How to Unpivot a Python Pandas Dataframe

1. Use the melt() Function to Unpivot a Python Pandas Dataframe
2. Conclusion

Python is a preferred language for data analysis due to its great data-centric packages ecosystem. One of these packages is Pandas, which significantly simplifies data import and analysis.

This article uses the Pandas dataframe melt() function to convert a dataframe from wide to long format while optionally leaving identifier variables set.

Use the melt() Function to Unpivot a Python Pandas Dataframe

This function can convert a dataframe into a format in which one or more columns are identifier variables (id vars). All other columns, considered measured variables (value vars), are unpivoted to the row axis, leaving only two non-identifier columns, variable and value.

Syntax:

DataFrame.melt(id_vars=None, value_vars=None, var_name=None, value_name='value', col_level=None)

Parameters:

• ‎frame:‎ This is the DataFrame we use.‎
• ‎id_vars:‎‎ This is the column we use as an identifier variable.
• ‎value_vars:‎‎ This is used to unpivot.‎
• var_name: ‎This specifies a name for the variable column. If none is specified, the frame is used, variable, or columns name.
• ‎value_name:‎‎ The value column should be given a unique name.
• ‎col_level:‎ Use this level to melt the columns if they are MultiIndexes.
• Returns: Convert a DataFrame into a format with one or more identifier variables in each column.

Example 1:

First, create a dataframe with columns A, Bsc, Cse, and xyz.

import pandas as pd

df = pd.DataFrame(
    {
        "A": [12, 4, 5, 44, 1],
        "Bsc": [5, 2, 54, 3, 2],
        "Cse": [20, 16, 7, 3, 8],
        "xyz": [14, 3, 17, 2, 6],
    }
)
df

Output:

|      | A    | Bsc  | Cse  | xyz  |
| ---- | ---- | ---- | ---- | ---- |
| 0    | 12   | 5    | 20   | 14   |
| 1    | 4    | 2    | 16   | 3    |
| 2    | 5    | 54   | 7    | 17   |
| 3    | 44   | 3    | 3    | 2    |
| 4    | 1    | 2    | 8    |      |

The below function unpivots the dataframe. Here A is an identifier, and the Bsc column is the value variable.

df.melt(id_vars=["A"], value_vars=["Bsc"])

Output:

|      | A    | variable | value |
| ---- | ---- | -------- | ----- |
| 0    | 12   | Bsc      | 5     |
| 1    | 4    | Bsc      | 2     |
| 2    | 5    | Bsc      | 54    |
| 3    | 44   | Bsc      | 3     |
| 4    | 1    | Bsc      | 2     |

Example 2:

Set column A as the identifier variable and columns Bsc and Csc as the value variables using the dataframe.melt() function. ‎‎‎‎

df.melt(
    id_vars=["A"],
    value_vars=["Bsc", "Cse"],
    var_name="Variable_column",
    value_name="Value_column",
)

Output:

| index | A    | Variable_column | Value_column |
| ----- | ---- | --------------- | ------------ |
| 0     | 12   | Bsc             | 5            |
| 1     | 4    | Bsc             | 2            |
| 2     | 5    | Bsc             | 54           |
| 3     | 44   | Bsc             | 3            |
| 4     | 1    | Bsc             | 2            |
| 5     | 12   | Cse             | 20           |
| 6     | 4    | Cse             | 16           |
| 7     | 5    | Cse             | 7            |
| 8     | 44   | Cse             | 3            |
| 9     | 1    | Cse             |              |

Conclusion

In this article, we discussed how to unpivot a dataframe or convert a dataframe into a long format to increase the number of rows.

For this purpose, we used the melt() function. One column is designated as a variable identifier, while the other is the identifier’s value.