¿Cómo obtengo una lista de nombres de columnas de un cursor psycopg2?

Question

Me gustaría una forma general de generar etiquetas de columnas directamente desde los nombres de las columnas seleccionadas, y recordar ver que el módulo psycopg2 de python admite esta característica.

Answer 1

De "Python Programming" de Mark Lutz:

curs.execute("Select * FROM people") 
colnames = [desc[0] for desc in curs.description] 

Answer 2

Para obtener los nombres de columna en una consulta independiente, se puede consultar la tabla INFORMATION_SCHEMA.COLUMNS.

#!/usr/bin/env python3 

import psycopg2 

if __name__ == '__main__': 
    DSN = 'host=YOUR_DATABASE_HOST port=YOUR_DATABASE_PORT dbname=YOUR_DATABASE_NAME user=YOUR_DATABASE_USER' 

    column_names = [] 

    with psycopg2.connect(DSN) as connection: 
     with connection.cursor() as cursor: 
      cursor.execute("select column_name from information_schema.columns where table_schema = 'YOUR_SCHEMA_NAME' and table_name='YOUR_TABLE_NAME'") 
      column_names = [row[0] for row in cursor] 

    print("Column names: {}\n".format(column_names)) 

Para nombres de columna se interponen en la misma consulta como filas de datos, puede utilizar el campo de descripción del cursor:

#!/usr/bin/env python3 

import psycopg2 

if __name__ == '__main__': 
    DSN = 'host=YOUR_DATABASE_HOST port=YOUR_DATABASE_PORT dbname=YOUR_DATABASE_NAME user=YOUR_DATABASE_USER' 

    column_names = [] 
    data_rows = [] 

    with psycopg2.connect(DSN) as connection: 
    with connection.cursor() as cursor: 
     cursor.execute("select field1, field2, fieldn from table1") 
     column_names = [desc[0] for desc in cursor.description] 
     for row in cursor: 
     data_rows.append(row) 

    print("Column names: {}\n".format(column_names)) 

Answer 3

me he dado cuenta de que debe utilizar cursor.fetchone() después de la consulta a obtener la lista de columnas de cursor.description (es decir, en [desc[0] for desc in curs.description])

Answer 4

#!/usr/bin/python 
import psycopg2 
#note that we have to import the Psycopg2 extras library! 
import psycopg2.extras 
import sys 

def main(): 
    conn_string = "host='localhost' dbname='my_database' user='postgres' password='secret'" 
    # print the connection string we will use to connect 
    print "Connecting to database\n ->%s" % (conn_string) 

    # get a connection, if a connect cannot be made an exception will be raised here 
    conn = psycopg2.connect(conn_string) 

    # conn.cursor will return a cursor object, you can use this query to perform queries 
    # note that in this example we pass a cursor_factory argument that will 
    # dictionary cursor so COLUMNS will be returned as a dictionary so we 
    # can access columns by their name instead of index. 
    cursor = conn.cursor(cursor_factory=psycopg2.extras.DictCursor) 

    # tell postgres to use more work memory 
    work_mem = 2048 

    # by passing a tuple as the 2nd argument to the execution function our 
    # %s string variable will get replaced with the order of variables in 
    # the list. In this case there is only 1 variable. 
    # Note that in python you specify a tuple with one item in it by placing 
    # a comma after the first variable and surrounding it in parentheses. 
    cursor.execute('SET work_mem TO %s', (work_mem,)) 

    # Then we get the work memory we just set -> we know we only want the 
    # first ROW so we call fetchone. 
    # then we use bracket access to get the FIRST value. 
    # Note that even though we've returned the columns by name we can still 
    # access columns by numeric index as well - which is really nice. 
    cursor.execute('SHOW work_mem') 

    # Call fetchone - which will fetch the first row returned from the 
    # database. 
    memory = cursor.fetchone() 

    # access the column by numeric index: 
    # even though we enabled columns by name I'm showing you this to 
    # show that you can still access columns by index and iterate over them. 
    print "Value: ", memory[0] 

    # print the entire row 
    print "Row: ", memory 

if __name__ == "__main__": 
    main() 

Answer 5

I al tan acostumbrado a enfrentar un problema similar. Utilizo un truco simple para resolver esto. Suponga que tiene los nombres de columna en una lista como

col_name = ['a', 'b', 'c'] 

continuación, puede hacerlo siguiendo

for row in cursor.fetchone(): 
    print zip(col_name, row) 

Answer 6

Después de ejecutar consultas SQL escritura siguiente script en Python escrito en 2,7

total_fields = len(cursor.description)  
fields_names = [i[0] for i in cursor.description 
    Print fields_names 

Answer 7

Si desea para tener namedtuple obj de db query puede usar el siguiente fragmento de código:

from collections import namedtuple 

def create_record(obj, fields): 
    ''' given obj from db returns namedtuple with fields mapped to values ''' 
    Record = namedtuple("Record", fields) 
    mappings = dict(zip(fields, obj)) 
    return Record(**mappings) 

cur.execute("Select * FROM people") 
colnames = [desc[0] for desc in cur.description] 
rows = cur.fetchall() 
cur.close() 
result = [] 
for row in rows: 
    result.append(create_record(row, colnames)) 

Esto le allowes para evaluar los valores de registro como si fueran propiedades de clase, es decir,

record.id, record.other_table_column_name, etc.

o incluso más corto

from psycopg2.extras import NamedTupleCursor 
with cursor(cursor_factory=NamedTupleCursor) as cur: 
    cur.execute("Select * ...") 
    return cur.fetchall() 

Answer 8

Otra cosa que puedes hacer es crear un cursor con el que puedas hacer referencia a tus columnas por sus nombres (esa es una necesidad que me llevó a esta página en el primer lugar):

import psycopg2 
from psycopg2.extras import RealDictCursor 

ps_conn = psycopg2.connect(...) 
ps_cursor = psql_conn.cursor(cursor_factory=RealDictCursor) 

ps_cursor.execute('select 1 as col_a, 2 as col_b') 
my_record = ps_cursor.fetchone() 
print (my_record['col_a'],my_record['col_b']) 

>> 1, 2