diff --git a/misc/python3/ratchet-passing b/misc/python3/ratchet-passing
index 0265fc53a..6832b7fa7 100644
--- a/misc/python3/ratchet-passing
+++ b/misc/python3/ratchet-passing
@@ -58,6 +58,14 @@ allmydata.test.test_pipeline.Pipeline.test_errors2
 allmydata.test.test_python3.Python3PortingEffortTests.test_finished_porting
 allmydata.test.test_python3.Python3PortingEffortTests.test_ported_modules_distinct
 allmydata.test.test_python3.Python3PortingEffortTests.test_ported_modules_exist
+allmydata.test.test_spans.ByteSpans.test_basic
+allmydata.test.test_spans.ByteSpans.test_large
+allmydata.test.test_spans.ByteSpans.test_math
+allmydata.test.test_spans.ByteSpans.test_overlap
+allmydata.test.test_spans.ByteSpans.test_random
+allmydata.test.test_spans.StringSpans.test_basic
+allmydata.test.test_spans.StringSpans.test_random
+allmydata.test.test_spans.StringSpans.test_test
 allmydata.test.test_time_format.TimeFormat.test_epoch
 allmydata.test.test_time_format.TimeFormat.test_epoch_in_London
 allmydata.test.test_time_format.TimeFormat.test_format_delta
diff --git a/newsfragments/3351.minor b/newsfragments/3351.minor
new file mode 100644
index 000000000..e69de29bb
diff --git a/src/allmydata/test/test_spans.py b/src/allmydata/test/test_spans.py
new file mode 100644
index 000000000..f62d6e684
--- /dev/null
+++ b/src/allmydata/test/test_spans.py
@@ -0,0 +1,617 @@
+"""
+Tests for allmydata.util.spans.
+"""
+
+from __future__ import print_function
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import unicode_literals
+
+from future.utils import PY2
+if PY2:
+    from builtins import filter, map, zip, ascii, chr, hex, input, next, oct, open, pow, round, super, bytes, dict, int, list, object, range, str, max, min  # noqa: F401
+
+from past.builtins import long
+
+import binascii
+import hashlib
+
+from twisted.trial import unittest
+
+from allmydata.util.spans import Spans, overlap, DataSpans
+
+
+def sha256(data):
+    """
+    :param bytes data: data to hash
+
+    :returns: a hex-encoded SHA256 hash of the data
+    """
+    return binascii.hexlify(hashlib.sha256(data).digest())
+
+
+class SimpleSpans(object):
+    # this is a simple+inefficient form of util.spans.Spans . We compare the
+    # behavior of this reference model against the real (efficient) form.
+
+    def __init__(self, _span_or_start=None, length=None):
+        self._have = set()
+        if length is not None:
+            for i in range(_span_or_start, _span_or_start+length):
+                self._have.add(i)
+        elif _span_or_start:
+            for (start,length) in _span_or_start:
+                self.add(start, length)
+
+    def add(self, start, length):
+        for i in range(start, start+length):
+            self._have.add(i)
+        return self
+
+    def remove(self, start, length):
+        for i in range(start, start+length):
+            self._have.discard(i)
+        return self
+
+    def each(self):
+        return sorted(self._have)
+
+    def __iter__(self):
+        items = sorted(self._have)
+        prevstart = None
+        prevend = None
+        for i in items:
+            if prevstart is None:
+                prevstart = prevend = i
+                continue
+            if i == prevend+1:
+                prevend = i
+                continue
+            yield (prevstart, prevend-prevstart+1)
+            prevstart = prevend = i
+        if prevstart is not None:
+            yield (prevstart, prevend-prevstart+1)
+
+    def __bool__(self): # this gets us bool()
+        return bool(self.len())
+
+    def len(self):
+        return len(self._have)
+
+    def __add__(self, other):
+        s = self.__class__(self)
+        for (start, length) in other:
+            s.add(start, length)
+        return s
+
+    def __sub__(self, other):
+        s = self.__class__(self)
+        for (start, length) in other:
+            s.remove(start, length)
+        return s
+
+    def __iadd__(self, other):
+        for (start, length) in other:
+            self.add(start, length)
+        return self
+
+    def __isub__(self, other):
+        for (start, length) in other:
+            self.remove(start, length)
+        return self
+
+    def __and__(self, other):
+        s = self.__class__()
+        for i in other.each():
+            if i in self._have:
+                s.add(i, 1)
+        return s
+
+    def __contains__(self, start_and_length):
+        (start, length) = start_and_length
+        for i in range(start, start+length):
+            if i not in self._have:
+                return False
+        return True
+
+class ByteSpans(unittest.TestCase):
+    def test_basic(self):
+        s = Spans()
+        self.failUnlessEqual(list(s), [])
+        self.failIf(s)
+        self.failIf((0,1) in s)
+        self.failUnlessEqual(s.len(), 0)
+
+        s1 = Spans(3, 4) # 3,4,5,6
+        self._check1(s1)
+
+        s1 = Spans(long(3), long(4)) # 3,4,5,6
+        self._check1(s1)
+
+        s2 = Spans(s1)
+        self._check1(s2)
+
+        s2.add(10,2) # 10,11
+        self._check1(s1)
+        self.failUnless((10,1) in s2)
+        self.failIf((10,1) in s1)
+        self.failUnlessEqual(list(s2.each()), [3,4,5,6,10,11])
+        self.failUnlessEqual(s2.len(), 6)
+
+        s2.add(15,2).add(20,2)
+        self.failUnlessEqual(list(s2.each()), [3,4,5,6,10,11,15,16,20,21])
+        self.failUnlessEqual(s2.len(), 10)
+
+        s2.remove(4,3).remove(15,1)
+        self.failUnlessEqual(list(s2.each()), [3,10,11,16,20,21])
+        self.failUnlessEqual(s2.len(), 6)
+
+        s1 = SimpleSpans(3, 4) # 3 4 5 6
+        s2 = SimpleSpans(5, 4) # 5 6 7 8
+        i = s1 & s2
+        self.failUnlessEqual(list(i.each()), [5, 6])
+
+    def _check1(self, s):
+        self.failUnlessEqual(list(s), [(3,4)])
+        self.failUnless(s)
+        self.failUnlessEqual(s.len(), 4)
+        self.failIf((0,1) in s)
+        self.failUnless((3,4) in s)
+        self.failUnless((3,1) in s)
+        self.failUnless((5,2) in s)
+        self.failUnless((6,1) in s)
+        self.failIf((6,2) in s)
+        self.failIf((7,1) in s)
+        self.failUnlessEqual(list(s.each()), [3,4,5,6])
+
+    def test_large(self):
+        s = Spans(4, 2**65) # don't do this with a SimpleSpans
+        self.failUnlessEqual(list(s), [(4, 2**65)])
