DataFrame column assignment

While assigning to data can be a relatively expensive operation in pandas, assigning columns to a pd.DataFrame is a common operation.

How to do it

Let’s create a very simple pd.DataFrame:

df = pd.DataFrame({"col1": [1, 2, 3]})
df

    col1
0   1
1   2
2   3

New columns can be assigned using the pd.DataFrame[] operator. The simplest type of assignment can take a scalar value and broadcast it to every row of the pd.DataFrame:

df["new_column1"] = 42
df

    col1   new_column1
0   1      42
1   2      42
2   3      42

You can also assign a pd.Series or sequence as long as the number of elements matches the number of rows in the pd.DataFrame:

df["new_column2"] = list("abc")
df

    col1   new_column1   new_column2
0   1      42            a
1   2      42            b
2   3      42            c

df["new_column3"] = pd.Series(["dog", "cat&quot...