Pandas dataframe row-wise formula with function

Question

So I have quite big DataFrame and I create a new column by some equation based on other columns:

df['F'] = (params.a * params.b * df.A/1000 - param.C * (df.B + df.C - df.D) + param.D * df.E

and it works perfectly fine. Except I want to repeat this function throughout the code , so instead of error-prone copying and pasting I want to cast it into a reusable function.

So I casted it into lambda:

def fun(r):
     return (params.a * params.b * r.A/1000 - param.C * (r.B + r.C - r.D) + param.D * r.E   
df['F'] = r.apply(funy,axis =1)

yet this is 5x slower now ( 1.2s vs 6s for 10k rows).

What should I do if I want to have a neat function and still benefit from speed?

Answer 1

What's wrong with:

def fun():
    return params.a * params.b * df.A/1000 - param.C * (df.B + df.C - df.D) + param.D * df.E

df['F'] = fun()

So you get a reusable vectorized function.