import pytest

def mixstrings(s1, s2):
    """Given two strings s1 and s2, create and return a new
    string that contains the letters from s1 and s2 mixed:
    i.e. s[0] = s1[0], s[1] = s2[0], s[2] = s1[1],
    s[3] = s2[1], s[4] = s1[2], ...
    If one string is longer than the other, the extra
    characters in the longer string are ignored.

    Example:

    In []: mixstrings("Hello", "12345")
    Out[]: 'H1e2l3l4o5'
    """
    # what length to process
    n = min(len(s1), len(s2))
    # collect chars in this list
    s = []

    for i in range(n):
        s.append(s1[i])
        s.append(s2[i])
    return "".join(s)

def test_mixstrings_basics():
    assert mixstrings("hello", "world") == "hweolrllod"
    assert mixstrings("cat", "dog") == "cdaotg"

def test_mixstrings_empty():
    assert mixstrings("", "") == ""

def test_mixstrings_different_length():
    assert mixstrings("12345", "123") == "112233"
    assert mixstrings("", "hello") == ""

if __name__ == "__main__":                 
    pytest.main("-v " + __file__)