以变​​量为列名的 Pandas 查询

Question

Passing on new application of information I learned that was part of another question: Unable to query a local variable in pandas 0.14.0

Credit and thanks to user @choldgraf. I'm applying his answer from the above link differently.

Objective: To use a variable as the column name in a query

Failed examples:

import pandas as pd
fooframe = pd.DataFrame({'Size':['Large', 'Medium', 'Small', 'Tiny'], 'Color':[1, 2, 3, 4]})
myvar = 'Size'
subframe = fooframe.query("myvar == 'Large'")

The code above returns a key error for 'myvar'.

import pandas as pd
fooframe = pd.DataFrame({'Size':['Large', 'Medium', 'Small', 'Tiny'], 'Color':[1, 2, 3, 4]})
myvar = 'Size'
subframe = fooframe.query("@myvar == 'Large'")

The code above adds "@" before myvar in the query to reference myvar as a local variable. However, the code still returns an error.

Answer 1

Credit and thanks to user @choldgraf. I used the technique he mentioned in another post ( Unable to query a local variable in pandas 0.14.0 ) not for the value in the column but for the column name.

A variable can be used as the column name in a pandas query by inserting it into the query string like so:

import pandas as pd
fooframe = pd.DataFrame({'Size':['Large', 'Medium', 'Small', 'Tiny'], 'Color':[1, 2, 3, 4]})
myvar = 'Size'
subframe = fooframe.query("`{0}` == 'Large'".format(myvar))

(Where backticks are used to bracket the column name, dealing with special characters and spaces in column names.)