For a DataFrame already loaded in memory, use df = df.set_index(df.columns[0]). When loading a CSV, use pd.read_csv("data.csv", index_col=0). The first approach changes an existing DataFrame; the second sets the index as part of the import.
Set the first column as the index in an existing DataFrame
Select the first column by position with df.columns[0], then pass it to set_index:
df = df.set_index(df.columns[0])This works regardless of the column’s name. By default, set_index removes that column from the regular data columns after using its values as row labels. Assign the result back to df because the method returns a new DataFrame by default.
Keep the column in the data as well
Set drop=False if you want the first column to remain among the DataFrame’s columns while also becoming its index:
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df = df.set_index(df.columns[0], drop=False)Use a column name when it is known
If the column has a known name, specifying it directly is clearer and does not rely on its current position:
df = df.set_index("id")Set the first column as the index while reading a CSV
Pass index_col=0 to read_csv. The zero refers to the first column’s position:
import pandas as pd
df = pd.read_csv("data.csv", index_col=0)index_col can also take a column label. If a CSV has extra delimiters at line ends and you do not want pandas to interpret the first column as the index, the documented option is index_col=False. See the official pandas read_csv API reference.
Use more than one column for the index
When two fields together define a row label, pass both columns as a list. Pandas creates a MultiIndex:
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# Existing DataFrame: use its first two columns
df = df.set_index([df.columns[0], df.columns[1]])
# CSV: use its first two columns during import
df = pd.read_csv("data.csv", index_col=[0, 1])Choose multiple columns only when their combination represents the row labels you need. The DataFrame.set_index API reference and read_csv reference document list-based index selection.
What to know about the set_index result
The method also has an inplace option. With inplace=True, it changes the calling DataFrame and returns None, so do not assign that return value to df. For example:
df.set_index(df.columns[0], inplace=True)The API reference for pandas 3.0.6 marks verify_integrity deprecated since pandas 3.0.0. Check the documentation for the pandas version installed in your project before relying on version-specific parameters. See the current set_index API documentation.
Excel files: consider setting the index after import
read_excel accepts index_col, including a zero-based column position. However, its documentation notes that missing values in index columns may be forward-filled to support roundtripping with merged cells. If that behavior would alter your data in an unwanted way, read the sheet first and then call set_index. The pandas I/O guide covers CSV and Excel import behavior.
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