+        self.failUnless(s)
+        self.failUnlessEqual(s.len(), 2**65)
+        self.failIf((0,1) in s)
+        self.failUnless((4,2) in s)
+        self.failUnless((2**65,2) in s)
+
+    def test_math(self):
+        s1 = Spans(0, 10) # 0,1,2,3,4,5,6,7,8,9
+        s2 = Spans(5, 3) # 5,6,7
+        s3 = Spans(8, 4) # 8,9,10,11
+
+        s = s1 - s2
+        self.failUnlessEqual(list(s.each()), [0,1,2,3,4,8,9])
+        s = s1 - s3
+        self.failUnlessEqual(list(s.each()), [0,1,2,3,4,5,6,7])
+        s = s2 - s3
+        self.failUnlessEqual(list(s.each()), [5,6,7])
+        s = s1 & s2
+        self.failUnlessEqual(list(s.each()), [5,6,7])
+        s = s2 & s1
+        self.failUnlessEqual(list(s.each()), [5,6,7])
+        s = s1 & s3
+        self.failUnlessEqual(list(s.each()), [8,9])
+        s = s3 & s1
+        self.failUnlessEqual(list(s.each()), [8,9])
+        s = s2 & s3
+        self.failUnlessEqual(list(s.each()), [])
+        s = s3 & s2
+        self.failUnlessEqual(list(s.each()), [])
+        s = Spans() & s3
+        self.failUnlessEqual(list(s.each()), [])
+        s = s3 & Spans()
+        self.failUnlessEqual(list(s.each()), [])
+
+        s = s1 + s2
+        self.failUnlessEqual(list(s.each()), [0,1,2,3,4,5,6,7,8,9])
+        s = s1 + s3
+        self.failUnlessEqual(list(s.each()), [0,1,2,3,4,5,6,7,8,9,10,11])
+        s = s2 + s3
+        self.failUnlessEqual(list(s.each()), [5,6,7,8,9,10,11])
+
+        s = Spans(s1)
+        s -= s2
+        self.failUnlessEqual(list(s.each()), [0,1,2,3,4,8,9])
+        s = Spans(s1)
+        s -= s3
+        self.failUnlessEqual(list(s.each()), [0,1,2,3,4,5,6,7])
+        s = Spans(s2)
+        s -= s3
+        self.failUnlessEqual(list(s.each()), [5,6,7])
+
+        s = Spans(s1)
+        s += s2
+        self.failUnlessEqual(list(s.each()), [0,1,2,3,4,5,6,7,8,9])
+        s = Spans(s1)
+        s += s3
+        self.failUnlessEqual(list(s.each()), [0,1,2,3,4,5,6,7,8,9,10,11])
+        s = Spans(s2)
+        s += s3
+        self.failUnlessEqual(list(s.each()), [5,6,7,8,9,10,11])
+
+    def test_random(self):
+        # attempt to increase coverage of corner cases by comparing behavior
+        # of a simple-but-slow model implementation against the
+        # complex-but-fast actual implementation, in a large number of random
+        # operations
+        S1 = SimpleSpans
+        S2 = Spans
+        s1 = S1(); s2 = S2()
+        seed = b""
+        def _create(subseed):
+            ns1 = S1(); ns2 = S2()
+            for i in range(10):
+                what = sha256(subseed+bytes(i))
+                start = int(what[2:4], 16)
+                length = max(1,int(what[5:6], 16))
+                ns1.add(start, length); ns2.add(start, length)
+            return ns1, ns2
+
+        #print
+        for i in range(1000):
+            what = sha256(seed+bytes(i))
+            op = what[0:1]
+            subop = what[1:2]
+            start = int(what[2:4], 16)
+            length = max(1,int(what[5:6], 16))
+            #print what
+            if op in b"0":
+                if subop in b"01234":
+                    s1 = S1(); s2 = S2()
+                elif subop in b"5678":
+                    s1 = S1(start, length); s2 = S2(start, length)
+                else:
+                    s1 = S1(s1); s2 = S2(s2)
+                #print "s2 = %s" % s2.dump()
+            elif op in b"123":
+                #print "s2.add(%d,%d)" % (start, length)
+                s1.add(start, length); s2.add(start, length)
+            elif op in b"456":
+                #print "s2.remove(%d,%d)" % (start, length)
+                s1.remove(start, length); s2.remove(start, length)
+            elif op in b"78":
+                ns1, ns2 = _create(what[7:11])
+                #print "s2 + %s" % ns2.dump()
+                s1 = s1 + ns1; s2 = s2 + ns2
+            elif op in b"9a":
+                ns1, ns2 = _create(what[7:11])
+                #print "%s - %s" % (s2.dump(), ns2.dump())
+                s1 = s1 - ns1; s2 = s2 - ns2
+            elif op in b"bc":
+                ns1, ns2 = _create(what[7:11])
+                #print "s2 += %s" % ns2.dump()
+                s1 += ns1; s2 += ns2
+            elif op in b"de":
+                ns1, ns2 = _create(what[7:11])
+                #print "%s -= %s" % (s2.dump(), ns2.dump())
+                s1 -= ns1; s2 -= ns2
+            else:
+                ns1, ns2 = _create(what[7:11])
+                #print "%s &= %s" % (s2.dump(), ns2.dump())
+                s1 = s1 & ns1; s2 = s2 & ns2
+            #print "s2 now %s" % s2.dump()
+            self.failUnlessEqual(list(s1.each()), list(s2.each()))
+            self.failUnlessEqual(s1.len(), s2.len())
+            self.failUnlessEqual(bool(s1), bool(s2))
+            self.failUnlessEqual(list(s1), list(s2))
+            for j in range(10):
+                what = sha256(what[12:14]+bytes(j))
+                start = int(what[2:4], 16)
+                length = max(1, int(what[5:6], 16))
+                span = (start, length)
+                self.failUnlessEqual(bool(span in s1), bool(span in s2))
+
+
+    # s()
+    # s(start,length)
+    # s(s0)
+    # s.add(start,length) : returns s
+    # s.remove(start,length)
+    # s.each() -> list of byte offsets, mostly for testing
+    # list(s) -> list of (start,length) tuples, one per span
+    # (start,length) in s -> True if (start..start+length-1) are all members
+    #  NOT equivalent to x in list(s)
+    # s.len() -> number of bytes, for testing, bool(), and accounting/limiting
+    # bool(s)  (__nonzeron__)
+    # s = s1+s2, s1-s2, +=s1, -=s1
+
+    def test_overlap(self):
+        for a in range(20):
+            for b in range(10):
+                for c in range(20):
+                    for d in range(10):
+                        self._test_overlap(a,b,c,d)
+
+    def _test_overlap(self, a, b, c, d):
+        s1 = set(range(a,a+b))
+        s2 = set(range(c,c+d))
+        #print "---"
+        #self._show_overlap(s1, "1")
+        #self._show_overlap(s2, "2")
+        o = overlap(a,b,c,d)
+        expected = s1.intersection(s2)
+        if not expected:
+            self.failUnlessEqual(o, None)
+        else:
+            start,length = o
+            so = set(range(start,start+length))
+            #self._show(so, "o")
