Index out of range when using lambda [duplicate]

Question

I'm a little confused about lambda operators in Python right now. The following (working) code sorts a list of tuples after the number of occurrences of a tuples first element over the first elements of all tuples:

tuples = [(2, 1, 8, 4), (3, 4, 8, 1), (3, 8, 1, 4), (4, 1, 8, 3),
              (4, 8, 1, 3), (8, 8, 3, 1), (8, 1, 3, 4), (8, 4, 1, 3),
              (8, 4, 3, 1)]

temp = list(zip(*tuples))    
tuples.sort(key=lambda x: temp[0].count(x[0])
                ,reverse=True)

print(tuples)

However, if I now try to skip the creating of "temp", i.e write this:

tuples = [(2, 1, 8, 4), (3, 4, 8, 1), (3, 8, 1, 4), (4, 1, 8, 3),
              (4, 8, 1, 3), (8, 8, 3, 1), (8, 1, 3, 4), (8, 4, 1, 3),
              (8, 4, 3, 1)]

tuples.sort(key=lambda x: list(zip(*tuples))[0].count(x[0])
                ,reverse=True)

print(tuples)

It throws an error:

Traceback (most recent call last):
  File "E:\Python-Programms\Sorting", line 6, in <module>
    ,reverse=True)
  File "E:\Python-Programms\Sorting", line 5, in <lambda>
    tuples.sort(key=lambda x: list(zip(*tuples)) [0].count(x[0])
IndexError: list index out of range

Why does this error occur?

Answer 1

If you used a vanilla function and printed the list while it is being sorted, you'll notice the list is cleared out during the sort operation (AFAIK this applies to CPython). There isn't an index zero for an empty list:

def f(x):
  print (tuples)
  return ...

tuples.sort(key=f ,reverse=True)

[]
[]
[]
[]
[]
[]
[]
[]
[]

A peek into the CPython source leaves us with a useful comment that explains this behaviour:

static PyObject *
list_sort_impl(PyListObject *self, PyObject *keyfunc, int reverse)
{
    ...
    /* The list is temporarily made empty, so that mutations performed
     * by comparison functions can't affect the slice of memory we're
     * sorting (allowing mutations during sorting is a core-dump
     * factory, since ob_item may change).
     */
    ...
}

---

To your original problem, instead of calling list.count repeatedly, which is very inefficient, you can build a counter and then use that for sorting:

from collections import Counter

c = Counter([x[0] for x in tuples])
tuples.sort(key=lambda x: c[x[0]], reverse=True)