Pandas query()

import pandas as pd

# sample DataFrame
data = {'A': [1, 2, 3],
        'B': [4, 5, 6]}

df = pd.DataFrame(data)

# query the DataFrame where column 'A' is less than 3
result = df.query('A < 3')

print(result)

'''
Output

   A  B
0  1  4
1  2  5
'''

query() Syntax

The syntax of the query() method in Pandas is:

df.query(expr, inplace=False, **kwargs)

query() Arguments

The query() method has the following arguments:

• expr: the query string to evaluate
• inplace (optional): whether the query should modify the data in place or return a modified copy
• **kwargs: additional keyword arguments to pass to the query.

query() Return Value

The query() method returns a DataFrame resulting from the query expression.

Example 1: Basic Query

import pandas as pd

data = {'A': [1, 3, 5, 7],
        'B': [2, 4, 6, 8]}
df = pd.DataFrame(data)

# query the DataFrame for rows where 'A' is greater than 4
result = df.query('A > 4')

print(result)

Output

   A  B
2  5  6
3  7  8

In this example, we returned the rows with A > 4.

Example 2: Query Using Index

import pandas as pd

data = {'A': [1, 3, 5, 7],
        'B': [2, 4, 6, 8]}
df = pd.DataFrame(data)

# query the DataFrame using index
result = df.query('index > 1')

print(result)

Output

   A  B
2  5  6
3  7  8

In this example, we returned the rows with index > 1.

Example 3: Query with a Variable

import pandas as pd

data = {'A': [1, 3, 5, 7],
        'B': [2, 4, 6, 8]}
df = pd.DataFrame(data)

# set a threshold
threshold = 4

# query the DataFrame with a variable
result = df.query('A > @threshold')

print(result)

Output

   A  B
2  5  6
3  7  8

In this example, we returned the rows with A > @threshold.

Notice the use of @ before the variable name.

Example 4: String Matching Using Query

import pandas as pd

data = {'A': [1, 3, 5, 7],
        'B': [2, 4, 6, "bat"]}
df = pd.DataFrame(data)

# query the DataFrame for rows where 'B' matches a string
result = df.query('B == "bat"')

print(result)

Output

   A    B
3  7  bat

In this example, we performed string matching with query() to return the rows where B=="bat".