Data Science with Python Certification Data Science with Python Pandas DataFrame Operations 3

Question 1

What is the default behavior of pd.merge() when no 'how' argument is specified?

Accepted Answer

Inner join

Answer

pd.merge() defaults to an inner join, returning only rows with matching keys in both DataFrames.

Question 2

Which method converts a DataFrame column to the 'category' dtype?

Accepted Answer

df['col'].astype('category')

Answer

astype('category') is the standard way to convert a column to the memory-efficient category dtype.

Question 3

What does df.groupby('dept')['salary'].transform('mean') return?

Accepted Answer

A Series the same length as df with each row replaced by its group mean

Answer

transform broadcasts the group-level result back to the original DataFrame's index, preserving its length.

Question 4

How do you forward-fill missing values in a DataFrame?

Accepted Answer

df.fillna(method='ffill')

Answer

fillna(method='ffill') propagates the last valid observation forward to fill NaN values.

Question 5

What is the output type of df.groupby('category').agg({'price': ['min','max']})?

Accepted Answer

A DataFrame with MultiIndex columns

Answer

Passing a list of functions to agg creates a DataFrame with MultiIndex columns (column name, function name).

Data Science with Python Certification Practice Test