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bytes Type Complexity

The bytes type is an immutable sequence of bytes: one fixed buffer, never resized or edited after it is built. Its searches and predicates only read that buffer, and the operations that produce bytes copy what they return - except that, because nothing can change it, an exact bytes returns itself from most operations whose result would be an equal copy: a full slice, a strip that strips nothing, a replace() that finds nothing. For the mutable counterpart, which always copies, see bytearray.

n is the bytes in the receiver, k is the bytes in the other operand (the data concatenated, the slice covered, or the characters of a hex string), m is the bytes in a pattern (sub, old, prefix, suffix, sep), p is the parts handed to join(), and w is the bytes in the result where that differs from n. Reading one byte, or one item of a bytes, is O(1). The decode() row assumes a codec and error handler whose cost and output are proportional to their input, as the byte-oriented codecs are, and the % row assumes values whose formatting costs what it emits.

Complexity Reference

Sequence operations

Operation Time Space Notes
len(b) O(1) O(1)
b[i] O(1) O(1) An int in range(256); b[i] = v raises TypeError
b[i:j], b[i:j:step] O(k) O(k) A copy of the k bytes covered; a step-1 slice covering all of b is b itself, O(1). memoryview(b) is the O(1) alternative
x in b O(n) O(1) x is an int or bytes-like; one longer than b returns at once
b + other, b += other O(n + k) O(n + k) other is any bytes-like. += cannot grow b in place, so a loop of them copies everything accumulated each time
b * count O(n·count) O(n·count) O(1) for a count of 1, which returns b itself, and for a count of 0 or less, which returns b''
b == other, b != other O(n) O(1) O(1) when the lengths differ; < and the other orderings walk to the first difference
for x in b O(n) O(1) One int per byte; iter(b) on its own is O(1)
hash(b) O(n) the first time, then O(1) O(1) Cached on the object; an equal bytes built separately hashes its n bytes again
b % values O(n + w) O(n + w) w = result length
memoryview(b) O(1) O(1) A read-only view of the same buffer; slicing the view is O(1) as well

Searching

Operation Time Space Notes
find(sub, start, end), index(sub, start, end) O(n) O(1) sub is bytes-like or an int; index() raises ValueError when absent. With start and end, n is the region searched, read in place; a sub longer than it returns at once
rfind(sub, start, end), rindex(sub, start, end) O(n·m) O(1) Worst case, absent patterns included: the reverse search has no linear-time fallback, where the forward one does. n is the region searched, as for find()
count(sub, start, end) O(n) O(1) Non-overlapping occurrences; n is the region searched, as for find()
startswith(prefix, start, end), endswith(suffix, start, end) O(m + q) O(1) Independent of n. Either accepts a tuple of q candidates, and then m is their total length

Transforming

Where the Notes say b itself, a call that changes nothing returns the receiver rather than a copy; the others always build a new object, except that the case-changing methods and expandtabs() return the shared empty bytes for an empty b.

Operation Time Space Notes
replace(old, new, count=-1) O(n + w) O(w) w = result length. b itself, in O(1) space, when old does not occur
translate(table, /, delete=b'') O(n + d) O(n) table is a 256-byte table from maketrans(), or None; d = bytes in delete. b itself when no byte changes, though the n-byte result is built before that is known
bytes.maketrans(from, to) O(k) O(1) A 256-byte bytes whatever k is; the two arguments must be the same length. Static method
upper(), lower(), capitalize(), title(), swapcase() O(n) O(n) ASCII letters only. A new object even when nothing changes
strip(chars), lstrip(chars), rstrip(chars) O(n + (s + 1)·c) O(n) s = bytes stripped, c = bytes in chars, which is searched once per stripped byte and once more to stop; without chars, c is O(1). b itself when nothing is stripped, which costs only that O(c) search and O(1) space
removeprefix(prefix), removesuffix(suffix) O(n) O(n) b itself when the prefix or suffix is absent: O(m) time and O(1) space
center(width, fillbyte), ljust(width, fillbyte), rjust(width, fillbyte), zfill(width) O(width) O(width) b itself, O(1), when width ≤ n
expandtabs(tabsize=8) O(n + w) O(w) A new object even with no tabs; a tabsize below 1 drops the tabs, so w can be smaller than n

