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What does print(df['b'].tolist()) print? pandas Copy-on-Write, Views and Assignment
Block 7, Copy-on-Write, Views and Assignment, question 16 of the pandas sample
import pandas as pd
df = pd.DataFrame({'a': [1, 2, 3], 'b': [7, 8, 9]})
df.loc[df['a'] > 1, 'b'] = 0
print(df['b'].tolist())What does this print?
Show the answer and why each option is right or wrong
Answer, C. [7, 0, 0]
- A. ChainedAssignmentError
- Assumes this recommended single step pattern still triggers the warning that chained assignment triggers.
- B. [7, 8, 9]
- Assumes a loc based write is chained the same way df[mask][col] is, and gets silently dropped.
- C. [7, 0, 0]
- Correct. df.loc[mask, 'b'] = 0 addresses both the rows and the column in one call on df itself, so it is a direct write that succeeds, giving [7, 0, 0].
- D. [0, 0, 0]
- Assumes the write reaches every row of the column instead of only the rows selected by the mask.
The documentation for chained assignment recommends exactly this pattern, .loc[row_indexer, col_indexer] = value, because it resolves the row selection and the column selection in one call rather than two separate bracket lookups. It writes straight into df with no intermediate object involved.
The answer above is what pandas 3.0.5 printed when the snippet was run, not what someone expected it to do.
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