How to pass a variable to a re.sub callback?

Question

I am using a re.sub callback to replace substrings with random values, but I would like the random values to be the same across different strings. Since the re.sub callback does

Answer 1

You could use a function object.

 class A(object):
  def __init__(self, mappings):
    self.mappings = mappings
  def __call__(self, match):
    return str(eval(match.group(0)[2:-1], self.mappings))

 evaluate = A({'A': 1, 'B': 2})

Answer 2

You could create a closure.

def evaluator(mappings):
  def f(match):
    return str(eval(match.group(0)[2:-1], mappings))
  return f

evaluate = evaluator({'A': 1, 'B': 2})

Since f is just a single statement, you could simply use lambda:

def evaluator(mappings):
  return lambda match: str(eval(match.group(0)[2:-1], mappings))

Answer 3

The variant I would recommend:

mappings = {'A': 1, 'B': 2}  # Or whatever ...
def evaluate(match):
    return str(eval(match.group(0)[2:-1], mappings))
newstring = sub(r'\#\{([^#]+)\}', evaluate, string)

i.e. simply put the def evaluate(..) just before the sub(), as a local function.

Answer 4

The easiest way I guess is to make use of functools.partial, which allows you create a "partially evaluated" function:

from functools import partial

def evaluate(match, mappings):
    return str(eval(match.group(0)[2:-1], mappings))

mappings = {'A': 1, 'B': 2}  # Or whatever ...

newstring = sub(r'\#\{([^#]+)\}', partial(evaluate, mappings=mappings), string)