Splitting and joining

Operation Time Space Notes
split(sep=None, maxsplit=-1) O(n + m) O(n) A list of new bytes holding at most n bytes between them; [b] holding b itself when no separator occurs
rsplit(sep=None, maxsplit=-1) O(n·m) O(n) Worst case: a multi-byte sep is searched backwards, with the rfind() worst case at each match. None, and a one-byte sep, scan. [b] when no separator occurs
splitlines(keepends=False) O(n) O(n) [b] holding b itself when there is no line break
partition(sep) O(n + m) O(n) A hit copies the bytes either side of sep; a miss returns (b, b'', b'') with b itself, in O(1) space
rpartition(sep) O(n·m) O(n) Worst case, as for rfind(). A miss returns (b'', b'', b) with b itself
join(iterable) O(p + w) O(p + w) Every part must be bytes-like, and each costs a buffer descriptor whatever its length. A lone part that is an exact bytes is returned itself

Predicates and conversion

Operation Time Space Notes
isalnum(), isalpha(), isascii(), isdigit(), islower(), isspace(), istitle(), isupper() O(n) O(1) Stop at the first byte that settles the answer
decode(encoding='utf-8', errors='strict') O(n) O(n)
hex(sep, bytes_per_sep) O(n) O(n) Two characters per byte, plus any separators
bytes.fromhex(string) O(k) O(k) k = characters, one byte per pair; whitespace between pairs is skipped. Class method

Unchanged Results Are the Receiver

A bytes cannot change, so handing back the same object is as good as a copy. Where a method finds nothing to do, an exact bytes usually returns itself instead of copying, which a bytearray never does. The case-changing methods and expandtabs() are the exceptions: they always build a new object from a non-empty bytes.

data = b"Hello"

assert data[:] is data  # O(1) - the whole buffer, step 1
assert bytes(data) is data  # O(1) - see bytes()
assert data.strip() is data  # O(1) - nothing to strip, only the ends are read
assert data.replace(b"z", b"y") is data  # O(n) to search, O(1) space
assert data.removeprefix(b"z") is data  # O(m)
assert data.center(3) is data  # O(1) - width <= n
assert data.split(b",")[0] is data  # O(n + m) - no separator, so no copy
assert data.partition(b",")[0] is data  # O(n + m)

assert data.upper() is not data  # O(n) - always a new object
assert data.upper().upper() == b"HELLO"
mutable = bytearray(data)
assert mutable.strip() is not mutable  # a bytearray always copies


class Packet(bytes):
    pass


packet = Packet(b"Hello")
assert packet.strip() is not packet  # a subclass gets a copy
assert type(packet.strip()) is bytes

Building Bytes Incrementally

b += other cannot extend b in place: once b is non-empty, each non-empty other builds a new object holding both, so a loop that accumulates p chunks copies everything gathered so far on every pass, O(w·p) in total. Collect the parts and join() them once, O(p + w), or grow a bytearray and convert it at the end.

chunks = [b"header", b":", b"payload", b"\n"] * 3

# QUADRATIC: every += copies the whole accumulated value
slow = b""
for chunk in chunks:
    slow += chunk  # O(len(slow) + len(chunk))

# LINEAR: one pass to measure, one to copy
fast = b"".join(chunks)  # O(p + w)

# LINEAR: an amortized O(k) append per chunk, one copy at the end
buffer = bytearray()
for chunk in chunks:
    buffer += chunk  # O(k) amortized
built = bytes(buffer)  # O(w)

assert slow == fast == built
assert len(fast) == 3 * len(b"header:payload\n")
assert b"".join([fast]) is fast  # a single bytes part is returned itself

Hashing and Dictionary Keys

A bytes computes its hash the first time it is asked and keeps it, so a key used many times hashes its bytes once. The cache belongs to the object: a fresh slice or concatenation that produces an equal bytes is a new object, and hashing it reads every byte again. A successful lookup with a distinct but equal key also compares the two, reading all k bytes of the key.

