python 3: sorting with a comparison function

pruebauno at pruebauno at
Fri Oct 10 19:22:28 CEST 2008

On Oct 10, 8:35 am, Kay Schluehr <kay.schlu... at> wrote:
> On 9 Okt., 22:36, bearophileH... at wrote:
> > Yes, that's a wonderful thing, because from the code I see around
> > 99.9% of people see the cmp and just use it, totally ignoring the
> > presence of the 'key' argument, that allows better and shorter
> > solutions of the sorting problem.
> Me too because I don't get this:
> "key specifies a function of one argument that is used to extract a
> comparison key from each list element: key=str.lower. The default
> value is None."
> Kay

Don't know if further explanation is needed, but here is the deal:

cmp is a function that receives two values and you return -1, 0 or 1
depending if the first is smaller, equal or bigger. 99% of the time
you will do some operation on the values that come in and then do a if
statement with ">" or "<" and return -1,0,1.

key is a function that receives one value and you return the value
that you would normally compare against.

Let me show an example:

>>> data=[(4,'v'),(2,'x'),(1,'a')]
>>> sorted(data)
[(1, 'a'), (2, 'x'), (4, 'v')]

OK, we sorted the data, but What if we want to sort by the letter
instead of the number? Let's use cmp:

>>> def comp(x, y):
      if key_of_x < key_of_y:
        return -1
      elif key_of_x > key_of_y:
        return 1
        return 0 #key_of_x == key_of_y

>>> sorted(data,cmp=comp)
[(1, 'a'), (4, 'v'), (2, 'x')]

Very well, so how do we do this using key?

>>> def keyfunc(x):
      return key_of_x

>>> sorted(data,key=keyfunc)
[(1, 'a'), (4, 'v'), (2, 'x')]

Same output. Very good.

(Of course a smart python developer would use the operator module so
he doesn't even have to write keyfunc but this was just an example)

In summary to transform most cmp functions to a key function you just
take the code that calculates the first value to be compared and leave
out the rest of the logic.

Hope that was helpful.

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