Dunder Wonder

Python Magic Methods

• Joe Mann

Topics Covered

Global special variables (__name__, __all__, etc)

Private and Protected Methods

Magic Methods

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Terminology

Under (_) and Dunder (__) methods

_asdict, __supersecretmethod, __eq__

Duck Typing

If it walks like a duck and quacks like a duck, then it must be a duck

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Global Variables

Special Global Variables

• __name__

if __name__ == ‘__main__’:

pass # Do something

• __builtins__ / __builtin__
• __doc__
• __all__

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Private and Protected Methods

Private Methods

• Using the single underscore to designate
• A convention rather than a strongly enforced standard
• You can still access class methods that start with ‘_’ outside said class
• For functions, these won’t be included in ‘from spam import *’
• Some IDEs and code checkers will warn about access outside of class / module

Protected Methods

• Using a double underscore (dunder) to designate
• Also more of a convention than a standard
• Names are mangled, but it is a predictable pattern
• This is done to eliminate overriding rather than hiding these methods
• Really bad (and hacky) to call these outside the class / module

Magic Methods

Magic Methods

• Construction and initialization: __new__, __init__, __del__
• Object Representation: __str__, __repr__, __format__
• Container Methods: __len__, __getitem__, __setitem__
• Also…. __dict__

Questions?

Magic Methods, cont

• Binary Operators: __add__, __sub__
• Uniary Operators: __neg__, __pos__, __abs__
• Attributes: __getattr__, __setattr__
• Extended Operators (+=, -=, etc): __iadd__, __isub__
• Type Conversions: __int__, __long__