Python3 TypeError: can only concatenate list (not "str") to list

Question

I'm porting odoo 11, python 2.7 to python 3. I have been edited an addon which has belongs to odoo, python code.

The code is:

vat = invoice.partner_id.vat or ''
vat = list(filter(lambda x: x.isnumeric(), vat[:2])) + vat[2:]

Error is:

TypeError: can only concatenate list (not "str") to list

How can I fix that, what is wrong with this code? Please help me.

Answer 1

list(filter(lambda x: x.isnumeric(), vat[:2]))

This operation above always returns the list.

vat = invoice.partner_id.vat or '' 

• seems, this operation returns str (because of or '').

If you expect that your type(vat)==list, you should use

vat = invoice.partner_id.vat or []

If you expect type(vat)==str, you should convert your filtered list to str, for example

"".join(list(filter(lambda x: x.isnumeric(), vat[:2]))) + vat[2:]