+            self.failUnlessEqual(so, expected)
+
+    def _show_overlap(self, s, c):
+        import sys
+        out = sys.stdout
+        if s:
+            for i in range(max(s)):
+                if i in s:
+                    out.write(c)
+                else:
+                    out.write(" ")
+        out.write("\n")
+
+def extend(s, start, length, fill):
+    if len(s) >= start+length:
+        return s
+    assert len(fill) == 1
+    return s + fill*(start+length-len(s))
+
+def replace(s, start, data):
+    assert len(s) >= start+len(data)
+    return s[:start] + data + s[start+len(data):]
+
+class SimpleDataSpans(object):
+    def __init__(self, other=None):
+        self.missing = "" # "1" where missing, "0" where found
+        self.data = b""
+        if other:
+            for (start, data) in other.get_chunks():
+                self.add(start, data)
+
+    def __bool__(self): # this gets us bool()
+        return bool(self.len())
+
+    def len(self):
+        return len(self.missing.replace("1", ""))
+
+    def _dump(self):
+        return [i for (i,c) in enumerate(self.missing) if c == "0"]
+
+    def _have(self, start, length):
+        m = self.missing[start:start+length]
+        if not m or len(m)<length or int(m):
+            return False
+        return True
+    def get_chunks(self):
+        for i in self._dump():
+            yield (i, self.data[i:i+1])
+    def get_spans(self):
+        return SimpleSpans([(start,len(data))
+                            for (start,data) in self.get_chunks()])
+    def get(self, start, length):
+        if self._have(start, length):
+            return self.data[start:start+length]
+        return None
+    def pop(self, start, length):
+        data = self.get(start, length)
+        if data:
+            self.remove(start, length)
+        return data
+    def remove(self, start, length):
+        self.missing = replace(extend(self.missing, start, length, "1"),
+                               start, "1"*length)
+    def add(self, start, data):
+        self.missing = replace(extend(self.missing, start, len(data), "1"),
+                               start, "0"*len(data))
+        self.data = replace(extend(self.data, start, len(data), b" "),
+                            start, data)
+
+
+class StringSpans(unittest.TestCase):
+    def do_basic(self, klass):
+        ds = klass()
+        self.failUnlessEqual(ds.len(), 0)
+        self.failUnlessEqual(list(ds._dump()), [])
+        self.failUnlessEqual(sum([len(d) for (s,d) in ds.get_chunks()]), 0)
+        s1 = ds.get_spans()
+        self.failUnlessEqual(ds.get(0, 4), None)
+        self.failUnlessEqual(ds.pop(0, 4), None)
+        ds.remove(0, 4)
+
+        ds.add(2, b"four")
+        self.failUnlessEqual(ds.len(), 4)
+        self.failUnlessEqual(list(ds._dump()), [2,3,4,5])
+        self.failUnlessEqual(sum([len(d) for (s,d) in ds.get_chunks()]), 4)
+        s1 = ds.get_spans()
+        self.failUnless((2,2) in s1)
+        self.failUnlessEqual(ds.get(0, 4), None)
+        self.failUnlessEqual(ds.pop(0, 4), None)
+        self.failUnlessEqual(ds.get(4, 4), None)
+
+        ds2 = klass(ds)
+        self.failUnlessEqual(ds2.len(), 4)
+        self.failUnlessEqual(list(ds2._dump()), [2,3,4,5])
+        self.failUnlessEqual(sum([len(d) for (s,d) in ds2.get_chunks()]), 4)
+        self.failUnlessEqual(ds2.get(0, 4), None)
+        self.failUnlessEqual(ds2.pop(0, 4), None)
+        self.failUnlessEqual(ds2.pop(2, 3), b"fou")
+        self.failUnlessEqual(sum([len(d) for (s,d) in ds2.get_chunks()]), 1)
+        self.failUnlessEqual(ds2.get(2, 3), None)
+        self.failUnlessEqual(ds2.get(5, 1), b"r")
+        self.failUnlessEqual(ds.get(2, 3), b"fou")
+        self.failUnlessEqual(sum([len(d) for (s,d) in ds.get_chunks()]), 4)
+
+        ds.add(0, b"23")
+        self.failUnlessEqual(ds.len(), 6)
+        self.failUnlessEqual(list(ds._dump()), [0,1,2,3,4,5])
+        self.failUnlessEqual(sum([len(d) for (s,d) in ds.get_chunks()]), 6)
+        self.failUnlessEqual(ds.get(0, 4), b"23fo")
+        self.failUnlessEqual(ds.pop(0, 4), b"23fo")
+        self.failUnlessEqual(sum([len(d) for (s,d) in ds.get_chunks()]), 2)
+        self.failUnlessEqual(ds.get(0, 4), None)
+        self.failUnlessEqual(ds.pop(0, 4), None)
+
+        ds = klass()
+        ds.add(2, b"four")
+        ds.add(3, b"ea")
+        self.failUnlessEqual(ds.get(2, 4), b"fear")
+
+        ds = klass()
+        ds.add(long(2), b"four")
+        ds.add(long(3), b"ea")
+        self.failUnlessEqual(ds.get(long(2), long(4)), b"fear")
+
+
+    def do_scan(self, klass):
+        # do a test with gaps and spans of size 1 and 2
+        #  left=(1,11) * right=(1,11) * gapsize=(1,2)
+        # 111, 112, 121, 122, 211, 212, 221, 222
+        #    211
+        #      121
+        #         112
+        #            212
+        #               222
+        #                   221
+        #                      111
+        #                        122
+        #  11 1  1 11 11  11  1 1  111
+        # 0123456789012345678901234567
+        # abcdefghijklmnopqrstuvwxyz-=
+        pieces = [(1, b"bc"),
+                  (4, b"e"),
+                  (7, b"h"),
+                  (9, b"jk"),
+                  (12, b"mn"),
+                  (16, b"qr"),
+                  (20, b"u"),
+                  (22, b"w"),
+                  (25, b"z-="),
+                  ]
+        p_elements = set([1,2,4,7,9,10,12,13,16,17,20,22,25,26,27])
+        S = b"abcdefghijklmnopqrstuvwxyz-="
+        # TODO: when adding data, add capital letters, to make sure we aren't
+        # just leaving the old data in place
+        l = len(S)
+        def base():
+            ds = klass()
+            for start, data in pieces:
+                ds.add(start, data)
+            return ds
+        def dump(s):
+            p = set(s._dump())
+            d = b"".join([((i not in p) and b" " or S[i]) for i in range(l)])
+            assert len(d) == l
+            return d
+        DEBUG = False
+        for start in range(0, l):
+            for end in range(start+1, l):
+                # add [start-end) to the baseline
+                which = "%d-%d" % (start, end-1)
+                p_added = set(range(start, end))
+                b = base()
+                if DEBUG:
