How to make dataframe filter using .query method work with a variable?

Question

I'd like to use the .query method to filter a column in a dataframe by a variable but it won't work with a variable, only a string. Anybody know how to make it work with a variable? Thank you.

import pandas as pd    
var="A"

source = {'COL1': ['A','B','C'], 'COL2': ['D','E','F']}
dfsource=pd.DataFrame(source)
print(dfsource)

df2=dfsource.query('COL1=="A"') #Example, this works filtering for value A but not what I need.
df3=dfsource.query('COL1'==var)

print(df2)
print(df3)

Andrej Kesely · Accepted Answer · 2021-06-02 22:43:10Z

1

Use @:

import pandas as pd

var = "A"

source = {"COL1": ["A", "B", "C"], "COL2": ["D", "E", "F"]}
dfsource = pd.DataFrame(source)
print(dfsource)

df2 = dfsource.query(
    'COL1=="A"'
)  
df3 = dfsource.query("COL1 == @var") # <-- @

print(df2)
print(df3)

Prints:

  COL1 COL2
0    A    D
1    B    E
2    C    F
  COL1 COL2
0    A    D
  COL1 COL2
0    A    D

answered Jun 2, 2021 at 22:43

Andrej Kesely

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1 Comment

Randall Over a year ago

Such a simple but elusive fix! Thank you Andrej!

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