Understanding lambda functions in python

Question

I am new to python and I created a script to sort through "show ip accounting" information from a cisco router. The script reads a file and breaks up every line into a list, then it creates a list of every line. So I end up with a list of lists:

list a = [[192.168.0.1,172.16.0.1,3434,12222424],[192.168.2.1,172.12.0.1,33334,12667896722424]]

I want to be able to sort by the third column or 4th columns of the list within the list.

I was able to do it using a lambda function, but my question is how to duplicate this using a standard function?

here is my code below:

from sys import argv

script, option, filename = argv
a=[]
b=[]

def openfile(filename):
  file = open(filename)
  for line in file:
    if not line.startswith("  "):
      a.append((line.split()))
  return a

def sort(a,num):
  b = sorted(a, reverse=True, key=lambda x: int(x[num]))
  return b

def top5(b):
  print "Source     Destination Packets     Bytes"
  for i in b[:4]: 
    print i[0]+"    "+i[1]+"    "+i[2]+"        "+i[3]

def main(option):
  a = openfile(filename)
  if option == "--bytes":
    b = sort(a,3)
    top5(b)
  elif option == "--packets":
    b = sort(a,2)
    top5(b)
  else:
    print """
    Not a valid switch, 
    --bytes to sort by bytes 
    --packets to sort by packets."""


main(option)

So my question is how can I duplicate the lambda function as a standard custom sort function? I am trying to figure out how this works.

b = sorted(a, reverse=True, key=lambda x: int(x[num]))

Check out the Python Sorting HOWTO's section on "key functions". — Francis Avila
– Francis Avila, Commented Jan 25, 2013 at 23:06

NPE · Accepted Answer · 2013-01-25 22:47:10Z

4

how can I duplicate the lambda function as a standard custom sort function?

Do you mean this:

def sort(a, num):
  def key(x):
    return int(x[num])
  return sorted(a, reverse=True, key=key)

or perhaps this:

from functools import partial

def key(num, x):
  return int(x[num])

def sort(a, num):
  return sorted(a, reverse=True, key=partial(key, num))

?

edited Jan 25, 2013 at 22:47

answered Jan 25, 2013 at 22:41

NPE

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10 Comments

georg Over a year ago

I guess they're actually looking for itemgetter.

NPE Over a year ago

@thg435: Maybe. Hard to say really. "Standard custom function" is a bit of an oxymoron.

bufo333 Over a year ago

OK, so I tried both snippets of code and they both work, My question is when you made the function key inside of the function sort, what value does it get for x, if you don't send it any values when you call it with key=key? I am trying to understand how this works,

NPE Over a year ago

@user2010044: x refers to the list item whose key is being requested. sorted() calls key() for every element in the list, and then uses the keys it got to do the actual sorting.

abarnert Over a year ago

@user2010044: A key function doesn't necessarily have to be defined inside the calling function. But it you want to create a closure over a local variable inside that function, then you need to define it there (or use functools.partial, or wrap it in a local function or a lambda, etc.).

|

cyfur01 · Accepted Answer · 2013-01-25 23:07:39Z

1

Python provides operator.itemgetter for doing this kind of thing:

def sort(a, num):
    return sorted(a, reverse=True, key=operator.itemgetter(num))

Edit

As @NPE pointed out, that doesn't convert the key to an int for sorting. For that, you're best off to stick with a lambda.

edited Jan 25, 2013 at 23:07

answered Jan 25, 2013 at 22:49

cyfur01

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2 Comments

cyfur01 Over a year ago

@NPE True. I missed the cast to int

abarnert Over a year ago

You could do compose(int, operator.itemgetter(num)), if you had a compose function from somewhere (e.g., functional). But I don't think this is what the OP is asking for.

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