surprising error message with Poly('a-a')

[georgesk]

Hello, I hereby forward a bug report raised by "Kingsley G. Morse Jr." kingsley@loaner.com, which was sent to the Debian bug tracking system (see https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1052596)

Here is a minimal example to trigger the error message within ipython3's shell:

In [1]: from sympy import Poly

In [2]: Poly('a-a')
---------------------------------------------------------------------------
GeneratorsNeeded                          Traceback (most recent call last)
Cell In[2], line 1
----> 1 Poly('a-a')

File /usr/lib/python3/dist-packages/sympy/polys/polytools.py:182, in Poly.__new__(cls, rep, *gens, **args)
    180     return cls._from_poly(rep, opt)
    181 else:
--> 182     return cls._from_expr(rep, opt)

File /usr/lib/python3/dist-packages/sympy/polys/polytools.py:312, in Poly._from_expr(cls, rep, opt)
    310 """Construct a polynomial from an expression. """
    311 rep, opt = _dict_from_expr(rep, opt)
--> 312 return cls._from_dict(rep, opt)

File /usr/lib/python3/dist-packages/sympy/polys/polytools.py:249, in Poly._from_dict(cls, rep, opt)
    246 gens = opt.gens
    248 if not gens:
--> 249     raise GeneratorsNeeded(
    250         "Cannot initialize from 'dict' without generators")
    252 level = len(gens) - 1
    253 domain = opt.domain

GeneratorsNeeded: Cannot initialize from 'dict' without generators

In [3]: p = Poly('a')

In [4]: p-p
Out[4]: Poly(0, a, domain='ZZ')

Best regards, Georges.

[smichr]

When 'a-a' is parsed, the result is 0 and the history of how it came to be so is lost. It might be possible to sympify with evaluate false and then let Polys handle that expression.