from SO
import pandas as pd
import numpy as np
df = pd.DataFrame({'group': np.repeat(np.arange(1000), 10),
'B': np.nan,
'C': np.nan})
df.ix[4::10, 'B':'C'] = 5 # every 4th row of a group is non-null
df.groupby('group').transform('first')
This is then iterating over groups. Last I can see this was changed is: here. My recollection is that this was ONLY supposed to hit in a special case, and the general case is simply a repeat based on the indices.
This seems to be hitting in all cases makes transform back to super SLOW.
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