Most idiomatic way to provide default value in python?

Question

Many discussions and SO searches have been unable to conclusively answer the question of the best/safest/most Pythonic way of providing a default value if a Python function receives None in a parameter. This specifically came up in regards to a datetime parameter, in case that matters, but ideally we standardize our approach for all parameter types.

Here are the two approaches that both keep coming up as the "correct" way to do this:

1. myval if myval else defaultval
2. myval or defaultval

Are they both functionally equivalent, or are there subtle differences between the two? I vastly prefer the brevity and clarity of the second option, but adherents to the first one say it's not always safe. Any guidance from someone with more python experience than me (e.g. nearly anyone) would be greatly appreciated.

Answer 1

1 is by far the preffered method of conditional assignment (Explicit is better than implicit)

and even better to be explicit

myval = myval if myval is not None else defaultval

or even better

def some_function(arg1,arg2="defaultValue"):
    myval = arg2

the main problem is

x = x or y

can never be assigned an x of 0 or an empty array or any other falsy value

Answer 2

They are exactly equivalent, however I suggest

defaultval if myval is None else myval

(This behaves properly when passed ie myval = []).