PrivateConstructorMeta

class lsst.daf.butler.core.utils.PrivateConstructorMeta

Bases: type

A metaclass that disables regular construction syntax.

A class that uses PrivateConstructorMeta may have an __init__ and/or __new__ method, but these can’t be invoked by “calling” the class (that will always raise TypeError). Instead, such classes can be called by calling the metaclass-provided _construct class method with the same arguments.

As is usual in Python, there are no actual prohibitions on what code can call _construct; the purpose of this metaclass is just to prevent instances from being created normally when that can’t do what users would expect.

..note:

Classes that inherit from PrivateConstructorMeta also inherit
the hidden-constructor behavior.  If you just want to disable
construction of the base class, `abc.ABCMeta` may be a better
option.

Examples

Given this class definition::

class Hidden(metaclass=PrivateConstructorMeta):

def __init__(self, a, b):

self.a = a self.b = b

This doesn’t work:

>>> instance = Hidden(a=1, b="two")
TypeError: Hidden objects cannot be constructed directly.

But this does:

>>> instance = Hidden._construct(a=1, b="two")

Methods Summary

__call__(*args, **kwds)	Disabled class construction interface; always raises TypeError.
mro()	return a type’s method resolution order

Methods Documentation

__call__(*args, **kwds)

Disabled class construction interface; always raises TypeError.

mro() → list

return a type’s method resolution order