Series.replace not working on slices of heterogeneous types

Core Problem

The Series.replace function in pandas does not work as expected when applied to a slice of a heterogeneous type series. This issue can lead to unexpected behavior and incorrect results.

Solution & Analysis

To understand this issue, let's first look at the code sample provided:

import pandas as pd
import numpy as np 

c = pd.Series([
    np.nan,
    1,
    "hello",
])
c_replaced_1 = c[0:3].replace({np.nan: None})
c_replaced_2 = c[0:2].replace({np.nan: None})
print(c_replaced_1)
print(c_replaced_2)

As expected, c_replaced_1 prints:

0     None
1        1
2    hello
dtype: object

However, c_replaced_2 does not print as expected, instead it prints:

0    NaN
1    1.0
dtype: float64

The problem here is that when we apply replace to a slice of the series, pandas tries to replace each value in the slice separately. However, if the values in the slice are of different types (as seen in c_replaced_2), this can lead to unexpected behavior.

To fix this issue, you can use the inplace=True parameter when applying replace to a slice of the series. Here's how you can modify the code:

import pandas as pd
import numpy as np 

c = pd.Series([
    np.nan,
    1,
    "hello",
])
c_replaced_1 = c[0:3].replace({np.nan: None}, inplace=True)
c_replaced_2 = c[0:2].replace({np.nan: None}, inplace=True)
print(c_replaced_1)
print(c_replaced_2)

Alternatively, you can convert the series to a homogeneous type before applying replace. Here's how you can do it:

import pandas as pd
import numpy as np 

c = pd.Series([
    np.nan,
    1,
    "hello",
])
# Convert the series to float64 before applying replace
c_float = c.astype(float)
c_replaced_2 = c_float[0:2].replace({np.nan: None})
print(c_replaced_2)

Conclusion

The Series.replace function in pandas can be finicky when applied to slices of heterogeneous type series. By using the inplace=True parameter or converting the series to a homogeneous type, you can avoid this issue and ensure that your code produces the expected results.

Reference

• Source