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### How to check if a symbolic expression is numerically evaluable?

Context: I want to evaluate a symbolic expression numerically; however sometimes this expression still depends on some symbolic parameters not yet assigned a value.

Question: What is the best way to avoid "TypeError: cannot evaluate symbolic expression numerically" in such a situation?

In other words, is there a function (or an equivalent construction) for

 if isThisSymbolicExpressionNumericallyEvaluabel(exp):
n = exp.n()
else:
print "sorry, no"