+                    print()
+                    print(dump(b), which)
+                    add = klass(); add.add(start, S[start:end])
+                    print(dump(add))
+                b.add(start, S[start:end])
+                if DEBUG:
+                    print(dump(b))
+                # check that the new span is there
+                d = b.get(start, end-start)
+                self.failUnlessEqual(d, S[start:end], which)
+                # check that all the original pieces are still there
+                for t_start, t_data in pieces:
+                    t_len = len(t_data)
+                    self.failUnlessEqual(b.get(t_start, t_len),
+                                         S[t_start:t_start+t_len],
+                                         "%s %d+%d" % (which, t_start, t_len))
+                # check that a lot of subspans are mostly correct
+                for t_start in range(l):
+                    for t_len in range(1,4):
+                        d = b.get(t_start, t_len)
+                        if d is not None:
+                            which2 = "%s+(%d-%d)" % (which, t_start,
+                                                     t_start+t_len-1)
+                            self.failUnlessEqual(d, S[t_start:t_start+t_len],
+                                                 which2)
+                        # check that removing a subspan gives the right value
+                        b2 = klass(b)
+                        b2.remove(t_start, t_len)
+                        removed = set(range(t_start, t_start+t_len))
+                        for i in range(l):
+                            exp = (((i in p_elements) or (i in p_added))
+                                   and (i not in removed))
+                            which2 = "%s-(%d-%d)" % (which, t_start,
+                                                     t_start+t_len-1)
+                            self.failUnlessEqual(bool(b2.get(i, 1)), exp,
+                                                 which2+" %d" % i)
+
+    def test_test(self):
+        self.do_basic(SimpleDataSpans)
+        self.do_scan(SimpleDataSpans)
+
+    def test_basic(self):
+        self.do_basic(DataSpans)
+        self.do_scan(DataSpans)
+
+    def test_random(self):
+        # attempt to increase coverage of corner cases by comparing behavior
+        # of a simple-but-slow model implementation against the
+        # complex-but-fast actual implementation, in a large number of random
+        # operations
+        S1 = SimpleDataSpans
+        S2 = DataSpans
+        s1 = S1(); s2 = S2()
+        seed = b""
+        def _randstr(length, seed):
+            created = 0
+            pieces = []
+            while created < length:
+                piece = sha256(seed + bytes(created))
+                pieces.append(piece)
+                created += len(piece)
+            return b"".join(pieces)[:length]
+        def _create(subseed):
+            ns1 = S1(); ns2 = S2()
+            for i in range(10):
+                what = sha256(subseed+bytes(i))
+                start = int(what[2:4], 16)
+                length = max(1,int(what[5:6], 16))
+                ns1.add(start, _randstr(length, what[7:9]));
+                ns2.add(start, _randstr(length, what[7:9]))
+            return ns1, ns2
+
+        #print
+        for i in range(1000):
+            what = sha256(seed+bytes(i))
+            op = what[0:1]
+            subop = what[1:2]
+            start = int(what[2:4], 16)
+            length = max(1,int(what[5:6], 16))
+            #print what
+            if op in b"0":
+                if subop in b"0123456":
+                    s1 = S1(); s2 = S2()
+                else:
+                    s1, s2 = _create(what[7:11])
+                #print "s2 = %s" % list(s2._dump())
+            elif op in b"123456":
+                #print "s2.add(%d,%d)" % (start, length)
+                s1.add(start, _randstr(length, what[7:9]));
+                s2.add(start, _randstr(length, what[7:9]))
+            elif op in b"789abc":
+                #print "s2.remove(%d,%d)" % (start, length)
+                s1.remove(start, length); s2.remove(start, length)
+            else:
+                #print "s2.pop(%d,%d)" % (start, length)
+                d1 = s1.pop(start, length); d2 = s2.pop(start, length)
+                self.failUnlessEqual(d1, d2)
+            #print "s1 now %s" % list(s1._dump())
+            #print "s2 now %s" % list(s2._dump())
+            self.failUnlessEqual(s1.len(), s2.len())
+            self.failUnlessEqual(list(s1._dump()), list(s2._dump()))
+            for j in range(100):
+                what = sha256(what[12:14]+bytes(j))
+                start = int(what[2:4], 16)
+                length = max(1, int(what[5:6], 16))
+                d1 = s1.get(start, length); d2 = s2.get(start, length)
+                self.failUnlessEqual(d1, d2, "%d+%d" % (start, length))
diff --git a/src/allmydata/test/test_util.py b/src/allmydata/test/test_util.py
index 8c0e937f3..1519c42da 100644
--- a/src/allmydata/test/test_util.py
+++ b/src/allmydata/test/test_util.py
@@ -1,8 +1,6 @@
 from __future__ import print_function
 
-import binascii
 import six
-import hashlib
 import os, time, sys
 import yaml
 
@@ -16,7 +14,6 @@ from allmydata.util import fileutil
 from allmydata.util import limiter, pollmixin
 from allmydata.util import statistics, dictutil, yamlutil
 from allmydata.util import log as tahoe_log
-from allmydata.util.spans import Spans, overlap, DataSpans
 from allmydata.util.fileutil import EncryptedTemporaryFile
 from allmydata.test.common_util import ReallyEqualMixin
 
@@ -24,15 +21,6 @@ if six.PY3:
     long = int
 
 
-def sha256(data):
-    """
-    :param bytes data: data to hash
-
-    :returns: a hex-encoded SHA256 hash of the data
-    """
-    return binascii.hexlify(hashlib.sha256(data).digest())
-
-
 class IDLib(unittest.TestCase):
     def test_nodeid_b2a(self):
         self.failUnlessEqual(idlib.nodeid_b2a("\x00"*20), "a"*32)
@@ -764,589 +752,6 @@ class Log(unittest.TestCase):
         self.flushLoggedErrors(SampleError)
 
 
-class SimpleSpans(object):
-    # this is a simple+inefficient form of util.spans.Spans . We compare the
-    # behavior of this reference model against the real (efficient) form.
