I have a long code that at some point has a very small dataframe with 813 rows and 16 columns.
To this dataframe i apply the groupby function

fm = fm.groupby(['Tower_ID'                 ,'Cell_ID'                 ,'Alarm ID'                 ,'Severity'                 ,'Alarm Type'                 ,'Alarm Text'                 ,'Supplementary Info'                 ,'PERIOD_START_TIME'                 #,'File_Name'                ]
               ).agg({'Full_Start_Date': 'min'
                      ,'Full_End_Date': 'max'
                      ,'Alarm hold time (sec)': 'sum'
                      ,'End_Date': 'max'
                      ,'Cross_Over': 'max'
                      ,'Cross_Over_diff': 'max'
                     }
                    )

This results in an error of out of memory
numpy.core._exceptions._ArrayMemoryError: Unable to allocate 11.1 GiB for an array with shape (1485993600,) and data type int64

Things that i have tried.
1 – Instead of fm = fm i used different variable like bananas = fm.groupby ….. same result
2 – Tried changing the formats of the columns to the exact type i need, category, int, etc…. same result

What worked

Before the groupby i save the fm dataframe into a file and then read the file back into fm

fm.to_excel('C:\home\fm_data.xlsx')
fm = pd.read_excel('C:\home\fm_data.xlsx')

And this works!!!

Can anyone have an ideia of why?
This is a very very poor solution and i would want to understand what can be the problem.
I appreciate the help.