Length of binary data in python

Question

I am using Python. I am trying to determine the correct length of bytes in a binary set of data.

If I assign a variable the binary data...

x = "aabb".decode("hex")

is that the same as

x = b'aabb'

And if so, how do you get how many bytes that is? (It should be 2 bytes)

When I try:

len(x)

I get 4 instead of 2 though...

I am worried that x is turned into a string or something else I don't understand because the data types are so fluid in Python...

When I do x = "aabb".decode("hex"), then len(x) returns 2. Is that not what it returns on your machine? — Kevin
– Kevin, Commented Nov 21, 2016 at 17:46
"aabb".decode("hex") == b'aabb' returns False. Your assumption on these two forms being equal is wrong. — Łukasz Rogalski
– Łukasz Rogalski, Commented Nov 21, 2016 at 17:49
Kevin, you are correct, It is only when I do x = b'aabb' do I get the length of 4, so the question is I guess what does b'aabb' actually do? — Matthew
– Matthew, Commented Nov 21, 2016 at 17:57
In Python 2, b'aabb' is identical to 'aabb', but they're different in Python 3. Also, "aabb".decode("hex") in Python 3 raises AttributeError: 'str' object has no attribute 'decode'. And b"aabb".decode("hex") in Python 3 raises LookupError: 'hex' is not a text encoding; use codecs.decode() to handle arbitrary codecs — PM 2Ring
– PM 2Ring, Commented Nov 21, 2016 at 17:58
To convert 'aabb' to the bytestring b'\xaa\xbb' in a way that works in both versions you can do: import binascii;binascii.unhexlify("aabb") — PM 2Ring
– PM 2Ring, Commented Nov 21, 2016 at 18:04

wim · Accepted Answer · 2018-10-15 16:55:43Z

20

The length of binary data is just the len, and the type is str in Python-2.x (or bytes in Python-3.x). However, your object 'aabb' does not contain the two bytes 0xaa and 0xbb, rather it contains 4 bytes corresponding with ASCII 'a' and 'b' characters:

>>> bytearray([0x61, 0x61, 0x62, 0x62])
bytearray(b'aabb')
>>> bytearray([0x61, 0x61, 0x62, 0x62]) == 'aabb'
True

This is probably the equivalence you were actually looking for:

>>> 'aabb'.decode('hex') == b'\xaa\xbb' 
True

The following items are all equal (and length 2):

>>> s1 = 'aabb'.decode('hex')
>>> s2 = b'\xaa\xbb'
>>> s3 = bytearray([0xaa, 0xbb])
>>> s4 = bytearray([170, 187])
>>> s1 == s2 == s3 == s4
True

edited Oct 15, 2018 at 16:55

answered Nov 21, 2016 at 17:55

wim

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2 Comments

Elliot Woods Over a year ago

hmm.. would be good to answer the title though rather than the body text

wim Over a year ago

@ElliotWoods I've edited to put it first and foremost.

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