-
-    def __init__(self, _span_or_start=None, length=None):
-        self._have = set()
-        if length is not None:
-            for i in range(_span_or_start, _span_or_start+length):
-                self._have.add(i)
-        elif _span_or_start:
-            for (start,length) in _span_or_start:
-                self.add(start, length)
-
-    def add(self, start, length):
-        for i in range(start, start+length):
-            self._have.add(i)
-        return self
-
-    def remove(self, start, length):
-        for i in range(start, start+length):
-            self._have.discard(i)
-        return self
-
-    def each(self):
-        return sorted(self._have)
-
-    def __iter__(self):
-        items = sorted(self._have)
-        prevstart = None
-        prevend = None
-        for i in items:
-            if prevstart is None:
-                prevstart = prevend = i
-                continue
-            if i == prevend+1:
-                prevend = i
-                continue
-            yield (prevstart, prevend-prevstart+1)
-            prevstart = prevend = i
-        if prevstart is not None:
-            yield (prevstart, prevend-prevstart+1)
-
-    def __nonzero__(self): # this gets us bool()
-        return self.len()
-
-    def len(self):
-        return len(self._have)
-
-    def __add__(self, other):
-        s = self.__class__(self)
-        for (start, length) in other:
-            s.add(start, length)
-        return s
-
-    def __sub__(self, other):
-        s = self.__class__(self)
-        for (start, length) in other:
-            s.remove(start, length)
-        return s
-
-    def __iadd__(self, other):
-        for (start, length) in other:
-            self.add(start, length)
-        return self
-
-    def __isub__(self, other):
-        for (start, length) in other:
-            self.remove(start, length)
-        return self
-
-    def __and__(self, other):
-        s = self.__class__()
-        for i in other.each():
-            if i in self._have:
-                s.add(i, 1)
-        return s
-
-    def __contains__(self, start_and_length):
-        (start, length) = start_and_length
-        for i in range(start, start+length):
-            if i not in self._have:
-                return False
-        return True
-
-class ByteSpans(unittest.TestCase):
-    def test_basic(self):
-        s = Spans()
-        self.failUnlessEqual(list(s), [])
-        self.failIf(s)
-        self.failIf((0,1) in s)
-        self.failUnlessEqual(s.len(), 0)
-
-        s1 = Spans(3, 4) # 3,4,5,6
-        self._check1(s1)
-
-        s1 = Spans(long(3), long(4)) # 3,4,5,6
-        self._check1(s1)
-
-        s2 = Spans(s1)
-        self._check1(s2)
-
-        s2.add(10,2) # 10,11
-        self._check1(s1)
-        self.failUnless((10,1) in s2)
-        self.failIf((10,1) in s1)
-        self.failUnlessEqual(list(s2.each()), [3,4,5,6,10,11])
-        self.failUnlessEqual(s2.len(), 6)
-
-        s2.add(15,2).add(20,2)
-        self.failUnlessEqual(list(s2.each()), [3,4,5,6,10,11,15,16,20,21])
-        self.failUnlessEqual(s2.len(), 10)
-
-        s2.remove(4,3).remove(15,1)
-        self.failUnlessEqual(list(s2.each()), [3,10,11,16,20,21])
-        self.failUnlessEqual(s2.len(), 6)
-
-        s1 = SimpleSpans(3, 4) # 3 4 5 6
-        s2 = SimpleSpans(5, 4) # 5 6 7 8
-        i = s1 & s2
-        self.failUnlessEqual(list(i.each()), [5, 6])
-
-    def _check1(self, s):
-        self.failUnlessEqual(list(s), [(3,4)])
-        self.failUnless(s)
-        self.failUnlessEqual(s.len(), 4)
-        self.failIf((0,1) in s)
-        self.failUnless((3,4) in s)
-        self.failUnless((3,1) in s)
-        self.failUnless((5,2) in s)
-        self.failUnless((6,1) in s)
-        self.failIf((6,2) in s)
-        self.failIf((7,1) in s)
-        self.failUnlessEqual(list(s.each()), [3,4,5,6])
-
-    def test_large(self):
-        s = Spans(4, 2**65) # don't do this with a SimpleSpans
-        self.failUnlessEqual(list(s), [(4, 2**65)])
-        self.failUnless(s)
-        self.failUnlessEqual(s.len(), 2**65)
-        self.failIf((0,1) in s)
-        self.failUnless((4,2) in s)
-        self.failUnless((2**65,2) in s)
-
-    def test_math(self):
-        s1 = Spans(0, 10) # 0,1,2,3,4,5,6,7,8,9
-        s2 = Spans(5, 3) # 5,6,7
-        s3 = Spans(8, 4) # 8,9,10,11
-
-        s = s1 - s2
-        self.failUnlessEqual(list(s.each()), [0,1,2,3,4,8,9])
-        s = s1 - s3
-        self.failUnlessEqual(list(s.each()), [0,1,2,3,4,5,6,7])
-        s = s2 - s3
-        self.failUnlessEqual(list(s.each()), [5,6,7])
-        s = s1 & s2
-        self.failUnlessEqual(list(s.each()), [5,6,7])
-        s = s2 & s1
-        self.failUnlessEqual(list(s.each()), [5,6,7])
-        s = s1 & s3
-        self.failUnlessEqual(list(s.each()), [8,9])
-        s = s3 & s1
-        self.failUnlessEqual(list(s.each()), [8,9])
-        s = s2 & s3
-        self.failUnlessEqual(list(s.each()), [])
-        s = s3 & s2
-        self.failUnlessEqual(list(s.each()), [])
-        s = Spans() & s3
-        self.failUnlessEqual(list(s.each()), [])
-        s = s3 & Spans()
-        self.failUnlessEqual(list(s.each()), [])
-
-        s = s1 + s2
-        self.failUnlessEqual(list(s.each()), [0,1,2,3,4,5,6,7,8,9])
-        s = s1 + s3
-        self.failUnlessEqual(list(s.each()), [0,1,2,3,4,5,6,7,8,9,10,11])
-        s = s2 + s3
-        self.failUnlessEqual(list(s.each()), [5,6,7,8,9,10,11])
-
-        s = Spans(s1)
-        s -= s2
-        self.failUnlessEqual(list(s.each()), [0,1,2,3,4,8,9])
-        s = Spans(s1)
-        s -= s3
-        self.failUnlessEqual(list(s.each()), [0,1,2,3,4,5,6,7])
-        s = Spans(s2)
-        s -= s3
-        self.failUnlessEqual(list(s.each()), [5,6,7])
-
-        s = Spans(s1)
-        s += s2
-        self.failUnlessEqual(list(s.each()), [0,1,2,3,4,5,6,7,8,9])
-        s = Spans(s1)
-        s += s3
