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

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.

It's worth noting that putting spaces between the = and an argument is against PEP-8's recommendations. — Gareth Latty
– Gareth Latty, Commented Mar 29, 2013 at 22:16

unutbu · Accepted Answer · 2013-03-30 16:35:02Z

66

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])

edited Mar 30, 2013 at 16:35

answered Mar 29, 2013 at 22:13

unutbu

886k197 gold badges1.9k silver badges1.7k bronze badges

Sign up to request clarification or add additional context in comments.

7 Comments

Blender Over a year ago

Why was this syntax removed?

Gareth Latty Over a year ago

@Blender It was shown to be little used, and made things like function annotations awkward, as well as adding complexity to some introspection stuff. It's also very easily replaced. See PEP-3113.

Zack Over a year ago

Thank you so much! this works perfectly. Also, not that it matters, but would you happen to know why I had to change iteritems() to items() to run it with python3 as well?

Gareth Latty Over a year ago

@Zack iteritems() was there as items() in 2.x returns a list, which is memory-inefficient. In 3.x, items() returns an iterator, removing the need for iteritems().

MEMark Over a year ago

@NarenBabuR That lambda would require two different arguments when called, not a single tuple.

|

Collectives™ on Stack Overflow

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

1 Answer 1

7 Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

7 Comments

Linked

Related