Re: subclassable types

Discussion in 'Python' started by Wolfgang Maier, Feb 24, 2013.

  1. Terry Reedy <tjreedy <at> udel.edu> writes:

    >
    > On 2/22/2013 4:35 AM, Wolfgang Maier wrote:
    > > Dear all,
    > > I am wondering what the rules are that determine whether a built-in type is
    > > subclassable or not.
    > > As examples, why can you base your classes on int or set,
    > > but not on bool or range?
    > > Also: can you use introspection to find out whether a type is valid as a
    > > base type?

    >
    > You are not the first to ask about this. I opened a doc issue.
    > http://bugs.python.org/issue17279
    >


    Thanks, Terry,
    using the __flags__ & (1 << 10) trick from Daniel, I did something similar to
    what you just added to the doc issue.
    The complete list of types classes that *cannot* be subclassed is:

    <class 'builtin_function_or_method'>
    <class 'code'>
    <class 'frame'>
    <class 'function'>
    <class 'generator'>
    <class 'getset_descriptor'>
    <class 'function'>
    <class 'mappingproxy'>
    <class 'member_descriptor'>
    <class 'method'>
    <class 'traceback'>
    <class 'range'>
    <class 'memoryview'>
    <class 'slice'>
    <class 'bool'>

    of these range, memoryview, slice and bool are the only ones in builtins.

    Just thought it might be interesting.
    Wolfgang Maier, Feb 24, 2013
    #1
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