-        self.failUnlessEqual(list(s.each()), [0,1,2,3,4,5,6,7,8,9,10,11])
-        s = Spans(s2)
-        s += s3
-        self.failUnlessEqual(list(s.each()), [5,6,7,8,9,10,11])
-
-    def test_random(self):
-        # attempt to increase coverage of corner cases by comparing behavior
-        # of a simple-but-slow model implementation against the
-        # complex-but-fast actual implementation, in a large number of random
-        # operations
-        S1 = SimpleSpans
-        S2 = Spans
-        s1 = S1(); s2 = S2()
-        seed = ""
-        def _create(subseed):
-            ns1 = S1(); ns2 = S2()
-            for i in range(10):
-                what = sha256(subseed+str(i))
-                start = int(what[2:4], 16)
-                length = max(1,int(what[5:6], 16))
-                ns1.add(start, length); ns2.add(start, length)
-            return ns1, ns2
-
-        #print
-        for i in range(1000):
-            what = sha256(seed+str(i))
-            op = what[0]
-            subop = what[1]
-            start = int(what[2:4], 16)
-            length = max(1,int(what[5:6], 16))
-            #print what
-            if op in "0":
-                if subop in "01234":
-                    s1 = S1(); s2 = S2()
-                elif subop in "5678":
-                    s1 = S1(start, length); s2 = S2(start, length)
-                else:
-                    s1 = S1(s1); s2 = S2(s2)
-                #print "s2 = %s" % s2.dump()
-            elif op in "123":
-                #print "s2.add(%d,%d)" % (start, length)
-                s1.add(start, length); s2.add(start, length)
-            elif op in "456":
-                #print "s2.remove(%d,%d)" % (start, length)
-                s1.remove(start, length); s2.remove(start, length)
-            elif op in "78":
-                ns1, ns2 = _create(what[7:11])
-                #print "s2 + %s" % ns2.dump()
-                s1 = s1 + ns1; s2 = s2 + ns2
-            elif op in "9a":
-                ns1, ns2 = _create(what[7:11])
-                #print "%s - %s" % (s2.dump(), ns2.dump())
-                s1 = s1 - ns1; s2 = s2 - ns2
-            elif op in "bc":
-                ns1, ns2 = _create(what[7:11])
-                #print "s2 += %s" % ns2.dump()
-                s1 += ns1; s2 += ns2
-            elif op in "de":
-                ns1, ns2 = _create(what[7:11])
-                #print "%s -= %s" % (s2.dump(), ns2.dump())
-                s1 -= ns1; s2 -= ns2
-            else:
-                ns1, ns2 = _create(what[7:11])
-                #print "%s &= %s" % (s2.dump(), ns2.dump())
-                s1 = s1 & ns1; s2 = s2 & ns2
-            #print "s2 now %s" % s2.dump()
-            self.failUnlessEqual(list(s1.each()), list(s2.each()))
-            self.failUnlessEqual(s1.len(), s2.len())
-            self.failUnlessEqual(bool(s1), bool(s2))
-            self.failUnlessEqual(list(s1), list(s2))
-            for j in range(10):
-                what = sha256(what[12:14]+str(j))
-                start = int(what[2:4], 16)
-                length = max(1, int(what[5:6], 16))
-                span = (start, length)
-                self.failUnlessEqual(bool(span in s1), bool(span in s2))
-
-
-    # s()
-    # s(start,length)
-    # s(s0)
-    # s.add(start,length) : returns s
-    # s.remove(start,length)
-    # s.each() -> list of byte offsets, mostly for testing
-    # list(s) -> list of (start,length) tuples, one per span
-    # (start,length) in s -> True if (start..start+length-1) are all members
-    #  NOT equivalent to x in list(s)
-    # s.len() -> number of bytes, for testing, bool(), and accounting/limiting
-    # bool(s)  (__nonzeron__)
-    # s = s1+s2, s1-s2, +=s1, -=s1
-
-    def test_overlap(self):
-        for a in range(20):
-            for b in range(10):
-                for c in range(20):
-                    for d in range(10):
-                        self._test_overlap(a,b,c,d)
-
-    def _test_overlap(self, a, b, c, d):
-        s1 = set(range(a,a+b))
-        s2 = set(range(c,c+d))
-        #print "---"
-        #self._show_overlap(s1, "1")
-        #self._show_overlap(s2, "2")
-        o = overlap(a,b,c,d)
-        expected = s1.intersection(s2)
-        if not expected:
-            self.failUnlessEqual(o, None)
-        else:
-            start,length = o
-            so = set(range(start,start+length))
-            #self._show(so, "o")
-            self.failUnlessEqual(so, expected)
-
-    def _show_overlap(self, s, c):
-        import sys
-        out = sys.stdout
-        if s:
-            for i in range(max(s)):
-                if i in s:
-                    out.write(c)
-                else:
-                    out.write(" ")
-        out.write("\n")
-
-def extend(s, start, length, fill):
-    if len(s) >= start+length:
-        return s
-    assert len(fill) == 1
-    return s + fill*(start+length-len(s))
-
-def replace(s, start, data):
-    assert len(s) >= start+len(data)
-    return s[:start] + data + s[start+len(data):]
-
-class SimpleDataSpans(object):
-    def __init__(self, other=None):
-        self.missing = "" # "1" where missing, "0" where found
-        self.data = ""
-        if other:
-            for (start, data) in other.get_chunks():
-                self.add(start, data)
-
-    def __nonzero__(self): # this gets us bool()
-        return self.len()
-    def len(self):
-        return len(self.missing.replace("1", ""))
-    def _dump(self):
-        return [i for (i,c) in enumerate(self.missing) if c == "0"]
-    def _have(self, start, length):
-        m = self.missing[start:start+length]
-        if not m or len(m)<length or int(m):
-            return False
-        return True
-    def get_chunks(self):
-        for i in self._dump():
-            yield (i, self.data[i])
-    def get_spans(self):
-        return SimpleSpans([(start,len(data))
