import pandas as pd

summary = (
    df.groupby('category')
      .agg(
          n=('price', 'size'),
          avg_price=('price', 'mean'),
          top_price=('price', 'max'),
      )
      .reset_index()
)

# Rank WITHIN each category, best first
df['rank'] = (
    df.groupby('category')['price']
      .rank(method='dense', ascending=False)
      .astype(int)
)

# Safe conditional assignment — one .loc, never chained indexing
df.loc[df['price'] > 100, 'tier'] = 'premium'