python pandas

df.pivot_table(index=[인덱스컬럼],
               columns=[컬럼1,컬럼2],
               values=[값],
               aggfunc='sum').reset_index()

• string일 때는 aggfunc='max'
• index에 NULL 있으면 안됨

df[['a','b','c']] = df[['a','b','c']].fillna('')

df.groupby([컬럼들]).agg({'컬럼':sum}).reset_index()

df = df.assign(date=pd.to_numeric(df['date'], errors='coerce')).groupby(['코드', '종목명']).agg({'date':np.min}).reset_index().drop_duplicates()

df = df[['코드', 'date']].groupby(['코드']).agg({'date': [np.min, np.max]}).reset_index(level='종목코드')
df.columns = df.columns.droplevel()

df['rank'] = df.groupby('code')['value'].rank(ascending=False)

df_out = df_out.merge(df, on=['no', 'name'], how='outer')	#left_on right_on

df = df.append(pd.DataFrame(pd.Series(dict_row)).transpose(), ignore_index=True)

df.rename(columns = {'컬럼':'new name'}, index={0:'new index'})
df.rename({1: 2, 2: 4}, axis='index')

df.columns = [컬럼들..]
df.columns = ['1'] + df.columns[1:].tolist()

#1
df = df.sort_index(axis='columns', level = 'MULTILEVEL INDEX NAME/no')
#2
col_order = ['a','b','c']
df = df.reindex(col_order, axis='columns')

df['코드'] = 'A' + df['코드'].map(lambda x: f'{x:0>6}')  #6글자로

df.shape
df.describe()
df.info()

#use the dataframe.nunique() function to find the unique values
unique_counts = pd.DataFrame.from_records([(col, df[col].nunique()) for col in df.columns],columns=['Column_Name', 'Num_Unique']).sort_values(by=['Num_Unique'])

df.loc[df.sex=='W', 'year_born'].hist()

df['value'] = pd.to_numeric(df['value'], errors='coerce')
df['value'].dtype

iloc: Select by position
loc: Select by label

for idx,row in anime[:2].iterrows():
    print(idx, row)

#1
df = pd.read_csv('file.csv', encoding='utf-8', index_col=0, converters={'col':int, 'col2':str})
#2
df['col'] = pd.to_numeric(df[col].str.replace(',',''), errors='coerce')

writer = pd.ExcelWriter('file.xlsx', engine = 'xlsxwriter')
df.to_excel(writer, sheet_name = '시트이름')
writer.save()
writer.close()

for index, chunk in enumerate( pd.read_csv(filename, chunksize=chunk_size, dtype=str), start=1):
  chunk.memory_usage()

df_chunk = pd.read_csv(r'../input/data.csv', chunksize=1000000) # chink : # of lines
for chunk in df_chunk:  
    # perform data filtering 
    chunk_filter = chunk_preprocessing(chunk)

df3=pd.read_csv("abc.txt", skiprows=1, header=None,
               sep="|", nrows=100, encoding="utf-16") #, error_bad_lines=False)

with open("abc.txt", encoding="utf-16") as f:
    i=0
    for line in f:
        i=i+1
        print(line)
        if i >3 : break

file open/ append lines : https://thispointer.com/how-to-append-text-or-lines-to-a-file-in-python/