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  • What is __qualname__ in python and how is it useful?

  • Why would I need to use it over __name__?

I read the docs, but they didn't help me get a clear understanding on it's usefulness.

I have read Get fully qualified name of a Python class (Python 3.3+).
That question asks "how to get a qualified name", which presumes that one knows the meaning of "qualified name". Obviously, the answer to that question is to use the __qualname__ attribute.

My question asks what __qualname__ is, and why should I use it over __name__.

Max Vollmer
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Lord Elrond
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    @Selcuk How is that a duplicate? I am asking what `__qualname__` is and how it is used, not *How do I get the fully qualified name*... – Lord Elrond Sep 26 '19 at 01:49
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    The question you linked has no explanation about what it does or what it is used for. – Lord Elrond Sep 26 '19 at 01:50
  • It's a 100% duplicate, but feel free to nominate it for reopening. I don't think you have read the question and the accepted answer. Also I find the [docs](https://docs.python.org/3/glossary.html#term-qualified-name) very clear on that too. – Selcuk Sep 26 '19 at 01:52
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    `You are looking for __qualname__ (introduced in Python 3.3):` – Lord Elrond Sep 26 '19 at 01:53
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    I don't consider that an explanation for what `__qualname__` is. – Lord Elrond Sep 26 '19 at 01:53
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    You might want to elaborate on why the docs didn't help you understand what a qualified name is. I find the explanation pretty straight forward. Helping us understand your lack of understanding helps us provide a proper explanation that hopefully helps you understand. – Max Vollmer Sep 26 '19 at 04:48

2 Answers2

116

__qualname__ gives more complete information than __name__ and therefore can be more helpful in debugging, for example.

Example:

>>> def f(): pass
... class A:
...    def f(self): pass
...    class A:
...        def f(self): pass
...
>>> # __name__ is not showing the path, so these functions look equal
>>> f.__name__
'f'
>>> A.f.__name__
'f'
>>> A.A.f.__name__
'f'
>>> # And these classes looks equal
>>> A.__name__
'A'
>>> A.A.__name__
'A'
>>>
>>> # __qualname__ shows the path, so these functions are distinguishable
>>> f.__qualname__
'f'
>>> A.f.__qualname__
'A.f'
>>> A.A.f.__qualname__
'A.A.f'
>>> # And these classes are distinguishable
>>> A.__qualname__
'A'
>>> A.A.__qualname__
'A.A'

__qualname__ is also adding some backwards compatibility with Python 2's .im_class.

More details in the rationale for PEP 3155

Mattias Wallin
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    Not only does this help with debugging, but it's also tremendously useful when monkey patching in order to avoid false positives when checking whether the original function is the one you expect. – blubberdiblub Sep 22 '21 at 06:43
38

Just to (potentially) add to the previous answer, __qualname__ can also be called from inside a class, without it having to be bound to any methods. This allows to get a class' name from inside the class when you don't have an __init__ method defined:

class myClass:
    print(__qualname__)

This will return:

myClass

A practical scenario where I found this useful is when working with logging. If you want to implement it in a module with a class that, as stated before, doesn't have an __init__ method, i.e. consists only on class methods, then to add the class' name to the dot-notation path that logging requires to join logs generated by different modules into a single one, __qualname__ seems like an easy solution.

rmoralesdelgado
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