-                            for (start,data) in self.get_chunks()])
-    def get(self, start, length):
-        if self._have(start, length):
-            return self.data[start:start+length]
-        return None
-    def pop(self, start, length):
-        data = self.get(start, length)
-        if data:
-            self.remove(start, length)
-        return data
-    def remove(self, start, length):
-        self.missing = replace(extend(self.missing, start, length, "1"),
-                               start, "1"*length)
-    def add(self, start, data):
-        self.missing = replace(extend(self.missing, start, len(data), "1"),
-                               start, "0"*len(data))
-        self.data = replace(extend(self.data, start, len(data), " "),
-                            start, data)
-
-
-class StringSpans(unittest.TestCase):
-    def do_basic(self, klass):
-        ds = klass()
-        self.failUnlessEqual(ds.len(), 0)
-        self.failUnlessEqual(list(ds._dump()), [])
-        self.failUnlessEqual(sum([len(d) for (s,d) in ds.get_chunks()]), 0)
-        s1 = ds.get_spans()
-        self.failUnlessEqual(ds.get(0, 4), None)
-        self.failUnlessEqual(ds.pop(0, 4), None)
-        ds.remove(0, 4)
-
-        ds.add(2, "four")
-        self.failUnlessEqual(ds.len(), 4)
-        self.failUnlessEqual(list(ds._dump()), [2,3,4,5])
-        self.failUnlessEqual(sum([len(d) for (s,d) in ds.get_chunks()]), 4)
-        s1 = ds.get_spans()
-        self.failUnless((2,2) in s1)
-        self.failUnlessEqual(ds.get(0, 4), None)
-        self.failUnlessEqual(ds.pop(0, 4), None)
-        self.failUnlessEqual(ds.get(4, 4), None)
-
-        ds2 = klass(ds)
-        self.failUnlessEqual(ds2.len(), 4)
-        self.failUnlessEqual(list(ds2._dump()), [2,3,4,5])
-        self.failUnlessEqual(sum([len(d) for (s,d) in ds2.get_chunks()]), 4)
-        self.failUnlessEqual(ds2.get(0, 4), None)
-        self.failUnlessEqual(ds2.pop(0, 4), None)
-        self.failUnlessEqual(ds2.pop(2, 3), "fou")
-        self.failUnlessEqual(sum([len(d) for (s,d) in ds2.get_chunks()]), 1)
-        self.failUnlessEqual(ds2.get(2, 3), None)
-        self.failUnlessEqual(ds2.get(5, 1), "r")
-        self.failUnlessEqual(ds.get(2, 3), "fou")
-        self.failUnlessEqual(sum([len(d) for (s,d) in ds.get_chunks()]), 4)
-
-        ds.add(0, "23")
-        self.failUnlessEqual(ds.len(), 6)
-        self.failUnlessEqual(list(ds._dump()), [0,1,2,3,4,5])
-        self.failUnlessEqual(sum([len(d) for (s,d) in ds.get_chunks()]), 6)
-        self.failUnlessEqual(ds.get(0, 4), "23fo")
-        self.failUnlessEqual(ds.pop(0, 4), "23fo")
-        self.failUnlessEqual(sum([len(d) for (s,d) in ds.get_chunks()]), 2)
-        self.failUnlessEqual(ds.get(0, 4), None)
-        self.failUnlessEqual(ds.pop(0, 4), None)
-
-        ds = klass()
-        ds.add(2, "four")
-        ds.add(3, "ea")
-        self.failUnlessEqual(ds.get(2, 4), "fear")
-
-        ds = klass()
-        ds.add(long(2), "four")
-        ds.add(long(3), "ea")
-        self.failUnlessEqual(ds.get(long(2), long(4)), "fear")
-
-
-    def do_scan(self, klass):
-        # do a test with gaps and spans of size 1 and 2
-        #  left=(1,11) * right=(1,11) * gapsize=(1,2)
-        # 111, 112, 121, 122, 211, 212, 221, 222
-        #    211
-        #      121
-        #         112
-        #            212
-        #               222
-        #                   221
-        #                      111
-        #                        122
-        #  11 1  1 11 11  11  1 1  111
-        # 0123456789012345678901234567
-        # abcdefghijklmnopqrstuvwxyz-=
-        pieces = [(1, "bc"),
-                  (4, "e"),
-                  (7, "h"),
-                  (9, "jk"),
-                  (12, "mn"),
-                  (16, "qr"),
-                  (20, "u"),
-                  (22, "w"),
-                  (25, "z-="),
-                  ]
-        p_elements = set([1,2,4,7,9,10,12,13,16,17,20,22,25,26,27])
-        S = "abcdefghijklmnopqrstuvwxyz-="
-        # TODO: when adding data, add capital letters, to make sure we aren't
-        # just leaving the old data in place
-        l = len(S)
-        def base():
-            ds = klass()
-            for start, data in pieces:
-                ds.add(start, data)
-            return ds
-        def dump(s):
-            p = set(s._dump())
-            d = "".join([((i not in p) and " " or S[i]) for i in range(l)])
-            assert len(d) == l
-            return d
-        DEBUG = False
-        for start in range(0, l):
-            for end in range(start+1, l):
-                # add [start-end) to the baseline
-                which = "%d-%d" % (start, end-1)
-                p_added = set(range(start, end))
-                b = base()
-                if DEBUG:
-                    print()
-                    print(dump(b), which)
-                    add = klass(); add.add(start, S[start:end])
-                    print(dump(add))
-                b.add(start, S[start:end])
-                if DEBUG:
-                    print(dump(b))
-                # check that the new span is there
-                d = b.get(start, end-start)
-                self.failUnlessEqual(d, S[start:end], which)
-                # check that all the original pieces are still there
-                for t_start, t_data in pieces:
-                    t_len = len(t_data)
-                    self.failUnlessEqual(b.get(t_start, t_len),
-                                         S[t_start:t_start+t_len],
-                                         "%s %d+%d" % (which, t_start, t_len))
-                # check that a lot of subspans are mostly correct
-                for t_start in range(l):
-                    for t_len in range(1,4):
-                        d = b.get(t_start, t_len)
-                        if d is not None:
-                            which2 = "%s+(%d-%d)" % (which, t_start,
