[Fixed]-In Django, how can I get an exception's message?

If you are importing ValidationError from django.core.exceptions you can simply use messages method as err.messages.

https://github.com/django/django/blob/main/django/core/exceptions.py#L124

If you are importing ValidationError from rest_framework.serializers , there is no messages property for the ValidationError but there is detail property. so you can use err.detail which will give you a dictionary.

In order to concatenate all the error messages as a string, you can use
"".join(["".join(v) for v in err.detail.values()])
or
"".join([k+" - "+"".join(v) for k,v in err.detail.items()])

https://github.com/encode/django-rest-framework/blob/master/rest_framework/exceptions.py#L143

A more general way to always know where the message is to call the dir function on your err variable to show you all the attribute of this object.
from that list you can infer where the messages are, in my case (with django 3.2) i found out that the message dict is in an attribute called args.
so to use it (with a print for example)

try:
    url = request.POST.get('u', '')
    if len(url) == 0:
        raise ValidationError('Empty URL')
except ValidationError, err:
    print(err.args) # HERE THE ERROR DICT MESSAGE IS ACCESSIBLE

I fixed it by changing ValidationError to BaseException.