Flask: conditional validation on multiple form fields

Let me start with usual - I'm new to both Python & Flask. Before posting this question (my first ever here) I have spent number of hours searching and experimenting, unfortunately with no luck so far.

I am building web form where user can define firewall rules, which subsequently get recorded in the database. I am at the validation stage and I came up to a wall here... hopefully someone will be able to help me out.

My (simplified here) form has 2 fields - src_ip and dst_ip:

class FirewallRule(Form)
    src_ip = StringField('Source IP')
    dst_ip = StringField('Destination IP')

and my validation requirements are:

1. At least 1 field must be filled (both are also fine)


2. Fields that are filled must contain valid IP address

wtforms.validators.IPAddress() and custom validation function seems to be my friend, but I am struggling to find a way to plug them together.

Essentially I am trying to build conditional validation:

1. If not (src_ip OR dst_ip); return False


2. If src_ip AND src_ip not valid IP address; return False


3. If dst_ip AND dst_ip not valid IP address; return False


4. Return True

Obviously I would like to re-use IPAddress() [or generally any of the built-in validators] rather then write my own.

I am sure someone must have done it before.. unfortunately I could not find any pointers in the right direction.

Thanks in advance.