python - Access operator functions by symbol

Question

I need a function which takes one of python's operator symbols or keywords as a string, along with its operands, evaluates it, and returns the result. Like this:

>>> string_op('<=', 3, 3)
True
>>> string_op('|', 3, 5)
7
>>> string_op('and', 3, 5)
True
>>> string_op('+', 5, 7)
12
>>> string_op('-', -4)
4

The string cannot be assumed to be safe. I will be satisfied with just mapping the binary operators, but I'd be extra happy if I could get all of them.

My current implementation manually maps the symbols to the functions in the operator module:

import operator

def string_op(op, *args, **kwargs):
    """http://docs.python.org/2/library/operator.html"""
    symbol_name_map = {
        '<': 'lt',
        '<=': 'le',
        '==': 'eq',
        '!=': 'ne',
        '>=': 'ge',
        '>': 'gt',
        'not': 'not_',
        'is': 'is_',
        'is not': 'is_not',
        '+': 'add', # conflict with concat
        '&': 'and_', # (bitwise)
        '/': 'div',
        '//': 'floordiv',
        '~': 'invert',
        '%': 'mod',
        '*': 'mul',
        '|': 'or_', # (bitwise)
        'pos': 'pos_',
        '**': 'pow',
        '-': 'sub', # conflicts with neg
        '^': 'xor',
        'in': 'contains',
        '+=': 'iadd', # conflict with iconcat
        '&=': 'iand',
        '/=': 'idiv',
        '//=': 'ifloordiv',
        '<<=': 'ilshift',
        '%=': 'imod',
        '*=': 'imul',
        '|=': 'ior',
        '**=': 'ipow',
        '>>=': 'irshift',
        '-=': 'isub',
        '^=': 'ixor',
    }
    if op in symbol_name_map:
        return getattr(operator, symbol_name_map[op])(*args, **kwargs)
    else:
        return getattr(operator, op)(*args, **kwargs)

This solution fails on the overloaded operators -- add/concat and sub/neg. Checks could be added to detect those cases and detect types or count arguments to pick the right function name, but that feels a bit ugly. It's what I'll go with if I don't get a better idea here.

The thing that is bugging me is that python already does this. It already knows how to map symbols to operator functions, but so far as I can tell, that functionality is not exposed to the programmer. Seems like everything else in python, right down to the pickling protocol, is exposed to programmers. So where is this? or why isn't it?

Answer 1

Python does not map symbols to operator functions. It interprets symbols by calling special dunder methods.

For example, when you write 2 * 3, it doesn't call mul(2, 3); it calls some C code that figures out whether to use two.__mul__, three.__rmul__, or the C-type equivalents (the slots nb_multiply and sq_repeat are both equivalent to both __mul__ and __rmul__). You can call that same code from a C extension module as PyNumber_Multiply(two, three). If you look at the source to operator.mul, it's a completely separate function that calls the same PyNumber_Multiply.

So, there is no mapping from * to operator.mul for Python to expose.

If you want to do this programmatically, the best I can think of is to parse the docstrings of the operator functions (or, maybe, the operator.c source). For example:

runary = re.compile(r'Same as (.+)a')
rbinary = re.compile(r'Same as a (.+) b')
unary_ops, binary_ops = {}, {}
funcnames = dir(operator)
for funcname in funcnames:
    if (not funcname.startswith('_') and
        not (funcname.startswith('r') and funcname[1:] in funcnames) and
        not (funcname.startswith('i') and funcname[1:] in funcnames)):
        func = getattr(operator, funcname)
        doc = func.__doc__
        m = runary.search(doc)
        if m:
            unary_ops[m.group(1)] = func
        m = rbinary.search(doc)
        if m:
            binary_ops[m.group(1)] = func

I don't think this misses anything, but it definitely has some false positive, like "a + b, for a " as an operator that maps to operator.concat and callable( as an operator that maps to operator.isCallable. (The exact set depends on your Python version.) Feel free to tweak the regexes, blacklist such methods, etc. to taste.

However, if you really want to write a parser, you're probably better off writing a parser for your actual language than writing a parser for the docstrings to generate your language parser…

If the language you're trying to parse is a subset of Python, Python does expose the internals to help you there. See the ast module for the starting point. You might still be happier with something like pyparsing, but you should at least play with ast. For example:

sentinel = object()
def string_op(op, arg1, arg2=sentinel):
    s = '{} {}'.format(op, arg1) if arg2 is sentinel else '{} {} {}'.format(op, arg1, arg2)
    a = ast.parse(s).body

Print out a (or, better, ast.dump(a)), play with it, etc. You'll still need to map from _ast.Add to operator.add, however. But if you want to instead map to an actual Python code object… well, the code for that is available too.