python – How do operator.itemgetter() and sort() work?

python – How do operator.itemgetter() and sort() work?

Looks like youre a little bit confused about all that stuff.

operator is a built-in module providing a set of convenient operators. In two words operator.itemgetter(n) constructs a callable that assumes an iterable object (e.g. list, tuple, set) as input, and fetches the n-th element out of it.

So, you cant use key=a[x][1] there, because python has no idea what x is. Instead, you could use a lambda function (elem is just a variable name, no magic there):

a.sort(key=lambda elem: elem[1])

Or just an ordinary function:

def get_second_elem(iterable):
    return iterable[1]

a.sort(key=get_second_elem)

So, heres an important note: in python functions are first-class citizens, so you can pass them to other functions as a parameter.

Other questions:

  1. Yes, you can reverse sort, just add reverse=True: a.sort(key=..., reverse=True)
  2. To sort by more than one column you can use itemgetter with multiple indices: operator.itemgetter(1,2), or with lambda: lambda elem: (elem[1], elem[2]). This way, iterables are constructed on the fly for each item in list, which are than compared against each other in lexicographic(?) order (first elements compared, if equal – second elements compared, etc)
  3. You can fetch value at [3,2] using a[2,1] (indices are zero-based). Using operator… Its possible, but not as clean as just indexing.

Refer to the documentation for details:

  1. operator.itemgetter explained
  2. Sorting list by custom key in Python

Answer for Python beginners

In simpler words:

  1. The key= parameter of sort requires a key function (to be applied to be objects to be sorted) rather than a single key value and
  2. that is just what operator.itemgetter(1) will give you: A function that grabs the first item from a list-like object.

(More precisely those are callables, not functions, but that is a difference that can often be ignored.)

python – How do operator.itemgetter() and sort() work?

You are asking a lot of questions that you could answer yourself by reading the documentation, so Ill give you a general advice: read it and experiment in the python shell. Youll see that itemgetter returns a callable:

>>> func = operator.itemgetter(1)
>>> func(a)
[Paul, 22, Car Dealer]
>>> func(a[0])
8

To do it in a different way, you can use lambda:

a.sort(key=lambda x: x[1])

And reverse it:

a.sort(key=operator.itemgetter(1), reverse=True)

Sort by more than one column:

a.sort(key=operator.itemgetter(1,2))

See the sorting How To.

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