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base64 Module Complexity¶

The base64 module turns binary data into ASCII text and back: Base64 (standard and URL-safe), Base32, Base16, Ascii85, Base85 and Z85. Every encoder and decoder is linear in its input and returns a new bytes object, so each costs its input length in time and in memory. The legacy encode() and decode() copy between files and hold one line.

From Python 3.15 every encoding hands its pass to binascii, which is C. Before 3.15 only Base64, Base16 and the legacy file functions do; Base32, Ascii85, Base85 and Z85 loop over each 5- or 4-byte group in Python, so they are many times slower per byte at the same O(n), and the 85 family's peak memory is many times its output.

n is the input length: bytes for an encoder, encoded characters for a decoder. An encoder's unwrapped output is at most a fixed multiple of its input, plus padding - 4/3 for Base64, 8/5 for Base32, 2 for Base16 and 5/4 for the 85 family. L is the longest line in a file passed to decode(). The legacy functions' space is working memory; it excludes what the output file keeps.

Python 3.15 adds keyword arguments that change no bound: padded=False drops the = padding when encoding and stops requiring it when decoding, wrapcol breaks the output into lines of at most wrapcol characters, ignorechars names bytes a decoder skips, and canonical=True rejects an encoding binascii would not have produced. ignorechars is a short set of bytes and is priced at O(1).

Complexity Reference¶

Base64¶

Operation	Time	Space	Notes
`base64.b64encode(s, altchars=None)`	O(n)	O(n)	`altchars` replaces `+` and `/`
`base64.b64decode(s, altchars=None, validate=False)`	O(n)	O(n)	Discards characters outside the alphabet; `validate=True` raises `binascii.Error` instead
`base64.standard_b64encode(s)`, `base64.standard_b64decode(s)`	O(n)	O(n)	`b64encode()` and `b64decode()` with the standard alphabet
`base64.urlsafe_b64encode(s)`, `base64.urlsafe_b64decode(s)`	O(n)	O(n)	`-` and `_` instead of `+` and `/`; from Python 3.15 the decoder accepts input without its `=` padding

Base32 and Base16¶

Operation	Time	Space	Notes
`base64.b32encode(s)`	O(n)	O(n)	`binascii.b2a_base32()` from Python 3.15; a Python loop over 5-byte groups before
`base64.b32decode(s, casefold=False, map01=None)`	O(n)	O(n)	`binascii.a2b_base32()` from Python 3.15; a Python loop over 8-character groups before
`base64.b32hexencode(s)`, `base64.b32hexdecode(s, casefold=False)`	O(n)	O(n)	Base32 with the extended hex alphabet
`base64.b16encode(s)`	O(n)	O(n)	Uppercase hexadecimal
`base64.b16decode(s, casefold=False)`	O(n)	O(n)	Rejects lowercase unless `casefold=True`

Ascii85, Base85 and Z85¶

Operation	Time	Space	Notes
`base64.a85encode(b, *, foldspaces=False, wrapcol=0, pad=False, adobe=False)`	O(n)	O(n)	`binascii.b2a_ascii85()` from Python 3.15; a Python loop over 4-byte groups before
`base64.a85decode(b, *, foldspaces=False, adobe=False, ignorechars=b' \t\n\r\v')`	O(n)	O(n)	Skips `ignorechars`; `binascii.a2b_ascii85()` from Python 3.15, a Python loop over each character before
`base64.b85encode(b, pad=False)`, `base64.b85decode(b)`	O(n)	O(n)	`binascii.b2a_base85()` and `a2b_base85()` from Python 3.15; Python loops before
`base64.z85encode(s)`, `base64.z85decode(s)`	O(n)	O(n)	Python 3.13+. Base85 with the Z85 alphabet: one `binascii` call from Python 3.15, the Base85 loops plus one translation pass on 3.13 and 3.14

Legacy interface¶

Operation	Time	Space	Notes
`base64.encode(input, output)`	O(n)	O(1)	Reads 57 bytes at a time and writes one 76-character line for each
`base64.decode(input, output)`	O(n)	O(L)	Decodes one line at a time
`base64.encodebytes(s)`	O(n)	O(n)	Base64 with a newline after every 76 characters
`base64.decodebytes(s)`	O(n)	O(n)	Accepts the lines `encodebytes()` writes
`base64.main()`	O(n)	O(1) to encode, O(L) to decode	`python -m base64 [-d] [file]`: `encode()` or `decode()` from a file or pipe to stdout

Encoding and Decoding¶

Encoders take a bytes-like object. The b64, b32, b16, a85, b85 and z85 decoders also accept an ASCII str, which they encode first; decodebytes() does not.

import base64

data = b"Hello, World!"
encoded = base64.b64encode(data)  # O(n)
assert encoded == b'SGVsbG8sIFdvcmxkIQ=='

assert base64.b64decode(encoded) == data  # O(n)
assert base64.b64decode('SGVsbG8sIFdvcmxkIQ==') == data  # an ASCII str works too

# Encoders refuse str: encode the text first
try:
    base64.b64encode("Hello")
except TypeError as error:
    assert 'bytes-like object' in str(error)
else:
    raise AssertionError('an encoder accepted str')

text = base64.b64encode("Hello".encode('utf-8')).decode('ascii')
assert text == 'SGVsbG8='

