It's a bit nasty, but I suppose one solution to this might be creating a function like element.alterChild(oldElement, func).

I'm envisaging that alterChild(oldElement, func) would recurse down the ParseElement tree until it finds any instance of oldElement, and then replaces it with func(oldElement), allowing the user to attach a new setParseAction on it. I guess there would also have to be the facility to specify just one branch of the element tree in case they don't want to modify all instances of that class.

For example, it would work like this for the above example:

def python2Eval(expr):
    py2Expression = expression.copy().alterChild(op, lambda oldOp: oldOp.copy().setParseAction(lambda s, lok, toks: {
        '+': operator.add,
        '-': operator.sub,
        '*': operator.mul,
        '/': operator.floordiv
    }[toks[0]])
    
    return py2Expression.parseString(expr)

What if you use a Forward for the binary operator, as a late-binding expression to be defined just before parsing:

from pyparsing import *
import operator

number = Regex('[0-9]').setParseAction(lambda s, lok, toks: int(toks[0]))

# use oneOf, not Or
opSymbol = oneOf(['+', '-', '/', '*'])

py3op = opSymbol().setParseAction(lambda s, lok, toks: {
        '+': operator.add,
        '-': operator.sub,
        '*': operator.mul,
        '/': operator.truediv
    }[toks[0]])

# Use floordiv instead of truediv because Python 2 sucks
py2op = opSymbol().setParseAction(lambda s, lok, toks: {
        '+': operator.add,
        '-': operator.sub,
        '*': operator.mul,
        '/': operator.floordiv
    }[toks[0]])

# define operator using a Forward - an expression to be named later
op_fwd = Forward()
expression = (number + op_fwd + number).setParseAction(lambda s, loc, toks: toks[1](toks[0], toks[2]))

def python3Eval(expr):
    op_fwd <<= py3op
    return expression.parseString(expr)

def python2Eval(expr):
    op_fwd <<= py2op
    return expression.parseString(expr)

Would this work for you?

That's certainly an interesting solution. It's a tad ugly because I have to define the Forward() (op_fwd) and the actual tokens (opSymbol) as separate variables, but maybe that's fine (?). I'll try it out and see if I have any issues. Thanks for the answer!