Printing of UnevaluatedExpr needs parenthesis around Add terms

[bjodah]

Floating point "add" is non-associative, i.e. (x+y)+z often differs from x+(y+z). In SymPy Add (re)orders its arguments (for good reasons), but for representing e.g. an algorithm in finite precision this creates a problem (see e.g. Kahan summation on wikipedia).

Typically we use UnevaluatedExpr for situations like this:

>>> from sympy import UnevaluatedExpr as ue
>>> from sympy.abc import k,l,m
>>> ue(k+m)+l
l + k + m
>>> ue(k+m)+l + k
k + l + k + m

but, as you see the printer is not emitting the required parenthesis to make this useful for representing non-associative add.

Note that SymEngine gets this right:

>>> from symengine import Symbol as S, UnevaluatedExpr as UE
>>> x,y,z = map(S, 'xyz')
>>> UE(y + x) + z
z + (x + y)

I propose that we fix this in the printers (I suspect updating precedence for UnevaluatedExpr might be enough, I haven't tried yet though). At the very least, the code printers should output parenthesis, but I think it's reasonable for the StrPrinter to do so as well?

[vsuharnikov]

It seems, there is a related issue with LaTeX:

import sympy
from IPython.display import display
from sympy.abc import x, y, z
expr = x - sympy.UnevaluatedExpr(y - z)
print(expr) # prints: x - (y - z)
display(expr) # prints: x - y - z
print(sympy.__version__) # 1.8

[bjodah]

thanks for the report @vsuharnikov, have you tried with the master branch (or 1.9-rc1)?

[vsuharnikov]

@bjodah Same issue:

x - (y - z)
𝑥−𝑦−𝑧 
1.9rc1

[bjodah]

Thanks!