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 get_year function in your example uses an automatically unpacked tuple parameter (this is the feature gone from Python 3). To call it, you give it a single parameter, and that parameter is expected to be a sequence containing four values.

# Invocation
my_input = [2013, 'a', 'b', 'c'] # sequence does NOT have to be a tuple!
my_year = get_year(my_input) # returns 2013

To rewrite this for Python 3 but not alter the invocation (in other words, to not break existing code which calls get_year):

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

The above is essentially what tuple unpacking is doing for you automatically. In this particular case, you could simply write

def get_year(input_sequence):
    return input_sequence[0]

For further information, read PEP 3113.