python .get() and None

Question

I love python one liners:

u = payload.get("actor", {}).get("username", "")

Problem I face is, I have no control over what 'payload' contains, other than knowing it is a dictionary. So, if 'payload' does not have "actor", or it does and actor does or doesn't have "username", this one-liner is fine.

Problem of course arises when payload DOES have actor, but actor is not a dictionary.

Is there as pretty a way to do this comprehensively as a one liner, and consider the possibility that 'actor' may not be a dictionary?

Of course I can check the type using 'isinstance', but that's not as nice.

I'm not requiring a one liner per se, just asking for the most efficient way to ensure 'u' gets populated, without exception, and without prior knowledge of what exactly is in 'payload'.

Answer 1

Using EAFP

As xnx suggested, you can take advantage of the following python paradigm:

Easier to ask for forgiveness than permission

you can use it on KeyErrors as well:

try:
    u = payload["actor"]["username"]
except (AttributeError, KeyError):
    u = ""

Using a wrapper with forgiving indexing

Sometimes it would be nice to have something like null-conditional operators in Python. With some helper class this can be compressed into a one-liner expression:

class Forgive:
  def __init__(self, value = None):
    self.value = value
  def __getitem__(self, name):
    if self.value is None:
      return Forgive()
    try:
      return Forgive(self.value.__getitem__(name))
    except (KeyError, AttributeError):
      return Forgive()
  def get(self, default = None):
    return default if self.value is None else self.value

data = {'actor':{'username': 'Joe'}}
print(Forgive(data)['actor']['username'].get('default1'))
print(Forgive(data)['actor']['address'].get('default2'))

ps: one could redefine __getattr__ as well besides __getitem__, so you could even write Forgive(data)['actor'].username.get('default1').

Answer 2

Why not use an Exception:

try:
    u = payload.get("actor", {}).get("username", "")
except AttributeError:
    u = ""