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Write a data validation function simulating an Airflow TaskFlow task. The function must assert that row count is greater than zero and that no null values exist in the amt column, returning True if valid or raising ValueError on failure.
Operating on a DataFrame slice without `.copy()`, producing `SettingWithCopyWarning`, or using `.apply(axis=1)` with custom Python functions when vectorized NumPy / pandas column operations could run 50x faster with 0 memory overhead.
Interviewers assess whether you write idiomatic, vectorized pandas code instead of slow row-by-row procedural Python loops.
Construct the solution logically from first principles to avoid typical edge case pitfalls.
Use boolean masks with `.loc[row_mask, col_list]` to avoid chained assignment warnings.
subset = df.loc[df['status'] == 'completed'].copy()
Use numpy/pandas native operations like `.groupby()`, `.transform()`, or column arithmetic.
df['rev_share'] = df['revenue'] / df.groupby('country')['revenue'].transform('sum')Sort values and reset index to match the required evaluation output contract.
result = df.sort_values('rev_share', ascending=False).reset_index(drop=True)def validate_batch(records):
if not records:
raise ValueError('Batch is empty')
for r in records:
if r.get('amt') is None:
raise ValueError('Null amount detected')
return True
result = validate_batch(records)Real code patterns candidates submit that fail the grading suite.
results = []
for i, row in df.iterrows():
results.append(row['val'] * 2)Three recurring syntax and semantic traps relevant to this problem domain.
Modifying a filtered slice like df[df.age > 30]['salary'] = 5000 modifies an ephemeral copy instead of the underlying dataframe.
df[df['active'] == True]['status'] = 'verified' # ❌ SettingWithCopyWarning
df.loc[df['active'] == True, 'status'] = 'verified' # ✅ Idiomatic inplace assignment
Calling df.groupby('category').agg(...) places 'category' into the MultiIndex/Index, making subsequent column references fail.
res = df.groupby('dept')['salary'].mean(); print(res['dept']) # ❌ KeyErrorres = df.groupby('dept', as_index=False)['salary'].mean() # ✅ Keeps column intactIterating rows with for index, row in df.iterrows() is an anti-pattern in data science interviews.
for i, r in df.iterrows(): df.at[i, 'total'] = r['price'] * r['qty'] # ❌ Slow
df['total'] = df['price'] * df['qty'] # ✅ C-speed vectorization
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