flatten.py

#!/usr/bin/env python


def flatten(d_list):
    """Flatten arbitrarily nested arrays of integers/strings.
    Raises ValueError if the argument is not a list."""
    if not isinstance(d_list, list):
        raise ValueError("Argument must be a list type")
    # Make a copy to preserve the original
    d_list = list(d_list)
    i = 0
    while i < len(d_list):
        # Nothing to do if it is not a list
        if not isinstance(d_list[i], list):
            i += 1
            continue
        # Extract the elements from the array and replace the element
        d_list[i:i+1] = d_list[i]
    return d_list

if __name__ == "__main__":
    test_data = [[1, 2, [3]], 4]
    print "Original array: %s" % test_data
    print "Flattened array: %s" % flatten(test_data)

test_flatten.py

import pytest
import flatten


def test_flatten_2000_level():
    nested = []
    # We create a 2000 deep nested list
    for elem in xrange(2000):
        nested = [nested, elem]
    result = flatten.flatten(nested)
    expected = range(2000)
    assert cmp(result, expected) == 0


def test_flatten_empty():
    nested = []
    result = flatten.flatten(nested)
    expected = []
    assert cmp(result, expected) == 0


def test_invalid_type():
    data = "bad data"
    with pytest.raises(ValueError):
        flatten.flatten(data)

Running the test requires pytest. (pip install pytest && py.test)