Question 159 of 498
PCEP Practice Question: Functions, Tuples, Dictionaries and Exceptions
You are a developer for a financial application that processes transactions. The application uses a dictionary to store account balances where keys are account numbers (strings) and values are floats. A function `transfer(from_acc, to_acc, amount)` is supposed to subtract amount from `from_acc` and add it to `to_acc`. However, some transfers are resulting in incorrect balances: the `from_acc` balance is reduced but the `to_acc` balance is not increased. The code uses `try-except` to catch KeyError if an account does not exist. Upon inspection, the function first checks if both accounts exist, then performs subtraction, then addition, and finally returns success. No exceptions are raised during the problematic transfers. The accounts definitely exist. What is the most likely cause?
⚠ Common exam trap
Python Institute often tests the misconception that Python's GIL prevents all concurrency issues, but the trap here is that the GIL does not make compound operations atomic, so race conditions can still occur with dictionary updates.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The dictionary is being modified concurrently by multiple threads or processes without synchronization, leading to race conditions.
The described symptom — the `from_acc` balance is reduced but the `to_acc` balance is not increased — is a classic race condition. In Python, dictionary operations like `dict[key] -= amount` are not atomic; they involve a read, modify, and write sequence. If two threads execute the transfer function concurrently on overlapping accounts, one thread's write to `to_acc` can be overwritten by another thread's stale read, causing the addition to be lost. The `try-except` only catches `KeyError`, not data races, and since no exception is raised, the only plausible explanation is unsynchronized concurrent access.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The dictionary is being modified concurrently by multiple threads or processes without synchronization, leading to race conditions.
Why this is correct
Race condition can cause the second update to be lost.
- ✗
The function catches KeyError and silently returns without completing the transfer.
Why it's wrong here
No KeyError is raised.
- ✗
The function does not check if the from_acc has sufficient balance before subtracting.
Why it's wrong here
Insufficient balance would cause negative balance but not missing addition.
- ✗
The balances are stored as strings instead of floats, causing concatenation instead of arithmetic.
Why it's wrong here
If strings, concatenation would increase both balances, not cause missing addition.
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Last reviewed: Jun 30, 2026
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