, etc. (since we can still do it with abstract
> >> symbol and then replace).
> >>
> >> [1]
> >>
> https://github.com/melsophos/sympy/blob/units/doc/src/modules/physics/unitsystems/examples.rst
> >> [2]
> >>
> https://github.com/melsophos/sympy/blob/units/sympy/physics/unitsystems/units.py
> >> [3] https://github.com/sympy/sympy/pull/2628
> >> [4] https://github.com/melsophos/sympy/tree/units
> >>
> >> Best,
> >>
> >> Harold
> >>
> >> --
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