How do you find the minimum window substring?

from collections import Counter

def min_window(s, t):
    if not t or not s:
        return ""
    need = Counter(t)
    required = len(need)
    formed = 0
    window = {}
    best = (float('inf'), 0, 0)
    left = 0
    for right, ch in enumerate(s):
        window[ch] = window.get(ch, 0) + 1
        if ch in need and window[ch] == need[ch]:
            formed += 1
        while formed == required:
            if right - left + 1 < best[0]:
                best = (right - left + 1, left, right)
            lc = s[left]
            window[lc] -= 1
            if lc in need and window[lc] < need[lc]:
                formed -= 1
            left += 1
    return "" if best[0] == float('inf') else s[best[1]:best[2] + 1]


# --- demo ---
print(min_window("ADOBECODEBANC", "ABC"))   # BANC