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Concepts·Certified Associate Python Programmer

Python Instance and Class Variables

This is the most asked subject in PCAP's 34% object-oriented section, and it comes down to one rule: **reading an attribute searches the instance and then the class; assigning one always writes to the instance.** Everything surprising follows - the list shared between every instance, and the counter that increments to 1 three times over.

Object-Oriented Python Guide 15 of 25 Intermediate

Written against the versions above. Unchanged in Python 3. The attribute lookup order described here is the same mechanism that makes inheritance work - see {{guide:inheritance-and-overriding}}.

One machine, and any shell with Python 3 will do - these exams test the language, not a distribution.
Server NameIP AddressOSRolesCPURAMHDD
RUNNER01192.168.0.27Ubuntu 26.04 LTSPython 3.14.4 - the only machine this path needs2 Core4 GB50 GB

Before you start

  1. Where each variable lives

  2. Assigning through an instance creates an instance variable

  3. The trap: a mutable class variable

  4. The fix

  5. What a class variable is actually for

  6. The counter that does not count

  7. The rule, in one line

  8. What the exam does with this objective

Official sources

← All PCAP guides