Can you pass a dictionary when replacing strings in Python?

Question

In PHP, you have preg_replace($patterns, $replacements, $string), where you can make all your substitutions at once by passing in an array of patterns and replacements.

What is the equivalent in Python?

I noticed that the string and re functions replace() and sub() don't take dictionaries...

Edited to clarify based on a comment by rick: the idea is to have a dict with keys to be taken as regular expression patterns, such as '\d+S', and (hopefully) constant string values (hopefully w/o backreferences). Now editing my answer accordingly (i.e. to answer the actual question).

Answer 1

closest is probably:

somere.sub(lambda m: replacements[m.group()], text)

for example:

>>> za = re.compile('z\w')
>>> za.sub(lambda m: dict(za='BLU', zo='BLA')[m.group()], 'fa za zo bu')
'fa BLU BLA bu'

with a .get instead of []-indexing if you want to supply a default for matches that are missing in replacements.

Edit: what rick really wants is to have a dict with keys to be taken as regular expression patterns, such as '\d+S', and (hopefully) constant string values (hopefully w/o backreferences). The cookbook recipe can be adapted for this purpose:

def dict_sub(d, text): 
  """ Replace in 'text' non-overlapping occurences of REs whose patterns are keys
  in dictionary 'd' by corresponding values (which must be constant strings: may
  have named backreferences but not numeric ones). The keys must not contain
  anonymous matching-groups.
  Returns the new string.""" 

  # Create a regular expression  from the dictionary keys
  regex = re.compile("|".join("(%s)" % k for k in d))
  # Facilitate lookup from group number to value
  lookup = dict((i+1, v) for i, v in enumerate(d.itervalues()))

  # For each match, find which group matched and expand its value
  return regex.sub(lambda mo: mo.expand(lookup[mo.lastindex]), text)

Example use:

  d={'\d+S': 'wot', '\d+T': 'zap'}
  t='And 23S, and 45T, and 66T but always 029S!'
  print dict_sub(d, t)

emits:

And wot, and zap, and zap but always wot!

You could avoid building lookup and just use mo.expand(d.values()[mo.lastindex-1]), but that might be a tad slow if d is very large and there are many matches (sorry, haven't precisely measured/benchmarked both approaches, so this is just a guess;-).

Answer 2

It's easy enough to do this:

replacements = dict(hello='goodbye', good='bad')
s = "hello, good morning";
for old, new in replacements.items():
    s = s.replace(old, new)

You will find many places where PHP functions accept an array of values and there is no direct Python equivalent, but it is much easier to work with arrays (lists) in Python so it is less of an issue.