Question about __str()__ for custom class using sympy expressions

[Alan Bromborsky]

I am coding a str() function for a custom object.  The object is a liner
differential operator that consists of a linear combination of sympy
expressions and partial derivatives (defined in a class I have defined
and have their own str() function).  The problem is to know when to
represent a term in the linear differential operator str() with

str(sympy expression)+str(partial derivative)

or

'('+str(sympy expression)+')'+str(partial derivative)

that is does str(sympy expression) have low level instances of '+' or
'-' in it as opposed to '+' or '-' being used in functions within the
expression such as sin(x+y) or exp(-x)?

Any suggestions would be appreciated before I resort to using grep on 
str(sympy expression).

[Sangyub Lee]

I think that you should take a look at the StrPrinter interface and consider implementing a custom StrPrinter. You may need to implement a method like _print_LinearDifferentialOperator.

While simple string concatenation or interpolation might work for basic expressions, these approaches have significant limitations when dealing with expressions that require proper handling of operator precedence and parenthesizing.