Add new column in Pandas Data Frame Using a Dictionary

A Pandas DataFrame is a two-dimensional tabular data structure with rows and columns. You can add a new column by mapping values from a Python dictionary to an existing column using the map() function.

Creating a DataFrame

First, create a DataFrame from a Pandas Series ?

import pandas as pd

s = pd.Series([6, 8, 3, 1, 12])
df = pd.DataFrame(s, columns=['Month_No'])
print(df)

   Month_No
0         6
1         8
2         3
3         1
4        12

Adding a Column Using a Dictionary

Create a dictionary that maps month numbers to names, then use map() to add a new column ?

import pandas as pd

s = pd.Series([6, 8, 3, 1, 12])
df = pd.DataFrame(s, columns=['Month_No'])

# Dictionary mapping month numbers to names
months = {6: 'Jun', 8: 'Aug', 3: 'Mar', 1: 'Jan', 12: 'Dec'}

# Add new column using map()
df['Month_Name'] = df['Month_No'].map(months)
print(df)

   Month_No Month_Name
0         6        Jun
1         8        Aug
2         3        Mar
3         1        Jan
4        12        Dec

The map() function looks up each value in Month_No against the dictionary keys and fills the new column with the corresponding values. If a key is not found, NaN is inserted.

Other Methods to Add Columns

Using assign()

Returns a new DataFrame with the added column (does not modify original) ?

import pandas as pd

df = pd.DataFrame({'Month_No': [6, 8, 3, 1, 12]})
months = {6: 'Jun', 8: 'Aug', 3: 'Mar', 1: 'Jan', 12: 'Dec'}

df_new = df.assign(Month_Name=df['Month_No'].map(months))
print(df_new)

   Month_No Month_Name
0         6        Jun
1         8        Aug
2         3        Mar
3         1        Jan
4        12        Dec

Using insert() at a Specific Position

Insert the new column at a specific index position ?

import pandas as pd

df = pd.DataFrame({'Month_No': [6, 8, 3, 1, 12]})
months = {6: 'Jun', 8: 'Aug', 3: 'Mar', 1: 'Jan', 12: 'Dec'}

# Insert at position 0 (first column)
df.insert(0, 'Month_Name', df['Month_No'].map(months))
print(df)

  Month_Name  Month_No
0        Jun         6
1        Aug         8
2        Mar         3
3        Jan         1
4        Dec        12

Summary

Conclusion

Use map() with a dictionary to add a new column by mapping values from an existing column. Use assign() for immutable operations or insert() to place the new column at a specific position. If a dictionary key is missing for any row, NaN is automatically inserted.