Add, change and delete data

For many data sets, you may want to perform a transformation based on the values in an array, series or column in a DataFrame. For this, we look at the first Unicode characters:

import numpy as np
import pandas as pd

df = pd.DataFrame(
    {
        "Code": ["U+0000", "U+0001", "U+0002", "U+0003", "U+0004", "U+0005"],
        "Decimal": [0, 1, 2, 3, 4, 5],
        "Octal": ["001", "002", "003", "004", "004", "005"],
        "Key": ["NUL", "Ctrl-A", "Ctrl-B", "Ctrl-C", "Ctrl-D", "Ctrl-E"],
    }
)

df

	Code	Decimal	Octal	Key
0	U+0000	0	001	NUL
1	U+0001	1	002	Ctrl-A
2	U+0002	2	003	Ctrl-B
3	U+0003	3	004	Ctrl-C
4	U+0004	4	004	Ctrl-D
5	U+0005	5	005	Ctrl-E

Add data

Suppose you want to add a column where the characters are assigned to the C0 or C1 control code:

control_code = {
    "u+0000": "C0",
    "u+0001": "C0",
    "u+0002": "C0",
    "u+0003": "C0",
    "u+0004": "C0",
    "u+0005": "C0",
}

The map method for a series accepts a function or dict-like object that contains an assignment, but here we have a small problem because some of the codes in control_code are lower case, but not in our DataFrame. Therefore, we need to convert each value to lower case using the str.lower method:

lowercased = df["Code"].str.lower()

lowercased

0    u+0000
1    u+0001
2    u+0002
3    u+0003
4    u+0004
5    u+0005
Name: Code, dtype: object

df["Control code"] = lowercased.map(control_code)

df

	Code	Decimal	Octal	Key	Control code
0	U+0000	0	001	NUL	C0
1	U+0001	1	002	Ctrl-A	C0
2	U+0002	2	003	Ctrl-B	C0
3	U+0003	3	004	Ctrl-C	C0
4	U+0004	4	004	Ctrl-D	C0
5	U+0005	5	005	Ctrl-E	C0

We could also have passed a function that does all the work:

df["Code"].map(lambda x: control_code[x.lower()])

0    C0
1    C0
2    C0
3    C0
4    C0
5    C0
Name: Code, dtype: object

Using map is a convenient way to perform element-wise transformations and other data cleaning operations.

Change data

Note:

Replacing missing values is described in Managing missing data with pandas.

pd.Series(["Manpower", "man-made"]).str.replace("Man", "Personal", regex=False)

0    Personalpower
1         man-made
dtype: object

pd.Series(["Man-Power", "man-made"]).str.replace("[Mm]an", "Personal", regex=True)

0    Personal-Power
1     Personal-made
dtype: object

Note:

The replace method differs from str.replace in that it replaces strings element by element.

Delete data

Deleting one or more entries from an axis is easy if you already have an index array or a list without these entries.

To delete duplicates, see Deduplicating data.

Since this may require a bit of set theory, we return the drop method as a new object without the deleted values:

rng = np.random.default_rng()
s = pd.Series(rng.normal(size=7))

s

0    0.721314
1    0.566529
2    0.149298
3   -1.076892
4   -0.174988
5   -0.844397
6   -0.894065
dtype: float64

new = s.drop(2)

new

0    0.721314
1    0.566529
3   -1.076892
4   -0.174988
5   -0.844397
6   -0.894065
dtype: float64

new = s.drop([2, 3])

new

0    0.721314
1    0.566529
4   -0.174988
5   -0.844397
6   -0.894065
dtype: float64

With DataFrames, index values can be deleted on both axes. To illustrate this, we first create an example DataFrame:

data = {
    "Code": ["U+0000", "U+0001", "U+0002", "U+0003", "U+0004", "U+0005"],
    "Decimal": [0, 1, 2, 3, 4, 5],
    "Octal": ["001", "002", "003", "004", "004", "005"],
    "Key": ["NUL", "Ctrl-A", "Ctrl-B", "Ctrl-C", "Ctrl-D", "Ctrl-E"],
}

df = pd.DataFrame(data)

df

	Code	Decimal	Octal	Key
0	U+0000	0	001	NUL
1	U+0001	1	002	Ctrl-A
2	U+0002	2	003	Ctrl-B
3	U+0003	3	004	Ctrl-C
4	U+0004	4	004	Ctrl-D
5	U+0005	5	005	Ctrl-E

df.drop([0, 1])

	Code	Decimal	Octal	Key
2	U+0002	2	003	Ctrl-B
3	U+0003	3	004	Ctrl-C
4	U+0004	4	004	Ctrl-D
5	U+0005	5	005	Ctrl-E

You can also remove values from the columns by passing axis=1 or axis='columns':

df.drop("Decimal", axis=1)

	Code	Octal	Key
0	U+0000	001	NUL
1	U+0001	002	Ctrl-A
2	U+0002	003	Ctrl-B
3	U+0003	004	Ctrl-C
4	U+0004	004	Ctrl-D
5	U+0005	005	Ctrl-E

Many functions such as drop that change the size or shape of a row or DataFrame can manipulate an object in place without returning a new object:

df.drop(0, inplace=True)

df

	Code	Decimal	Octal	Key
1	U+0001	1	002	Ctrl-A
2	U+0002	2	003	Ctrl-B
3	U+0003	3	004	Ctrl-C
4	U+0004	4	004	Ctrl-D
5	U+0005	5	005	Ctrl-E

Warning:

Be careful with the inplace function, as the data will be irretrievably deleted.