Question

issue is this: in (pl)python code, we’ve calculated an integer = 26663.
Can easily convert this to hex using hex(myint) = 0x6827

So far so good!

Now, how to write this value -into a concatenation of strings- into a PostgreSQL (v9) bytea field? The DB is UTF8-encoded, if this matters.

EG, neither of these examples will work:

Here, of course, I cannot concatenate ‘str’ and ‘int’ objects:

rv = plpy.execute(plan, [ (string1 + 6827) ])

This one inputs the wrong hex code for 0x6827

rv = plpy.execute(plan, [ (string1 + str('6827')) ])

Help!

Answer 1

I’m not familiar with Postgres, but the hex(n) function returns a string representation of the numeric value of n in hexadecimal. The nicest way in my opinion to concatenate this with a string is to use format strings. For example:

rv = plpy.execute(plan, [ ( 'foo %s bar' % hex(6827) ) ] )

If the string is really in a variable called string1, and you only need to append it with the hex value, then simple concatenation using the + sign will work fine:

rv = plpy.execute(plan, [ ( string1 + hex(6827) ) ])

This works without conversion because the hex() function returns a string.

If you don’t actually want to store a printable string representation, but rather a binary string, use the struct module to create an array of bytes.

import struct
bytes = struct.pack('i', 6827) # Ignoring endianness

A lot of people are confused about what storing something as “binary” actually means, and since you are using a field type (bytea) which seems to be intended for binary storage, maybe this is what you actually want?

The returned value from bytes will be a string that you can either concatenate with another string, or continue to pack more binary values into.

See the struct module documentation for more information!