Output Size¶

Each encoding expands its input by a fixed ratio, plus padding, so the choice of encoding sets the size of what you store or send. Ascii85 can come out shorter: it writes an all-zero group as z.

import base64

original = b"x" * 100

assert len(base64.b64encode(original)) == 136  # 4/3, padded to a multiple of 4
assert len(base64.b32encode(original)) == 160  # 8/5
assert len(base64.b16encode(original)) == 200  # 2
assert len(base64.b85encode(original)) == 125  # 5/4
assert len(base64.a85encode(original)) == 125  # 5/4
assert base64.a85encode(b"\0" * 8) == b'zz'     # one character per zero group

Choosing an Alphabet¶

import base64

data = b"\xfb\xff"

assert base64.b64encode(data) == b'+/8='          # O(n)
assert base64.urlsafe_b64encode(data) == b'-_8='  # O(n) - safe in URLs and file names
assert base64.urlsafe_b64decode(b'-_8=') == data

assert base64.b32encode(b"Hello") == b'JBSWY3DP'     # O(n)
assert base64.b32hexencode(b"Hello") == b'91IMOR3F'  # O(n)
assert base64.b16encode(b"Hello") == b'48656C6C6F'   # O(n)
assert base64.b16decode(b'48656c6c6f', casefold=True) == b"Hello"
assert base64.b85encode(b"Hello") == b'NM&qnZv'      # O(n)
assert base64.a85encode(b"Hello") == b'87cURDZ'      # O(n)

Decoding Untrusted Input¶

By default b64decode() drops every character outside the alphabet, so stray whitespace or line breaks decode without complaint. validate=True rejects them instead. Either way, a missing = raises binascii.Error; the decoder does not guess the padding.

import base64
import binascii

assert base64.b64decode(b"SGVs bG8=") == b"Hello"  # the space is discarded

try:
    base64.b64decode(b"SGVs bG8=", validate=True)
except binascii.Error as error:
    assert 'base64' in str(error)
else:
    raise AssertionError('validate=True accepted a space')

try:
    base64.b64decode(b"SGVsbG8")
except binascii.Error as error:
    assert 'padding' in str(error)
else:
    raise AssertionError('missing padding was accepted')

# Restore the padding when a producer strips it
stripped = b"SGVsbG8"
assert base64.b64decode(stripped + b"=" * (-len(stripped) % 4)) == b"Hello"

Streaming Files¶

encode() and decode() copy from one binary file to another a line at a time, so memory follows the line rather than the file. encode() writes 76-character lines; decode() reads whatever lines it is given, so a file that is one long line is held whole.

import base64
import io

source = io.BytesIO(b"x" * 1000)
encoded = io.BytesIO()
base64.encode(source, encoded)  # O(n) time, O(1) working memory; `encoded` keeps the output
lines = encoded.getvalue().splitlines()
assert max(len(line) for line in lines) == 76

decoded = io.BytesIO()
base64.decode(io.BytesIO(encoded.getvalue()), decoded)  # O(n) time, O(L) working memory
assert decoded.getvalue() == b"x" * 1000

# encodebytes() is the in-memory form: the same 76-character lines
assert base64.encodebytes(b"x" * 1000) == encoded.getvalue()  # O(n)
assert base64.decodebytes(encoded.getvalue()) == b"x" * 1000  # O(n)

Common Patterns¶

Binary Data in JSON¶

import base64
import json

payload = json.dumps({'content': base64.b64encode(b"\x00\x01\x02").decode('ascii')})  # O(n)

content = base64.b64decode(json.loads(payload)['content'])  # O(n)
assert content == b"\x00\x01\x02"

Data URLs¶

import base64

image = b"\x89PNG\r\n\x1a\n"
data_url = "data:image/png;base64," + base64.b64encode(image).decode('ascii')  # O(n)

header, _, body = data_url.partition(',')
assert header == 'data:image/png;base64'
assert base64.b64decode(body) == image  # O(n)

Performance Best Practices¶

✅ Do:

Before Python 3.15, use Base64 or Base16 for bulk data: they run in C, while the other encodings loop in Python. From 3.15 all of them run in C
Stream large files with encode() and decode() when 76-character lines are acceptable output
Pass validate=True when stray characters in the input should be an error
Use the URL-safe alphabet for data that goes into URLs or file names

❌ Avoid:

Base32 or the 85 family for large payloads before Python 3.15, unless the format requires them
Encoding a large file in fixed-size chunks with b64encode() and joining the pieces: a chunk whose length is not a multiple of 3 puts padding in the middle
Treating Base64 as protection: anyone can decode it

Version Notes¶

Python 3.13+: Added z85encode() and z85decode()
Python 3.15+: Base32, Ascii85, Base85 and Z85 run in C, through binascii
Python 3.15+: Added the padded, wrapcol, ignorechars and canonical keyword arguments across the encoders and decoders, and pad to z85encode()
Python 3.15+: urlsafe_b64decode() no longer requires padding; pass padded=True to require it
Python 3.15+: a + or / that the alphabet in use replaced, in the input of urlsafe_b64decode() or of b64decode() with altchars, emits FutureWarning, or DeprecationWarning from b64decode() with validate=True

binascii - The C conversions under Base64 and Base16, and under every encoding from Python 3.15
codecs - codecs.encode(data, 'base64') uses encodebytes()
json - Carrying encoded binary data in text