The answer is a race condition caused by concurrent dictionary modifications without synchronization. This occurs because Python dictionary operations like `dict[key] -= amount` are not atomic—they execute as separate read, modify, and write steps. When multiple threads or processes run the transfer function simultaneously on overlapping accounts, one thread’s write to the destination balance can be overwritten by another thread’s stale read, causing the addition to vanish while the subtraction persists. On the Certified Entry-Level Python Programmer PCEP exam, this question tests your understanding of thread safety and the non-atomic nature of dictionary updates, a common trap where candidates assume `try-except` catches all errors. The key insight is that race conditions produce no exceptions—they silently corrupt data. Remember the mnemonic: “Read, modify, write—if threads collide, one update takes a ride.”
PCEP Practice Question: Functions, Tuples, Dictionaries and Exceptions
This PCEP practice question tests your understanding of functions, tuples, dictionaries and exceptions. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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.
Option A is correct because 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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Clue confirmation
The clue words "first", "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
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.
Detailed technical explanation
How to think about this question
Under the hood, Python's Global Interpreter Lock (GIL) does not protect against race conditions in dictionary operations because the GIL only ensures that one bytecode instruction runs at a time, but a single Python statement like `balances[to_acc] += amount` compiles to multiple bytecodes (LOAD, BINARY_OP, STORE). A context switch between these bytecodes can interleave operations from another thread, causing lost updates. In real-world financial systems, this is mitigated by using threading locks (e.g., `threading.Lock`) or atomic database transactions to ensure consistency.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Functions, Tuples, Dictionaries and Exceptions — This question tests Functions, Tuples, Dictionaries and Exceptions — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The dictionary is being modified concurrently by multiple threads or processes without synchronization, leading to race conditions. — Option A is correct because 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.
What should I do if I get this PCEP question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "first", "most likely". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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