我有一个大约1mn行的 Big Data 集，大约5000个缺少的坐标(我想用"city"类别的中值填充它们.除了fillna，一切都正常，如何实现？
city = ['London', 'Paris', 'Vienna', 'Milan','London', 'Paris', 'Vienna', 'Milan']
lat = [51.510843900000005, 48.8671391, 48.204465500000005, 45.4787357, 51.510843900000005, 48.8671391, None, None]
lng = [-0.1424476, 2.328075, 16.3686397, 9.1961308, -0.14244, 2.329, None, None]
data = pd.DataFrame(list(zip(city, lat, lng)),columns =['city', 'lat', 'lng'])
display(data['lat'].isna().sum()) # 2
display(data['lng'].isna().sum()) # 2
for city_name in set(data['city']):
data[data['city'] == city_name ]['lat'].fillna(data[data['city'] == city_name ]['lat'].median())
data[data['city'] == city_name ]['lng'].fillna(data[data['city'] == city_name ]['lng'].median())
print(city_name, data[data['city'] == city_name ]['lat'].median(),data[data['city'] == city_name ]['lng'].median())
display(data['lat'].isna().sum()) # 2
display(data['lng'].isna().sum()) # 2