Adding and removing columns in Data Analysis Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

How do you add a new column to a pandas DataFrame?

You can add a new column by assigning a list, array, or a single value to a new column name, like df['new_column'] = values.

beginner

What method removes a column from a pandas DataFrame?

Use df.drop('column_name', axis=1) to remove a column. Set inplace=True to change the DataFrame directly.

beginner

Can you add a column based on calculations from other columns? How?

Yes. For example, df['new_col'] = df['col1'] + df['col2'] adds a column by summing two existing columns.

beginner

What does axis=1 mean in the context of dropping columns?

axis=1 tells pandas to operate on columns. Without it, pandas assumes rows (axis=0).

intermediate

How to remove multiple columns at once from a DataFrame?

Pass a list of column names to df.drop(['col1', 'col2'], axis=1) to remove multiple columns.

Which code adds a new column 'age' with all values 30 to DataFrame df?

A df.remove('age')

B df['age'] = 30

C df.drop('age')

D df.add_column('age', 30)

Assigning a value to a new column name adds that column with the value repeated.

How do you remove the column 'salary' from DataFrame df without changing df itself?

A df.drop('salary')

B df.remove('salary')

C df.drop('salary', axis=1)

D df.delete('salary')

Use df.drop with axis=1 to remove a column. Without inplace=True, df stays unchanged.

What happens if you do df['total'] = df['price'] * df['quantity']?

A Adds a new column 'total' with product of 'price' and 'quantity'

B Deletes 'total' column

C Raises an error

D Replaces 'price' column

You create a new column by multiplying two existing columns element-wise.

Which axis value is used to drop rows in pandas?

A axis=0

B axis=1

C axis=2

D axis=-1

axis=0 refers to rows, axis=1 refers to columns.

How to remove columns 'A' and 'B' from DataFrame df in one command?

A df.remove(['A', 'B'])

B df.drop('A', 'B')

C df.delete('A', 'B')

D df.drop(['A', 'B'], axis=1)

Pass a list of column names to drop with axis=1 to remove multiple columns.

Explain how to add a new column to a pandas DataFrame using existing columns.

Think about creating a new column by combining or calculating from others.

Describe the difference between dropping a column with and without inplace=True.

Consider whether the original DataFrame changes or a new one is returned.