Python 3.2 Lambda Syntax Error

Zack picture Zack · Mar 29, 2013 · Viewed 35.8k times · Source
def sort_dictionary( wordDict ):
    sortedList = []
    for entry in sorted(wordDict.iteritems(), key = lambda (k, v): (-v, k) ):
        sortedList.append( entry )

    return sortedList

The function would be receiving a dictionary containing information such as: { 'this': 1, 'is': 1, 'a': 1, 'large': 2, 'sentence': 1 } I would like to have it generate a list of lists, with the elements ordered first by the dictionary's values from Largest to Smallest, then by the keys alphabetically.

The function works fine when run with python 2.7.2, but I receive the error:

  File "frequency.py", line 87
    for entry in sorted(wordDict.iteritems(), key = lambda (k, v): (-v, k)):
                                                           ^
SyntaxError: invalid syntax

when I run the program with python 3.2.3. I have been searching all over for a reason why, or syntax differences between 2.7 and 3.2, and have come up with nothing. Any help or fixes would be greatly appreciated.

Answer

unutbu picture unutbu · Mar 29, 2013

Using parentheses to unpack the arguments in a lambda is not allowed in Python3. See PEP 3113 for the reason why.

lambda (k, v): (-v, k)

Instead use:

lambda kv: (-kv[1], kv[0])