Key Invariance not ensured

[napyonsen]

"Every well-formed SymPy expression must either have empty args or satisfy expr == expr.func(*expr.args)."
However,
following returns false.
expr = r"(90-1)*2"
sym = latex2sympy(expr)
sym2 = sym.func(*sym.args)
print sym == sym2 // false

What is well-formed anyway?

[oscarbenjamin]

What exactly is sym in the above?

I don't know what version of SymPy you are using but I don't have a latex2sympy function on master. I guess that it is returning something unevaluated like

In [5]: e = Mul(Add(90, -1, evaluate=False), 2, evaluate=False)                                                                   

In [6]: e                                                                                                                         
Out[6]: 2⋅(-1 + 90)

In [7]: e.doit()                                                                                                                  
Out[7]: 178

In [8]: e.func(*e.args)                                                                                                           
Out[8]: -2 + 180

In [9]: e.func(*e.args, evaluate=False)                                                                                           
Out[9]: 2⋅(-1 + 90)

The invariant that x==x.func(*x.args) does depend on whether or not x was created with evaluate=True or evaluate=False.

[napyonsen]

latex2sympy is a third party function which is adopted into sympy in recent versions as "parse_latex". It converts latex expression to sympy.
Your point is correct. Exact identiity should be "x==x.func(*x.args, evaluate=False)".
Thanks