Why do some Python functions have an extra set of parenthesis around the argument list?

Question

I've seen some Python functions written like this:

def get_year((year,prefix,index,suffix)):
  return year

How does that differ (if at all) from other functions without the extra parentheses like this:

def do_format(yr,pfx,id,sfx):
  return "%s %s %s/%s"%(yr, id, pfx, sfx)

Or is it just a matter of taste in styles, or if they differ, can get_year() be rewritten to be in the style of do_format() or vice-versa without effecting existing caller's syntax?

Answer 1

The first function takes a single tuple argument, whereas the second function takes 4 arguments. You can pass those parameters individually, or as a tuple with splat operator, that will unpack the tuple into individual parameters.

E.g:

# Valid Invocations
print do_format(*('2001', '234', '12', '123'))  # Tuple Unpacking
print do_format('2001', '234', '12', '123')     # Separate Parameter
print get_year(('2001', '234', '12', '123'))

# Invalid invocation. 
print do_format(('2001', '234', '12', '123'))   # Passes tuple