changing ** operator to power function using parsing?

问题

My requirement is to change ** operator to power function

For example

1.Input -"B**2"
Output - power(B,2)
2."B**2&&T**2*X"
Output - power(B,2)

I have wrote following regular expression to address that problem

   rx=r"([a-zA-Z0-9]+)\*\*([a-zA-Z0-9()]+)"
        result = regex.sub(rx, r"power(\1,\2)", expression, 0, regex.IGNORECASE | regex.MULTILINE)

But above code successfully converting expression similar to the example 1 and example 2, but failed to convert expression like (a+1)**2 or ((a+b)*c)**2. I realized regular expression is not the best way to handle such scenarios. Instead of that parsing will be the best way to handle that. I bit new to python .Please guide me how to approach to solve this problem.

回答1:

This sounds very familiar, I think I dealt with a similar problem on the pyparsing mailing list, but I can't find it at the moment. But try something like this:

from pyparsing import *

# define some basic operand expressions
number = Regex(r'\d+(\.\d*)?([Ee][+-]?\d+)?')
ident = Word(alphas+'_', alphanums+'_')

# forward declare our overall expression, since a slice could 
# contain an arithmetic expression
expr = Forward()
slice_ref = '[' + expr + ']'

# define our arithmetic operand
operand = number | Combine(ident + Optional(slice_ref))

# parse actions to convert parsed items
def convert_to_pow(tokens):
    tmp = tokens[0][:]
    ret = tmp.pop(-1)
    tmp.pop(-1)
    while tmp:
        base = tmp.pop(-1)
        # hack to handle '**' precedence ahead of '-'
        if base.startswith('-'):
            ret = '-pow(%s,%s)' % (base[1:], ret)
        else:
            ret = 'pow(%s,%s)' % (base, ret)
        if tmp:
            tmp.pop(-1)
    return ret

def unary_as_is(tokens):
    return '(%s)' % ''.join(tokens[0])

def as_is(tokens):
    return '%s' % ''.join(tokens[0])

# simplest infixNotation - may need to add a few more operators, but start with this for now
arith_expr = infixNotation( operand,
    [
    ('-', 1, opAssoc.RIGHT, as_is),
    ('**', 2, opAssoc.LEFT, convert_to_pow),
    ('-', 1, opAssoc.RIGHT, unary_as_is),
    (oneOf("* /"), 2, opAssoc.LEFT, as_is),
    (oneOf("+ -"), 2, opAssoc.LEFT, as_is),
    ])

# now assign into forward-declared expr
expr <<= arith_expr.setParseAction(lambda t: '(%s)' % ''.join(t))

assert "2**3" == expr
assert "2**-3" == expr

# test it out
tests = [
    "2**3",
    "2**-3",
    "2**3**x5",
    "2**-3**x6[-1]",
    "2**-3**x5+1",
    "(a+1)**2",
    "((a+b)*c)**2",
    "B**2",
    "-B**2",
    "(-B)**2",
    "B**-2",
    "B**(-2)",
    "B**2&&T**2*X",
    ]

x5 = 2
a,b,c = 1,2,3
B = 4
x6 = [3,2]
for test in tests:
    print test
    xform = expr.transformString(test)[1:-1]
    print xform
    print '**' not in xform and eval(xform) == eval(test)
    print

prints:

2**3
pow(2,3)
True

2**-3
pow(2,-3)
True

2**3**x5
pow(2,pow(3,x5))
True

2**-3**x6[-1]
pow(2,-pow(3,x6[((-1))]))
True

2**-3**x5+1
pow(2,-pow(3,x5))+1
True

(a+1)**2
pow((a+1),2)
True

((a+b)*c)**2
pow(((a+b)*c),2)
True

B**2
pow(B,2)
True

-B**2
(-pow(B,2))
True

(-B)**2
pow(((-B)),2)
True

B**-2
pow(B,-2)
True

B**(-2)
pow(B,((-2)))
True

B**2&&T**2*X
pow(B,2))&&(pow(T,2)*X
Traceback (most recent call last):
  File "convert_to_pow.py", line 85, in <module>
    print '**' not in xform and eval(xform) == eval(test)
  File "<string>", line 1
    pow(B,2))&&(pow(T,2)*X
            ^
SyntaxError: invalid syntax

If you have more corner cases in the code that you are converting, it will probably just need a bit more tweaking of the operand expression, or adding more operators (like &&) to the infixNotation expression.

(Note that you have to convert a**b**c as if written a**(b**c), as chained exponentiation is evaluated right-to-left, not left-to-right.)

EDIT:

Introduced hack to properly handle precedence between '-' and '**'. Expanded tests to actually evaluate before/after strings. This looks more solid now.

回答2:

If you need to handle general Python syntax, take a look at the ast module. You'd probably want to ast.parse the string into an abstract syntax tree and use an ast.NodeTransformer to replace nodes. There's no ast.unparse, though, so you'd need to either use a third-party dependency or write the unparsing handling yourself.

If you just need to handle arithmetic operations (and particularly if you want to reject other Python syntax), you might want to write your own grammar with something like pyparsing.

来源：https://stackoverflow.com/questions/37077052/changing-operator-to-power-function-using-parsing