routes = {b"GET /": "index", b"GET /health": "health"}

request = b"GET /health HTTP/1.1"
key = request.partition(b" HTTP/")[0]  # O(n + m) - a new 11-byte object

assert routes[key] == "health"  # O(k) to hash the new key, O(k) to compare it
assert routes[key] == "health"  # O(k) - the hash is cached, the comparison is not
assert hash(key) == hash(b"GET /health")  # equal bytes, equal hashes

try:
    hash(bytearray(key))
except TypeError as error:
    assert "unhashable" in str(error)
else:
    raise AssertionError("a bytearray was hashed")

Slices Copy, Views Do Not

A slice copies the k bytes it covers. A memoryview shares the buffer, and so does every slice of the view, so parsing a large message field by field through a view copies only what is finally converted.

message = b"\x00\x05hello world"

length = int.from_bytes(message[:2], "big")  # O(k) - a 2-byte copy
assert length == 5

view = memoryview(message)  # O(1)
body = view[2 : 2 + length]  # O(1) - no bytes copied
assert body.tobytes() == b"hello"  # O(k) - the one copy, when it is needed
assert bytes(body) == b"hello"  # O(k) likewise

try:
    view[0] = 1  # the view is read-only, as the bytes is
except TypeError as error:
    assert "read-only" in str(error)
else:
    raise AssertionError("a view of a bytes was written to")
view.release()

Searching

find() is linear even in the worst case. rfind() searches backwards with a simpler algorithm that can compare most of the pattern at every position, and so can cost the pattern's length at each of the receiver's. rsplit(), rpartition() and rindex() search the same way.

haystack = b"a" * 10_000
needle = b"ab" + b"a" * 98  # mismatches only at its second byte

assert haystack.find(needle) == -1  # O(n) - linear in the worst case
assert haystack.rfind(needle) == -1  # O(n·m) - m comparisons at each of n positions
assert haystack.count(b"aa") == 5_000  # O(n) - non-overlapping
assert haystack.startswith(b"aaa")  # O(m) - the prefix's length, not the receiver's
assert 97 in haystack  # O(n) - an int is a byte value

try:
    haystack.index(b"b")  # O(n)
except ValueError as error:
    assert "not found" in str(error)
else:
    raise AssertionError("index() found a byte that is not there")

Common Patterns

Splitting Records

data = b"alice,30\nbob,25\ncara,41\n"

rows = []
for line in data.splitlines():  # O(n)
    name, _, age = line.partition(b",")  # O(n + m), n = this line's length
    rows.append((name.decode("ascii"), int(age)))  # O(n)

assert rows == [("alice", 30), ("bob", 25), ("cara", 41)]

Normalising Without Copying

Normalising input that is usually clean makes no copies of the clean inputs, since each step returns its receiver when it has nothing to do.

def normalise(field):
    return field.strip().removeprefix(b"0x")  # no copy when nothing changes


clean = b"ff00"
assert normalise(clean) is clean  # no copies made
assert normalise(b" 0xff00 ") == b"ff00"  # two copies, each O(n)
assert bytes.fromhex(normalise(b" 0xff00 ").decode("ascii")) == b"\xff\x00"  # O(k)

Performance Best Practices

✅ Do:

  • Build output with b"".join(parts) or a bytearray, not +=; both stay linear in the bytes produced
  • Slice through memoryview(b) when parsing a large buffer field by field; each bytes slice copies its k bytes
  • Reuse one bytes object as a repeated dictionary key; its hash is computed once and cached
  • Prefer the forward find(), index(), split() and partition() to their r counterparts on input you do not control; only the forward search is linear in the worst case

❌ Avoid:

  • b += chunk in a loop - each pass copies everything accumulated so far
  • Copying a bytes to protect it, with b[:] or bytes(b) - it cannot change, and both return b itself
  • Calling upper(), lower() or expandtabs() to test the contents - they always copy, where isupper(), find() and startswith() do not
  • rfind(), rsplit() or rpartition() with a long pattern on untrusted input - O(n·m) in the worst case

Version Notes

  • Python 3.14+: bytes.fromhex() accepts bytes-like input as well as str
  • bytes() - Every way of building a bytes, and what each costs
  • bytearray - The mutable counterpart: the same methods, plus amortized O(1) appends, but every transform copies
  • bytearray() - Building a bytearray, the linear way to accumulate bytes
  • memoryview() - O(1) views and slices over a bytes buffer
  • str - Text; str.encode() and bytes.decode() convert between the two in O(n)

Further Reading