Counter object adds up

What this problem practises

Object-oriented bugs hide in state: class attributes shared by every instance, mutable defaults declared in __init__ signatures, and methods that mutate self when the caller expected a copy.

This is an output-prediction problem: read the snippet carefully and write down exactly what it prints before anything runs. These questions are interview staples because they test whether you know the language rules, or merely recognise their syntax.

Difficulty: medium — expect to hold two interacting rules in your head at once, such as a boundary combined with a mutation.

The challenge

What will this program print? Type the exact output.

The code runs in PyDebug’s in-browser compiler. Open the problem, run the snippet, watch the exact error or output, and keep iterating until it is right. Signed-in solves count toward XP and your daily streak.

The snippet

The interactive editor pre-fills exactly this code — read it, then submit your prediction:

class Counter:
    def __init__(self):
        self.count = 0
    def add(self, n):
        self.count += n
c = Counter()
c.add(3)
c.add(10)
print(c.count)

More oop problems

• AttributeError: missing self medium
• AttributeError: missing self medium
• AttributeError: missing self medium

Keep going

• Output-prediction strategies — how to predict tricky snippets without running them.
• Browse every oop problem in this topic.
• Common Python errors and what they actually mean, plus the Python glossary for any term used on this page.