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Align object.__sizeof__ with CPython for ints #6516

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Align object.__sizeof__ with CPython for ints #6516

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Copilot AI commented Dec 25, 2025 •
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The base object.__sizeof__ returned only the type basic size, underreporting memory for variable-sized ints compared to CPython.

  • Int-aware sizing: object.__sizeof__ now calculates int sizes using the same bit-length-based formula as int.__sizeof__, ensuring parity for arbitrary-sized integers.
  • Regression guard: Added a snippet test to assert object.__sizeof__ is never smaller than int.__sizeof__ for a very large integer.

Example:

big = 10**200
assert object.__sizeof__(big) >= big.__sizeof__()  # matches CPython-style sizing
Original prompt

This section details on the original issue you should resolve

<issue_title>object.__sizeof__ is not compatible with CPython implementation</issue_title>
<issue_description>When you run the below code, you can see the difference. ⬇️

# RustPython
>>>>> object.__sizeof__(1223456789812391231291231231231212312312312312312312321321321321312312321123123123199129391239219394923912949213021949302194942130123949203912430392402139210492139123012940219394923942395943856228368385)
32

# CPython
>>> object.__sizeof__(1223456789812391231291231231231212312312312312312312321321321321312312321123123123199129391239219394923912949213021949302194942130123949203912430392402139210492139123012940219394923942395943856228368385)
116

Plan (tracking)

<agent_instructions>Investigate why the size is different and try to properly implement it</agent_instructions>

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@youknowone
fix object sizeof int handling
4c3e63c 
Co-authored-by: youknowone <69878+youknowone@users.noreply.github.com>
Copilot AI changed the title [WIP] Fix object.__sizeof__ compatibility with CPython implementation Align object.__sizeof__ with CPython for ints Dec 25, 2025
Copilot AI requested a review from youknowone December 25, 2025 10:23
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youknowone commented Dec 25, 2025

This is too ad-hoc. There must be the right way

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object.__sizeof__ is not compatible with CPython implementation

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