Lambda sorting issue in Python3

Question

My python2.7 code contains lambda sort technique to sort fetched elements according to the defined order. Yesterday i migrated from python 2.7 to python 3.6, The problem is after migration the line where i wrote lambda sort is throwing error in program. For migration from py2 to py3 i used lib2to3. I believe this is cause of some syntax miss match in py3, But couldn't figure it out. The error log is as mentioned below-

My code Fragment:


 property_sort_order = ['field', 'rename', 'selected', 'description']
        node_property_value_list = []
        node_property_value_list = node_property_value[nodekey]
        node_property_value_list = [OrderedDict(
             sorted(item.iteritems(), key=lambda (k, v): property_sort_order.index(k)))
                                for item in node_property_value_list]
                                    for item in node_property_value_list]
        #print node_property_value_list
        node_property_value[nodekey] = node_property_value_list

Errror:
        for item in node_property_value_list]
    TypeError: <lambda>() missing 1 required positional argument: 'v'

ephemient · Accepted Answer · 2018-01-12 06:48:35Z

2

In Python 2 it is possible to unpack the (k, v) tuple in lambda/function arguments, but not in Python 3. You can either unpack the lambda inline,

lambda item: property_sort_order.index(item[0])

or unpack as a separate statement (which can't be done in a lambda).

def sort_key(item):
    k, v = item
    return property_sort_order.index(k)
sorted(..., key=sort_key)

answered Jan 12, 2018 at 6:48

ephemient

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1 Comment

frank hk Over a year ago

Its not helped @ephemient can u clarify this answer

Nathan Vērzemnieks · Accepted Answer · 2018-01-12 06:50:16Z

1

You need to make key=lambda k, v: property_sort_order.index(k) into key=lambda k_v:property_sort_order.index(k_v[0]) - the lambda is getting passed a single tuple, which you can't destructure in python3.6.

answered Jan 12, 2018 at 6:50

Nathan Vērzemnieks

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4 Comments

frank hk Over a year ago

Its not happening @Nathan Vērzemnieks

Nathan Vērzemnieks Over a year ago

Sorry, not sure what you need. I wrote exactly what string to replace and how. What exactly is "not happening"? Do you get a different error?

frank hk Over a year ago

sry mr @Nathan Vērzemnieks. i was wrong. your answer was helpful,tq

Nathan Vērzemnieks Over a year ago

No problem! Glad it's sorted out.

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