What Does __flags__ In Python Type Used For

I have been read pickle source code recently. The following code in copy_reg make me confused: _HEAPTYPE = 1<<9 def _reduce_ex(self, proto): assert proto < 2 for

Solution 1:

__flags__ is a wrapper, to access CPython type object structure member tp_flags, constants used to compose this flag defined in object.h, following is quoted from the source:

Type flags (tp_flags) These flags are used to extend the type structure in a backwards-compatible fashion. Extensions can use the flags to indicate (and test) when a given type structure contains a new feature. The Python core will use these when introducing new functionality between major revisions (to avoid mid-version changes in the PYTHON_API_VERSION).

See more detail on python document on tp_flags.

But interesting thing is that class.flags & _HEAPTYPE is always 0 when the class is a python internal type. And the result will be 1 when class is a subclass of python internal type.

Subclass of python built-in type, like other user defined type, allocated on heap with PyType_GenericAlloc().

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to break down type.__flags__:

import re

defflags_to_name(type_obj):
    tp_flag_consts = {}        
    withopen('/path/to/Include/object.h') as f:
        for l in f:
            m = re.search(r'^#define (Py_TPFLAGS_\w+)\s+\(.+?<< (\d+)\)', l.strip())
            if m:
                tp_flag_consts[int(m.group(2))] = m.group(1)
    bin_str = bin(type_obj.__flags__)[2:][::-1]
    return', '.join(tp_flag_consts[n] for n, c inenumerate(bin_str) if c == '1')

print(flags_to_name(type))

yields:

Py_TPFLAGS_BASETYPE, Py_TPFLAGS_READY, Py_TPFLAGS_HAVE_GC, Py_TPFLAGS_HAVE_VERSION_TAG, Py_TPFLAGS_VALID_VERSION_TAG, Py_TPFLAGS_TYPE_SUBCLASS

Solution 2:

__flags__ is to be viewed as binary, if you want to see the flags.

These type flags are defined here in Python's source code.