Selecting Rows Using Labels (.loc)

While selecting columns by name is straightforward, Pandas provides more powerful mechanisms for selecting rows or combinations of rows and columns. The primary label-based selection method is the .loc accessor. Think of .loc as selecting data by its "name" or "label" - whether that's the name you've given to a row (its index label) or the name of a column.

This is different from selecting by numerical position, which we'll cover with .iloc in the next section. Using labels makes your code more readable and less prone to breaking if the order of your data changes, as long as the labels remain consistent.

The basic syntax involves passing the desired row label(s) and optionally column label(s) inside square brackets [] after .loc:

# General Syntax
# dataframe.loc[row_label_selector, column_label_selector]

Let's create a sample DataFrame to illustrate how .loc works. Imagine we have weather data for a few days:

import pandas as pd
import numpy as np

data = {'Temperature': [25, 28, 22, 31, 29],
        'Humidity': [60, 55, 70, 50, 65],
        'WindSpeed': [10, 12, 8, 15, 11]}
index_labels = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri']
weather_df = pd.DataFrame(data, index=index_labels)

print(weather_df)

     Temperature  Humidity  WindSpeed
Mon           25        60         10
Tue           28        55         12
Wed           22        70          8
Thu           31        50         15
Fri           29        65         11

Selecting a Single Row

To select a single row, pass its index label to .loc. The result will be a Pandas Series where the index is the original DataFrame's column names.

# Select the row with index label 'Wed'
wednesday_data = weather_df.loc['Wed']

print(wednesday_data)
print(type(wednesday_data))

Temperature    22
Humidity       70
WindSpeed       8
Name: Wed, dtype: int64
<class 'pandas.core.series.Series'>

Selecting Multiple Rows

You can select multiple specific rows by providing a list of index labels. The result is a new DataFrame containing only the specified rows.

# Select rows for 'Mon' and 'Fri'
mon_fri_data = weather_df.loc[['Mon', 'Fri']]

print(mon_fri_data)
print(type(mon_fri_data))

     Temperature  Humidity  WindSpeed
Mon           25        60         10
Fri           29        65         11
<class 'pandas.core.frame.DataFrame'>

Selecting a Range of Rows (Slicing by Label)

.loc also supports slicing using index labels. A significant difference from standard Python slicing or position-based slicing (which we'll see with .iloc) is that label-based slicing with .loc includes both the start and the end label.

# Select rows from 'Tue' up to and including 'Thu'
tue_to_thu_data = weather_df.loc['Tue':'Thu']

print(tue_to_thu_data)

     Temperature  Humidity  WindSpeed
Tue           28        55         12
Wed           22        70          8
Thu           31        50         15

Notice how the row labeled 'Thu' is included in the output. This inclusive behavior applies only when using labels for slicing.

Selecting Rows and Columns Simultaneously

The real power of .loc comes when you select both rows and columns by label. You provide the row selector first, followed by a comma, and then the column selector.

# Select Temperature for Wednesday
temp_wed = weather_df.loc['Wed', 'Temperature']
print(f"Temperature on Wednesday: {temp_wed}\n")

# Select Humidity and WindSpeed for Monday and Tuesday
hum_wind_mon_tue = weather_df.loc[['Mon', 'Tue'], ['Humidity', 'WindSpeed']]
print(hum_wind_mon_tue, "\n")

# Select all rows, but only 'Temperature' and 'Humidity' columns
temp_humidity = weather_df.loc[:, ['Temperature', 'Humidity']]
print(temp_humidity, "\n")

# Select rows 'Wed' through 'Fri' and columns 'Humidity' through 'WindSpeed' (inclusive slicing)
subset_slice = weather_df.loc['Wed':'Fri', 'Humidity':'WindSpeed']
print(subset_slice)

Temperature on Wednesday: 22

     Humidity  WindSpeed
Mon        60         10
Tue        55         12

     Temperature  Humidity
Mon           25        60
Tue           28        55
Wed           22        70
Thu           31        50
Fri           29        65

     Humidity  WindSpeed
Wed        70          8
Thu        50         15
Fri        65         11

In the third example above, the colon : used in the row position weather_df.loc[:, ['Temperature', 'Humidity']] indicates "select all rows". Similarly, you could use : in the column position to select all columns for specific rows.

Using Boolean Conditions with .loc

While there's a dedicated section on boolean indexing coming up, it's useful to know that .loc works with boolean arrays (Series) for row selection. You can create a condition, which results in a Series of True/False values, and pass this Series to .loc. It will return only the rows where the condition is True.

# Select days where Temperature was above 25 degrees
hot_days = weather_df.loc[weather_df['Temperature'] > 25]

print(hot_days)

     Temperature  Humidity  WindSpeed
Tue           28        55         12
Thu           31        50         15
Fri           29        65         11

Here, weather_df['Temperature'] > 25 creates a boolean Series:

Mon    False
Tue     True
Wed    False
Thu     True
Fri     True
Name: Temperature, dtype: bool

Passing this Series to weather_df.loc[...] effectively selects the rows corresponding to the True values.

Working with Different Index Types

Remember that .loc always operates on the labels of the index and columns. If your DataFrame has the default integer index (0, 1, 2, ...), then .loc will use these integers as labels.

df_int_index = pd.DataFrame({'A': [1, 2, 3], 'B': [4, 5, 6]})
print(df_int_index)
# Select row with index label 1
print("\nRow with label 1:")
print(df_int_index.loc[1])

   A  B
0  1  4
1  2  5
2  3  6

Row with label 1:
A    2
B    5
Name: 1, dtype: int64

Even though 1 looks like a position, .loc treats it as the name or label of that row in this context. This can sometimes be confusing, which is why the purely position-based .iloc accessor exists, as we will see next.

In summary, .loc is your tool for selecting data when you know the names (labels) of the rows and columns you want. It supports selecting single items, lists of items, and slices (inclusive) by label, making your selection logic clear.