Grouping and Summarizing
groupby splits the rows into groups, then you summarise each group: mean, sum, count, or several at once with agg. It is the Pandas version of SQL's GROUP BY.
Examplepython
Quick check
`df.groupby('city')['age'].mean()` returns...
Your turn: write the code
Print the number of people in each city (size).
Finish the quick check (and any code exercise) to complete this lesson, or mark it done yourself.
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