CODEWITHSUNDEEP
pythontuples
Phil Hannent
Phil Hannent

Reputation: 12317

How do I convert part of a python tuple (byte array) into an integer

I am trying to talk to a device using python. I have been handed a tuple of bytes which contains the storage information. How can I convert the data into the correct values:

response = (0, 0, 117, 143, 6)

The first 4 values are a 32-bit int telling me how many bytes have been used and the last value is the percentage used.

I can access the tuple as response[0] but cannot see how I can get the first 4 values into the int I require.

Upvotes: 8

Views: 42852

Answers (6)

Adrian Bool
Adrian Bool

Reputation:

Would,

num = (response[0] << 24) + (response[1] << 16) + (response[2] << 8) + response[3]

meet your needs?

aid

Upvotes: 13

Kristian Vinther
Kristian Vinther

Reputation: 608

This looks like a job for reduce!

What you basically need is to, bit-shift a byte at a time, and then add (addition) the next byte in the sequence. 

a = (0, 0, 117, 143, 6)
reduce(lambda x, y: (x<<8) + y, a)
7704326

Upvotes: 2

medigeek
medigeek

Reputation:

How about using the map function:

a = (0, 0, 117, 143, 6)
b = []
map(b.append, a)

Also, I don't know if this is you are looking for:

response = (0, 0, 117, 143, 6)
response[0:4]

Upvotes: 0

Jay
Jay

Reputation: 42692

See Convert Bytes to Floating Point Numbers in Python

You probably want to use the struct module, e.g.

import struct

response = (0, 0, 117, 143, 6)
struct.unpack(">I", ''.join([chr(x) for x in response[:-1]]))

Assuming an unsigned int. There may be a better way to do the conversion to unpack, a list comprehension with join was just the first thing that I came up with.

EDIT: See also ΤΖΩΤΖΙΟΥ's comment on this answer regarding endianness as well.

EDIT #2: If you don't mind using the array module as well, here is an alternate method that obviates the need for a list comprehension. Thanks to @JimB for pointing out that unpack can operate on arrays as well.

import struct
from array import array

response = (0, 0, 117, 143, 6)
bytes = array('B', response[:-1])
struct.unpack('>I', bytes)

Upvotes: 12

Mr_Pink
Mr_Pink

Reputation: 109417

You could also make use of the array module

import struct
from array import array
response = (0, 0, 117, 143, 6)
a = array('B', response[:4])
struct.unpack('>I', a)

(30095L,)

Upvotes: 4

Ad__
Ad__

Reputation:

OK, You don't specify the endinanness or whether the integer is signed or and it (perhaps) is faster to with the struct module but:

b = (8, 1, 0, 0)
sum(b[i] << (i * 8) for i in range(4))

Upvotes: 4

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