pythongroovynullnullpointerexception

Safe dereferencing in Python

Groovy has a nice operator for safe dereferencing, which helps to avoid NullPointerExceptions:

variable?.method()

The method will only be called, if variable is not null.

Is there a way to do the same in Python? Or do I have to write if variable: variable.method()?

Solution

  • EDIT 2021:

    There is a new package that is sort of a hack featuring exactly this functionality in Python. Here is the repo: https://github.com/iann838/nullsafe-python

    from nullsafe import undefined, _

value = _(variable).method()
assert value is undefined
assert not value
assert value == None

    Works with AttributeError and KeyError aswell

    dic = {}
assert _(dic)["nah"] is undefined
assert _(dic).nah is undefined

    The wrapped object typings will remain effective.

    1. No, there isn't.

    2. But to check for None, you don't write if x:, you write if x is None:. This is an important distinction - x evaluates to False for quite a few values that are propably perfectly valid (most notably 0-equivalent numbers and empty collections), whereas x is None only evaluates to True if the reference x points to the singleton object None.

    3. From personal experience, such an operator would be needed very rarely. Yes, None is sometimes used to indicate no value. But somehow - maybe because idiomatic code returns null objects where sensible or throws exceptions to indicate critical failure - I only get an AttributeError: 'NoneType' object has no attribute '...' twice a month.

    4. I would argue that this might be a misfeature. null has two meanings - "forgot to initialize" and "no data". The first is an error and should throw an exception. The second case usually requires more elaborate handling than "let's just not call this method". When I ask the database/ORM for a UserProfile, it's not there and I get null instead... do I want to silently skip the rest of the method? Or do I really want to (when in "library code") throw an approriate exception (so "the user (code)" knows the user isn't there and can react... or ignore it) or (when I'm coding a specific feature) show a sensible message ("That user doesn't exist, you can't add it to your friend list") to the user?