A line in a self define function not working in pandas

I try and defined a function to process a df (like adding columns and convert all cols head to lower case) before doing the analysis. All other line works fine except the line that I tried to rearrange the columns orders.

the function looks like this

def cleanDf(df):    
    df.columns = df.columns.str.replace(' ','_')
    df.columns = df.columns.str.lower()
    df['date1'] = pd.to_datetime(df['date'].astype(str) + ' ' + df['time'].astype(str))
    df['weekday'] =  df['date1'].dt.day_name()
    
    business_hour_mask = (df['date1'].dt.hour >=9) & (df['date1'].dt.hour <=18)
    df['business_hour'] = np.where(business_hour_mask, "Yes","No")
    df['week_number'] = df.date1.dt.week
    
    df = df.reindex(['date1','week_number','weekday','business_hour','changed_by','customer','field_name','new_value','old_value','new_value.1','old_value.1','date','time','company_code','sales_organization','distribution_channel','division'], axis=1) 
    #problem line, i've tried both with and without "df = " in front of this line
    
    return df

my current workaround is to insert that line after i call the function then it works

cleanDf(df)

df = df.reindex(['date1','week_number','weekday','business_hour','changed_by','customer','field_name','new_value','old_value','new_value.1','old_value.1','date','time','company_code','sales_organization','distribution_channel','division'], axis=1)

df.head()

Appreciate if you can advise why the line does not inside the function, but ok when executed separately.

thank you very much

>Solution :

It’s because you’re reassigning the df variable inside the function, where it’s just a parameter. Since you’re returing df though, it’s simple. Just write df = cleanDf(df) instead of just cleanDf(df):

df = cleanDf(df)
df.head()

Per @mozway’s comment, you should also define your cleanDf function like so:

def cleanDf(df):
    df = df.copy()
    # ... do your stuff ...
    return df