How to discover type hierarchy in Python

Given any type in Python, you can easily discover its ancestor and descendant types. This ease of discovery of the internals of the language is one of my favorite features of Python.

• Remember that all types are descended from the object type.

• Even type is a type and it is a child of the object type.

• The __base__ attribute of any type has a string value with the name of the parent type.

• The __subclasses__ method of any type lists the child types.

• To determine which are the standard types (or builtin types or builtins as they are called in Python), check the __module__ attribute of the type. If it is builtins in Python 3 or __builtin__ in Python 2, then that is a standard type.

• If you start from object, you can actually list the entire type hierarchy tree. A script that does just that can be found here:

#!/usr/bin/env python3

"""Print hierarchy of types present in Python."""

import argparse
import sys

def is_builtin(t):
    """Check if type is builtin."""
    builtin_s = "__builtin__" if sys.version_info.major <= 2 else "builtins"
    return t.__module__ == builtin_s

def process_a_type(t, level, options):

    # Not a builtin type
    if options.only_builtins and not is_builtin(t):
        return

    # Print type
    s = ""
    if level > 0:
        s += "    " * (level - 1)
        s += "+-- "
    s += t.__name__
    if options.show_module:
        s += " ({})".format(t.__module__)
    print(s)

    # -type- has no subclasses
    if t.__name__ == "type":
        return

    # Recurse through child types
    for st in t.__subclasses__():
        process_a_type(
            st,
            level + 1,
            options,
        )

def get_args():
    ap = argparse.ArgumentParser(description=__doc__, formatter_class=argparse.ArgumentDefaultsHelpFormatter)
    ap.add_argument("--only_builtins", default=False, action="store_true", help="Only show builtin types")
    ap.add_argument("--show_module", default=False, action="store_true", help="Show module name beside type")
    ap.add_argument("--import_module", default="", type=str, help="Import this module to get its types")
    args = ap.parse_args()
    return args

def main():
    args = get_args()

    # Import module wanted by user
    if args.import_module:
        __import__(args.import_module)

    # -object- type is root
    process_a_type(
        object,
        0,
        args,
    )

if __name__ == "__main__":
    main()

• In Python 3.5.2, I found that there are 143 builtin types (most of them are just types of Exception) in the tree:

object
+-- type
+-- dict_values
    +-- odict_values
+-- tuple_iterator
+-- set
+-- fieldnameiterator
+-- frame
+-- dict_keyiterator
+-- PyCapsule
+-- coroutine
+-- bytearray
+-- NoneType
+-- list
+-- dict
+-- getset_descriptor
+-- method-wrapper
+-- method
+-- str_iterator
+-- formatteriterator
+-- str
+-- set_iterator
+-- range_iterator
+-- memoryview
+-- cell
+-- generator
+-- map
+-- list_iterator
+-- stderrprinter
+-- reversed
+-- method_descriptor
+-- code
+-- weakproxy
+-- int
    +-- bool
+-- ellipsis
+-- module
+-- dict_items
    +-- odict_items
+-- bytearray_iterator
+-- Struct
+-- moduledef
+-- filter
+-- staticmethod
+-- tuple
+-- frozenset
+-- managedbuffer
+-- coroutine_wrapper
+-- function
+-- builtin_function_or_method
+-- odict_iterator
+-- float
+-- range
+-- super
+-- dict_keys
    +-- odict_keys
+-- list_reverseiterator
+-- bytes_iterator
+-- member_descriptor
+-- wrapper_descriptor
+-- property
+-- instancemethod
+-- zip
+-- weakref
+-- slice
+-- longrange_iterator
+-- dict_valueiterator
+-- EncodingMap
+-- callable_iterator
+-- mappingproxy
+-- BaseException
    +-- Exception
        +-- TypeError
        +-- StopAsyncIteration
        +-- SyntaxError
            +-- IndentationError
                +-- TabError
        +-- AttributeError
        +-- AssertionError
        +-- StopIteration
        +-- MemoryError
        +-- BufferError
        +-- NameError
            +-- UnboundLocalError
        +-- LookupError
            +-- IndexError
            +-- KeyError
        +-- EOFError
        +-- ImportError
        +-- ValueError
            +-- UnicodeError
                +-- UnicodeEncodeError
                +-- UnicodeDecodeError
                +-- UnicodeTranslateError
        +-- RuntimeError
            +-- RecursionError
            +-- NotImplementedError
        +-- SystemError
        +-- Warning
            +-- UserWarning
            +-- DeprecationWarning
            +-- BytesWarning
            +-- SyntaxWarning
            +-- PendingDeprecationWarning
            +-- FutureWarning
            +-- ResourceWarning
            +-- ImportWarning
            +-- RuntimeWarning
            +-- UnicodeWarning
        +-- ReferenceError
        +-- OSError
            +-- ConnectionError
                +-- BrokenPipeError
                +-- ConnectionAbortedError
                +-- ConnectionRefusedError
                +-- ConnectionResetError
            +-- BlockingIOError
            +-- NotADirectoryError
            +-- PermissionError
            +-- FileExistsError
            +-- TimeoutError
            +-- IsADirectoryError
            +-- InterruptedError
            +-- ProcessLookupError
            +-- FileNotFoundError
            +-- ChildProcessError
        +-- ArithmeticError
            +-- FloatingPointError
            +-- OverflowError
            +-- ZeroDivisionError
    +-- GeneratorExit
    +-- KeyboardInterrupt
    +-- SystemExit
+-- dict_itemiterator
+-- classmethod
+-- NotImplementedType
+-- iterator
+-- bytes
+-- enumerate
+-- classmethod_descriptor
+-- complex
+-- traceback
+-- weakcallableproxy

• Note how bool is a child type of the int type.

• In Python 2.7.12, I found that there are 60 builtin types in the tree:

object
+-- type
+-- weakref
+-- weakcallableproxy
+-- weakproxy
+-- int
    +-- bool
+-- basestring
    +-- str
    +-- unicode
+-- bytearray
+-- list
+-- NoneType
+-- NotImplementedType
+-- traceback
+-- super
+-- xrange
+-- dict
+-- set
+-- slice
+-- staticmethod
+-- complex
+-- float
+-- buffer
+-- long
+-- frozenset
+-- property
+-- memoryview
+-- tuple
+-- enumerate
+-- reversed
+-- code
+-- frame
+-- builtin_function_or_method
+-- instancemethod
+-- function
+-- classobj
+-- dictproxy
+-- generator
+-- getset_descriptor
+-- wrapper_descriptor
+-- instance
+-- ellipsis
+-- member_descriptor
+-- file
+-- PyCapsule
+-- cell
+-- callable-iterator
+-- iterator
+-- EncodingMap
+-- fieldnameiterator
+-- formatteriterator
+-- module
+-- classmethod
+-- dict_keys
+-- dict_items
+-- dict_values
+-- deque_iterator
+-- deque_reverse_iterator
+-- Struct

• Note how str and unicode are child types of the basestring type. Also observe how this differs from Python 3 builtin types.

• Also notice how in Python 2 the exception types are not builtin types.