[Python] Pandas astype becomes in-place operation for data loaded from pickle files

Pandas astype() appears to unexpectedly switch to performing in-place operations after loading data from a pickle file. Concretly, for astype(str), the data type of the input dataframe values is modified. What is causing this behavior?

Pandas version: 2.0.3

Minimal example:

import pandas as pd
import numpy as np

# create a test dataframe
df = pd.DataFrame({'col1': ['hi']*10 + [False]*20 + [np.nan]*30})

# print the data types of the cells, before and after casting to string
print(pd.unique([type(elem) for elem in df['col1'].values]))
_ = df.astype(str)
print(pd.unique([type(elem) for elem in df['col1'].values]))

# store the dataframe as pkl and directly load it again
outpath = 'C:/Dokumente/my_test_df.pkl'
df.to_pickle(outpath)
df2 = pd.read_pickle(outpath)

# print the data types of the cells, before and after casting to string
print(pd.unique([type(elem) for elem in df2['col1'].values]))
_ = df2.astype(str)
print(pd.unique([type(elem) for elem in df2['col1'].values]))


Output:



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