[{'role': 'system', 'content': "You are an expert in machine learning and your task is to classify the code snippets into categories from the provided list of categories. List of categories: ['helper_functions', 'load_data', 'data_exploration', 'data_preprocessing', 'evaluation', 'modelling', 'prediction', 'result_visualization', 'save_results', 'comment_only']"}, {'role': 'user', 'content': "Identify to which category the provided code belongs and assign the code snippet to at least 1 but up to 5 categories based on the probabilities, ignore if the probability of the category is negligible. Strictly provide your response as a list. Avoid providing any additional information, commentary, or personal opinions.\\nPython Code:df_train['NumDate'] = df_train.Date.astype(int)/((10**9)*60*60*24)\nfirst_date = df_train.NumDate.min()\ndf_train.NumDate -= first_date\ndf_train.head()\n\ndf_test['NumDate'] = df_test.Date.astype(int)/((10**9)*60*60*24)\ndf_test.NumDate -= first_date\n\noutbreak_dates = df_train[['Country', 'NumDate']][df_train.ConfirmedCases>0].groupby('Country', as_index=False).min()\noutbreak_dates.columns = ['Country', 'FirstOutbreak']\n\nfirst_death = df_train[['Country', 'NumDate']][df_train.Fatalities>0].groupby('Country', as_index=False).min()\nfirst_death.columns = ['Country', 'FirstDeath']\n\ndf_train = pd.merge(df_train, outbreak_dates, how='left')\ndf_test = pd.merge(df_test, outbreak_dates, how='left')\n\ndf_train = pd.merge(df_train, first_death, how='left')\ndf_train.FirstDeath.fillna(0,inplace=True)\n\ndf_test = pd.merge(df_test, first_death, how='left')\ndf_test.FirstDeath.fillna(0,inplace=True)\n\ndf_train['DaysSinceOutbreak'] = df_train.NumDate - df_train.FirstOutbreak\ndf_test['DaysSinceOutbreak'] = df_test.NumDate - df_test.FirstOutbreak\n\ndf_train['DaysSinceFirstDeath'] = df_train.NumDate - df_train.FirstDeath\ndf_test['DaysSinceFirstDeath'] = df_test.NumDate - df_test.FirstDeath\n\ndf_train.head()"}, {'role': 'assistant', 'content': "['data_preprocessing']"}]