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An ongoing list of pandas quirks
In just-pandas-things.ipynb
Note: You can also just use df.loc[0, "Review"] to calculate the memory address to retrieve the item. Its performance is comparable to gret row then get review.
gret row then get review
should be get review then get row
?
Hidden chaining is another frequent issue responsible for SettingWithCopyWarning
. It may occur the following way:
import pandas as pd
import numpy as np
d = {'col1': [1, 2, 3], 'col2': [3, 4, np.nan]}
df = pd.DataFrame(data=d)
# This will cause the warning
df_no_na = df.dropna()
df_no_na.loc[:, 'new_column'] = df_no_na['col1'] + df_no_na['col2']
# This won't cause the warning
df_no_na = df.dropna().copy()
df_no_na.loc[: ,'new_column'] = df_no_na['col1'] + df_no_na['col2']
The point is that it also occurs when using df.loc[:, 'new_column'] so using .loc[] does not always prevent this warning from happening. The reason for the warning is obviously the same as in the example in your notebook, however i think this issue of chaining a get and an assignment is less obvious, hence 'hidden' :-).
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