Method writable

src/python/PyImath/PyImathFixedArray.h:578–578

    Py_ssize_t len()      const { return _length;   }
    size_t     stride()   const { return _stride;   }
    bool       writable() const { return _writable; }

    // This method is mainly here for use in confidence tests, but there may
    // be other use-cases where a writable array needs to be made read-only.