Bug#1052596: python3-sympy: sympy.Poly() fails with: GeneratorsNeeded: Cannot initialize from 'dict' without generators

[Kingsley G. Morse Jr.]

Package: python3-sympy
Version: 1.12-6
Severity: normal

Thank you very mcuh for maintaining Debian Linux's
"python3-sympy" package.

It's interesting and sophisticated.

Please allow me to draw your attention a possible
bug.

I can elicit it with just 2 lines of code.

Here is how:

    $ ipython3

    In [1]: from sympy import Poly

    In [2]: Poly('((a - b) - (((a - b)  +  (c - d)  +  (e - f))/3)) + ((c - d) - (((a - b)  +  (c - d)  +  (e - f))/3)) + (e - f) - ((a - b)  +  (c - d)  +  (e - f))/3')

sympy.Poly() complains with

    ---------------------------------------------------------------------------
    GeneratorsNeeded                          Traceback (most recent call last)
    Cell In [2], line 1
    ----> 1 Poly('((a - b) - (((a - b)  +  (c - d)  +  (e - f))/3)) + ((c - d) - (((a - b)  +  (c - d)  +  (e - f))/3)) + (e - f) - ((a - b)  +  (c - d)  +  (e - f))/3')

    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


I expected it to work.

Especially because simply deleting the trailing "/3" from the input does, like this...

    In [3]: Poly('((a - b) - (((a - b)  +  (c - d)  +  (e - f))/3)) + ((c - d) - (((a - b)  +  (c - d)  +  (e - f))/3)) + (e - f) - ((a - b)  +  (c - d)  +  (e - f))')
    
    Out[3]: Poly(-2/3*a + 2/3*b - 2/3*c + 2/3*d - 2/3*e + 2/3*f, a, b, c, d, e, f, domain='QQ')


I suppose an approximate workaround is to replace the trailing

    .../3

with a leading

    0.333*...

like this...

    In [4]: Poly('((a - b) - (((a - b)  +  (c - d)  +  (e - f))/3)) + ((c - d) - (((a - b)  +  (c - d)  +  (e - f))/3)) + (e - f) - 0.333*((a - b)  + 
       ...:  (c - d)  +  (e - f))')
    Out[4]: Poly(0.000333333333333297*a - 0.000333333333333297*b + 0.000333333333333297*c - 0.000333333333333297*d + 0.000333333333333297*e - 0.000333333333333297*f, a, b, c, d, e, f, domain='RR')


I also noticed that replacing the trailing "/3" with division by 1, 2, 4, 5 and 6 worked.

Thanks again for maintaining Debian's python3-sympy package.

Kind regards,
Kingsley


-- System Information:
Debian Release: bullseye/sid
  APT prefers unstable-debug
  APT policy: (500, 'unstable-debug'), (500, 'unstable')
merged-usr: no
Architecture: i386 (i686)

Kernel: Linux 4.4.0-1-686-pae (SMP w/2 CPU threads)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8), LANGUAGE not set
Shell: /bin/sh linked to /bin/bash
Init: systemd (via /run/systemd/system)

Versions of packages python3-sympy depends on:
ii  python3         3.11.1-3
ii  python3-mpmath  1.2.1-1

Versions of packages python3-sympy recommends:
ii  isympy-common  1.7.1-2
ii  python3-numpy  1:1.24.2-1
ii  python3-pil    9.4.0-1.1+b1

Versions of packages python3-sympy suggests:
pn  dvipng               <none>
pn  python-sympy-doc     <none>
pn  texlive-fonts-extra  <none>

-- no debconf information