A (deceptively tricky?) Python question

I am writing a Scrabble AI, and I want to handle blank tiles. Right now, I want to expand a string containing blank tiles into a list of strings composed of words created by all possible interpretations of the blanks.

So, "H_LLO" ==> ["HALLO", "HBLLO", ... "HZLLO"] (length is 26)

However, Scrabble has 2 blank tiles, so I can't just do a simple for loop and replace.

For example, "H_LL_" ==> ["HALLA", "HALLB", ... "HBLLA", ... "HZLLZ"] (length is 26x26 = 676)

What is the cleanest way to do this? The best I could come up with is the following, but I don't much like it:

  import re
  from itertools import product

  w = "H_LL_"
  expanded = []
  product_args = list()
  for _ in xrange(w.count("_")):
      product_args.append( map(chr, range(65, 91)) )
  for repl_str in product(*tuple(product_args)):
      wtemp = w
      for c in repl_str:
          wtemp = re.sub('_', c, wtemp)
      expanded.append(wtemp)