How can I create byte values from integers in Python?

Question

Background: I need to send a numerical value as a byte to an external device, but I have run into a problem. My code is:

ser=serial.Serial("COM3",9600, timeout=0)
ser.write(value)

where "value" is an int that I read have read. The problem is, when I send this, it sends the character value, not the actual value (it sends the byte value 31 for the number 5, since that is the unicode position for it, I believe)

In reality, I want to be able to send it the character "\x05" for example. I guess my question is, how would I convert and int 5 to a char "\x05", or 37 to "\x37"

Amber · Accepted Answer · 2012-03-10 09:05:07Z

8

Use the built-in function chr().

If you have a list of such integers you need to send, you might consider using a bytearray().

Alternatively, in newer versions of Python you can simply use a byte type.

answered Mar 10, 2012 at 9:05

Amber

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shofee · Accepted Answer · 2012-03-10 09:12:17Z

5

you can use this..

bytes(chr(my_int))    # not strictly correct unless 0<=my_int<=255
bytes((my_int,))

answered Mar 10, 2012 at 9:12

shofee

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2 Comments

Jack O'Connor Over a year ago

The first one fails, at least in Python 3, because it's trying to convert a string into bytes without specifying an encoding. (Note to readers, see joelonsoftware.com/articles/Unicode.html if encoding issues are new to you.) The second one works great. If the trailing comma used to create a tuple looks bad to you, you can also do something like bytes([5]).

larsks Over a year ago

The second one fails under Python 2, where bytes((1,)) returns the string '(1,)'. Under Python 3 this returns b'\x01'. So don't use either of these if you expect to support both Python 2 and 3.

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