I don't know why it is wrong when I import ctypes

Question

I changed the definition of the builtin function len, then I tried to import the ctypes module.But I don't know why it occurs an error when I did that.Here is my code:

import builtins def func(obj): print("Length: %d" % len(obj)) builtins.len = lambda obj: "Bad!!!" import ctypes

Then here is the tracback of the error:

Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<frozen importlib._bootstrap>", line 969, in _find_and_load File "<frozen importlib._bootstrap>", line 958, in _find_and_load_unlocked File "<frozen importlib._bootstrap>", line 673, in _load_unlocked File "<frozen importlib._bootstrap_external>", line 661, in exec_module File "<frozen importlib._bootstrap_external>", line 772, in get_code File "<frozen importlib._bootstrap_external>", line 491, in _code_to_bytecode File "<frozen importlib._bootstrap_external>", line 42, in _w_long ValueError: invalid literal for int() with base 10: 'Bad!!!' Error in sys.excepthook: Traceback (most recent call last): File "/usr/lib/python3/dist-packages/apport_python_hook.py", line 53, in apport_excepthook if not enabled(): File "/usr/lib/python3/dist-packages/apport_python_hook.py", line 24, in enabled import re File "<frozen importlib._bootstrap>", line 969, in _find_and_load File "<frozen importlib._bootstrap>", line 958, in _find_and_load_unlocked File "<frozen importlib._bootstrap>", line 673, in _load_unlocked File "<frozen importlib._bootstrap_external>", line 661, in exec_module File "<frozen importlib._bootstrap_external>", line 772, in get_code File "<frozen importlib._bootstrap_external>", line 491, in _code_to_bytecode File "<frozen importlib._bootstrap_external>", line 42, in _w_long ValueError: invalid literal for int() with base 10: 'Bad!!!' Original exception was: Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<frozen importlib._bootstrap>", line 969, in _find_and_load File "<frozen importlib._bootstrap>", line 958, in _find_and_load_unlocked File "<frozen importlib._bootstrap>", line 673, in _load_unlocked File "<frozen importlib._bootstrap_external>", line 661, in exec_module File "<frozen importlib._bootstrap_external>", line 772, in get_code File "<frozen importlibbootstrap_external>", line 491, in _code_to_bytecode File "<frozen importlib._bootstrap_external>", line 42, in _w_long ValueError: invalid literal for int() with base 10: 'Bad!!!'

This puzzles me a lot. Thanks very much if someone can help me!

Why are you puzzled? Changing the functionality of a heavily-relied upon basic builtin then trying to use it (indirectly, through importing) as it was intended should cause an error, no? — jedwards
– jedwards, Commented Sep 9, 2017 at 10:19
Do you mean the changed len function is excuted when I tried to import ctypes? @jedwards — duan xingzhe
– duan xingzhe, Commented Sep 9, 2017 at 10:28
The changed len fun is definitely called when you tried to import ctypes (see ValueError: invalid literal for int() with base 10: 'Bad!!!'). I'm not sure why though, unless a version of ctypes has a dependency on the future package. — jedwards
– jedwards, Commented Sep 9, 2017 at 10:56

Arount · Accepted Answer · 2017-09-09 10:26:04Z

Because your len function returns non integer string.

You just ran in the exact same behavior you could have with:

>>> int("I should know playing with builtins is always a bad thing") Traceback (most recent call last): File "<stdin>", line 1, in <module> ValueError: invalid literal for int() with base 10: 'lol'

Try to update your code like:

builtins.len = lambda obj: "123"

To see it.

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