How does one check if a variable is a sympy expression?

Question

I wanted to check a python variable is a sympy expression. Its easy to check if its a sympy variable with:

isinstance(arg, symbol.Symbol)

but I can't find how to do:

isinstance(arg, sympy.Expression)

is it possible to check if a python variable holds a sympy expression or a variable?

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as a quick check I did:

expr2 = x-y
type(expr2)
<class 'sympy.core.add.Add'>

but I don't want to have a giant series if statement clause checking each possible type of maths expression. Seems redundant/silly.

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It would also nice to be able to detect when a variable is of any type of sympy related thing and then act on it (and then maybe later check if its an expression or something more detailed...)

Answer 1

I think you simply need sympy.Expr instead of sympy.Expression:

In [164]: expr2
Out[164]: x - y

In [165]: type(expr2)
Out[165]: sympy.core.add.Add

but if we look at the __bases__ of this type:

In [166]: type(expr2).__bases__
Out[166]: (sympy.core.expr.Expr, sympy.core.operations.AssocOp)

And so:

In [167]: isinstance(2, sympy.Expr)
Out[167]: False

In [168]: isinstance(x, sympy.Expr)
Out[168]: True

In [169]: isinstance(x-y, sympy.Expr)
Out[169]: True