Python:Pandas .replace()

The .replace() function returns a new DataFrame object with specified values replaced with another specified value. The original DataFrame object, used to call the method, remains unchanged unless explicitly declared.

Syntax

df = dataframe.replace(value_to_replace, new_value, inplace)

dataframe is the DataFrame with the source data and value_to_replace is the value being targeted within the DataFrame. new_value is the value used to replace the original value. inplace is False by default. The original DataFrame values will not be replaced unless inplace is explicitly declared to True within the parameters.

.replace() has the following parameters:

Example

In the following example, the .replace() method is used to replace ??? values in a DataFrame, first with a scalar, then with a dict. For the second example with dict, it will be shown with differing amounts of ?:

import pandas as pd
import numpy as np

d = {"col 1" : [1,2,3,"???"], "col 2" : ["A","B","???","D"], "col 3" : [5,"???",7,8], "col 4" : ["???","F","G","H"]}
df = pd.DataFrame(data = d)

print(f"Example 1 Original df:\n{df}\n")

first_replace = df.replace("???", np.nan)
print(f"After first replace():\n{first_replace}\n")

d = {"col 1" : [1,2,3,"?"], "col 2" : ["A","B","??","D"], "col 3" : [5,"???",7,8], "col 4" : ["????","F","G","H"]}
df = pd.DataFrame(data = d)

print(f"Example 2 Original df:\n{df}\n")

second_replace = df.replace({"?" : 4, "??" : "C", "???" : 6, "????" : "E"})
print(f"After second replace():\n{second_replace}\n")

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The output from these instances of the .replace() method is shown below:

Example 1 Original df:
  col 1 col 2 col 3 col 4
0     1     A     5   ???
1     2     B   ???     F
2     3   ???     7     G
3   ???     D     8     H

After first replace():
   col 1 col 2  col 3 col 4
0    1.0     A    5.0   NaN
1    2.0     B    NaN     F
2    3.0   NaN    7.0     G
3    NaN     D    8.0     H

Example 2 Original df:
  col 1 col 2 col 3 col 4
0     1     A     5  ????
1     2     B   ???     F
2     3    ??     7     G
3     ?     D     8     H

After second replace():
   col 1 col 2  col 3 col 4
0      1     A      5     E
1      2     B      6     F
2      3     C      7     G
3      4     D      8     H

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