Question

I noted this syntax listed as a gotchya but with no explanation as to why:

def func(x=None):
    #good
    if x == None:
        x = []

    #bad
    x = x or []

In what ways can this be a gotchya?

Answer 1

In this particular case, it’s bad because an empty list evaluates to false. If you mutate the list, then the results won’t be as expected

def func(x=None):
    x = x or []
    x.append('hello')

mylist = []
func(mylist)
print mylist[0] # doesn't work

Since you want to check to see if the caller passed None or not, you should just say that explicitly and not try to be clever. Objects like [], (), {}, 0, and False evaluate to false, as can any user-defined object that wants to — checking for falsehood really catches too many false positives.

(Also: Using is None instead of == None can be faster and more reliable.)