Why do parentheses in a lambda function cause syntax error on Python 3?

Question

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.

Asked By: Zack

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Source

Answer 1

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])

Answered By: unutbu

Why do parentheses in a lambda function cause syntax error on Python 3?

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