-                                                     t_start+t_len-1)
-                            self.failUnlessEqual(d, S[t_start:t_start+t_len],
-                                                 which2)
-                        # check that removing a subspan gives the right value
-                        b2 = klass(b)
-                        b2.remove(t_start, t_len)
-                        removed = set(range(t_start, t_start+t_len))
-                        for i in range(l):
-                            exp = (((i in p_elements) or (i in p_added))
-                                   and (i not in removed))
-                            which2 = "%s-(%d-%d)" % (which, t_start,
-                                                     t_start+t_len-1)
-                            self.failUnlessEqual(bool(b2.get(i, 1)), exp,
-                                                 which2+" %d" % i)
-
-    def test_test(self):
-        self.do_basic(SimpleDataSpans)
-        self.do_scan(SimpleDataSpans)
-
-    def test_basic(self):
-        self.do_basic(DataSpans)
-        self.do_scan(DataSpans)
-
-    def test_random(self):
-        # attempt to increase coverage of corner cases by comparing behavior
-        # of a simple-but-slow model implementation against the
-        # complex-but-fast actual implementation, in a large number of random
-        # operations
-        S1 = SimpleDataSpans
-        S2 = DataSpans
-        s1 = S1(); s2 = S2()
-        seed = ""
-        def _randstr(length, seed):
-            created = 0
-            pieces = []
-            while created < length:
-                piece = sha256(seed + str(created))
-                pieces.append(piece)
-                created += len(piece)
-            return "".join(pieces)[:length]
-        def _create(subseed):
-            ns1 = S1(); ns2 = S2()
-            for i in range(10):
-                what = sha256(subseed+str(i))
-                start = int(what[2:4], 16)
-                length = max(1,int(what[5:6], 16))
-                ns1.add(start, _randstr(length, what[7:9]));
-                ns2.add(start, _randstr(length, what[7:9]))
-            return ns1, ns2
-
-        #print
-        for i in range(1000):
-            what = sha256(seed+str(i))
-            op = what[0]
-            subop = what[1]
-            start = int(what[2:4], 16)
-            length = max(1,int(what[5:6], 16))
-            #print what
-            if op in "0":
-                if subop in "0123456":
-                    s1 = S1(); s2 = S2()
-                else:
-                    s1, s2 = _create(what[7:11])
-                #print "s2 = %s" % list(s2._dump())
-            elif op in "123456":
-                #print "s2.add(%d,%d)" % (start, length)
-                s1.add(start, _randstr(length, what[7:9]));
-                s2.add(start, _randstr(length, what[7:9]))
-            elif op in "789abc":
-                #print "s2.remove(%d,%d)" % (start, length)
-                s1.remove(start, length); s2.remove(start, length)
-            else:
-                #print "s2.pop(%d,%d)" % (start, length)
-                d1 = s1.pop(start, length); d2 = s2.pop(start, length)
-                self.failUnlessEqual(d1, d2)
-            #print "s1 now %s" % list(s1._dump())
-            #print "s2 now %s" % list(s2._dump())
-            self.failUnlessEqual(s1.len(), s2.len())
-            self.failUnlessEqual(list(s1._dump()), list(s2._dump()))
-            for j in range(100):
-                what = sha256(what[12:14]+str(j))
-                start = int(what[2:4], 16)
-                length = max(1, int(what[5:6], 16))
-                d1 = s1.get(start, length); d2 = s2.get(start, length)
-                self.failUnlessEqual(d1, d2, "%d+%d" % (start, length))
-
 class YAML(unittest.TestCase):
     def test_convert(self):
         data = yaml.safe_dump(["str", u"unicode", u"\u1234nicode"])
diff --git a/src/allmydata/util/_python3.py b/src/allmydata/util/_python3.py
index d939d08b3..21081926c 100644
--- a/src/allmydata/util/_python3.py
+++ b/src/allmydata/util/_python3.py
@@ -31,6 +31,7 @@ PORTED_MODULES = [
     "allmydata.util.pipeline",
     "allmydata.util.pollmixin",
     "allmydata.util._python3",
+    "allmydata.util.spans",
     "allmydata.util.time_format",
     "allmydata.test.common_py3",
 ]
@@ -48,6 +49,7 @@ PORTED_TEST_MODULES = [
     "allmydata.test.test_observer",
     "allmydata.test.test_pipeline",
     "allmydata.test.test_python3",
+    "allmydata.test.test_spans",
     "allmydata.test.test_time_format",
     "allmydata.test.test_version",
 ]
diff --git a/src/allmydata/util/spans.py b/src/allmydata/util/spans.py
index 1576f059a..814904d68 100644
--- a/src/allmydata/util/spans.py
+++ b/src/allmydata/util/spans.py
@@ -1,4 +1,11 @@
 from __future__ import print_function
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import unicode_literals
+
+from future.utils import PY2
+if PY2:
+    from builtins import filter, map, zip, ascii, chr, hex, input, next, oct, open, pow, round, super, bytes, dict, int, list, object, range, str, max, min  # noqa: F401
 
 
 class Spans(object):
@@ -155,9 +162,11 @@ class Spans(object):
         for s in self._spans:
             yield s
 
-    def __nonzero__(self): # this gets us bool()
+    def __bool__(self): # this gets us bool()
         return bool(self.len())
 
+    #__nonzero__ = __bool__  # Python 2 backwards compatibility
+
     def len(self):
         # guess what! python doesn't allow __len__ to return a long, only an
         # int. So we stop using len(spans), use spans.len() instead.
@@ -235,7 +244,7 @@ class DataSpans(object):
             for (start, data) in other.get_chunks():
                 self.add(start, data)
 
-    def __nonzero__(self): # this gets us bool()
+    def __bool__(self): # this gets us bool()
         return bool(self.len())
 
     def len(self):