subs with a string, how to make this work with inheritance? Change from 0.7.3 to 0.7.4.1 ?

[Robert]

Hey everyone,

Let's say I defined my own sympy.Symbol called MySymbol2. For some reason subs using a string doesn't always work, but it works for sympy.Symbol.

This is version 0.7.4.1. I have a 0.7.3 example below that works fine. I see that the function fallback in core.py was changed.

from is not test on line 992 in 0.7.3:

if arg is not args[i]

to not equal test on line 1036 in 0.7.4.1:

if arg != args[i]:

    >>> class MySymbol2(sympy.Symbol): pass

    >>> expr = MySymbol2('A') * (MySymbol2('X') > 5)

    >>> print expr.subs({'X':10})

    A*(X > 5)

    >>> expr = sympy.Symbol('A') * (sympy.Symbol('X') > 5)

    >>> print expr.subs({'X':10})

    A*True

Sometimes the subs work fine. Below X and Y are both substituted for my MySymbol

    >>> expr = sympy.sympify('3*X + Y')

    >>> subsDic2 = {'X': MySymbol2('X'), 'Y': MySymbol2('Y')}

    >>> expr2 = expr.subs(subsDic2)

    >>> print [(i, type(i)) for i in expr2.atoms()]

    [(X, <class 'stats.tests.modelSymbolicTest.MySymbol2'>), (3, <class 'sympy.core.numbers.Integer'>), (Y, <class 'stats.tests.modelSymbolicTest.MySymbol2'>)]

I've tried a fix like:

class MySymbolWithNameEq(sympy.Symbol):

    def __eq__(self, other):

        if isinstance(other, sympy.Symbol):

            return self.name.__eq__(other.name)

        else:

            return sympy.Symbol.__eq__(self, other)

but that causes the subs on 3*X+Y to fail and my A*(X>5) still fails. Notice that not all the atoms are replaced. 

    >>> expr = MySymbolWithNameEq('A') * (MySymbolWithNameEq('X') > 5)

    >>> print expr.subs({'X':10})

    A*(X > 5)

    >>> subsDic3 = {'X': MySymbolWithNameEq('X'), 'Y': MySymbolWithNameEq('Y')}

    >>> expr = sympy.sympify('3*X + Y')

    >>> expr2 = expr.subs(subsDic3)

    >>> print [(i, type(i)) for i in expr2.atoms()]

    [(Y, <class 'stats.tests.modelSymbolicTest.MySymbolWithNameEq'>), (X, <class 'sympy.core.symbol.Symbol'>), (3, <class 'sympy.core.numbers.Integer'>)]

#on 0.7.3 the above works correctly

    >>> expr2 = expr.subs(subsDic3)

    >>> print [(i, type(i)) for i in expr2.atoms()]

    [(X, <class 'stats.tests.modelSymbolicTest.MySymbolWithNameEq'>), (3, <class 'sympy.core.numbers.Integer'>), (Y, <class 'stats.tests.modelSymbolicTest.MySymbolWithNameEq'>)]

Any idea how to inherit from sympy.Symbol while still getting string substitution to work reliably?

thanks,

Rob

[Aaron Meurer]

I would recommend not using strings. It's better to convert everything
to SymPy objects and only use those.

Even so, the real fix for this would be to add a call to sympify()
somewhere in the subs code. But you shouldn't rely on functions in
SymPy calling sympify(). Some do, but there will always be those that
don't.

Aaron Meurer

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