How to execute a string containing condition in python dataframe

Question

I am filtering a DataFrame inside a function. I get multiple conditions as string arguments which I have to employ to filter the DataFrame. exec() usually helps to execute a string expression, but I am not able to fine tune it for the DataFrame.

Here is a small miniature example:

import pandas as pd
df = pd.DataFrame({'A':[30,60,90,40],'B':['X','X','X','Y']})
print(df)
      A  B
  0  30  X
  1  60  X
  2  90  X
  3  40  Y

Now, the code below gives an error naturally. My question is how we can evaluate this string expression in the function below like exectable(expression) and get the filtered DataFrame:

def func(df,cond1,cond2):
    expression = "(df.A "+cond1+") & (df.B"+cond2+")"
    print(expression) # This results in --> (df.A < 50) & (df.B == 'X')

    return df[expression]
func(df,"< 50"," == 'X'")

Desired output,

      A  B
  0  30  X

It works. Many thanks. Just post one line answer and I will accept it. Since exec() did not work, so I couldn't forsee that eval() would work. — cph_sto
– cph_sto, Commented Oct 2, 2020 at 12:42

go2nirvana · Accepted Answer · 2020-10-02 12:53:18Z

1

eval is what you are looking for.

def func(df,cond1,cond2):
    expression = eval(f"(df.A {cond1}) & (df.B{cond2})")

    return df[expression]

answered Oct 2, 2020 at 12:53

go2nirvana

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1 Comment

go2nirvana Over a year ago

True, but it exists and so has applications. Unless TS is not putting user's input into eval he is ok.

Andrej Kesely · Accepted Answer · 2020-10-02 12:42:51Z

1

Youi can use df.query:

import pandas as pd

def func(df,cond1,cond2):
    expression = "(A "+cond1+") & (B"+cond2+")"
    return df.query(expression)

df = pd.DataFrame({'A':[30,60,90,40],'B':['X','X','X','Y']})
print( func(df,"< 50"," == 'X'") )

Prints:

    A  B
0  30  X

answered Oct 2, 2020 at 12:42

Andrej Kesely

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