Today I learned the difference between __getattr__ and __getattribute__.

__getattr__ vs __getattribute__

Today I attended a tutorial at EuroPython 2024 where the speaker showed a key difference between the dunder methods __getattr__ and __getattribute__:

class Missing:
    attr = 42

    def __getattr__(self, name):
        print(f"In __getattr__, asked for {name}")
        return 73

m = Missing()
print(m.attr)  # 42
print(m.xyz)  # In __getattr__, asked for xyz; 73

The code above shows that the dunder method __getattr__ was called for the lookup m.xyz.

class Always:
    attr = 42

    def __getattribute__(self, name):
        print(f"In __getattribute__, asked for {name}")
        return 73

a = Always()
print(a.attr)  # In __getattribute__, asked for attr; 73
print(a.xyz)  # In __getattribute__, asked for xyz; 73

The snippet with the class Always shows that the dunder method __getattribute__ was called for both lookups: a.attr and a.xyz.

I posted about this on X (Twitter) and Fosstodon and someone briefly explained that __getattribute__ is the dunder method that governs all attribute lookups, whereas __getattr__ is what's called when __getattribute__ fails and it's __getattr__ that you typically want to implement. When playing with these two dunder methods, it is easy to create infinite recursion loops if you implement __getattribute__ poorly.

The same person who commented on my post also pointed to the blog article “Unravelling attribute access in Python” (also in the references) that goes into much more detail on how this actually works. It's a good article. Take a look if you're interested in understanding how this works.

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References

• Unravelling attribute access in Python, https://snarky.ca/unravelling-attribute-access-in-python/ [last accessed 09-07-2024];