How to format a python assert statement that complies with PEP8?

ERR_MESSAGE_01 = '''
Some really long error message
'''

assert condition(a,b), ERR_MESSAGE_01

Is how I do it ...and I think that complies fine ..

It's important to remember that PEP8 is only a guideline and even states that there are times when the rules should be broken.

But most importantly: know when to be inconsistent -- sometimes the style guide just doesn't apply.

With that in mind, I would probably write this with old style line continuation:

def afunc(some_param_name):
    assert isinstance(some_param_name, SomeClassName), \ 
           'some_param_name must be an instance of SomeClassName, silly goose!'

If that doesn't sit well with you (or your linter), you can always do:

def afunc(some_param_name):
    assert isinstance(some_param_name, SomeClassName), ( 
           'some_param_name must be an instance of SomeClassName, silly goose!')

or even:

def afunc(some_param_name):
    assert isinstance(some_param_name, SomeClassName), ( 
           'some_param_name must be an instance of SomeClassName, '
           'silly goose!')

It's worth noting that it is possible to wrap with parenthesis, just not in the way you are thinking.

assert isinstance(some_param_name, 
                  SomeClassName), ('some_param_name must be an instance of '
                                   'SomeClassName, silly goose!')

I wouldn't argue it's particularly readable, however. In some cases, it might be the right option.