Python csv module throws "Error: sequence expected" at writer.writerow

Question

I use psycopg2 to connect postgresql and python, and here's my script,

import sys

#set up psycopg2 environment
import psycopg2

#driving_distance module
query = """
    select *
    from driving_distance ($$
        select
            gid as id,
            start_id::int4 as source,
            end_id::int4 as target,
            shape_leng::double precision as cost
        from network
        $$, %s, %s, %s, %s
    )
"""

#make connection between python and postgresql
conn = psycopg2.connect("dbname = 'TC_routing' user = 'postgres' host = 'localhost' password = '****'")
cur = conn.cursor()

#count rows in the table
cur.execute("select count(*) from network")
result = cur.fetchone()
k = result[0] + 1

#run loops
rs = []
i = 1
while i <= k:
    cur.execute(query, (i, 1000000, False, False))
    rs.append(cur.fetchall())
    i = i + 1

h = 0
ars = []
element = list(rs)
while h <= 15:
    rp = element[0][h][2]
    ars.append(rp)
    h = h + 1

print ars
conn.close()

the output is fine,

[0.0, 11810.7956476379, 16018.6818979217, 18192.3576530232, 21507.7366792666, 25819.1955059578, 26331.2523709618, 49447.0908955008, 28807.7871013087, 39670.8579371438, 42723.0239515299, 38719.7320396044, 38265.4435766971, 40744.8813155033, 43770.2158657742, 46224.8748774639]

but if I add some lines below in order to export results to the csv file, I got this error,

import csv

with open('test.csv', 'wb') as f:
    writer = csv.writer(f, delimiter = ',')
    for row in ars:
        writer.writerow(row)

[0.0, 11810.7956476379, 16018.6818979217, 18192.3576530232, 21507.7366792666, 25819.1955059578, 
26331.2523709618, 49447.0908955008, 28807.7871013087, 39670.8579371438, 42723.0239515299, 38719.7320396044, 38265.4435766971, 40744.8813155033, 43770.2158657742, 46224.8748774639]

Traceback (most recent call last):
  File "C:/Users/Heinz/Desktop/python_test/distMatrix_test.py", line 54, in <module>
    writer.writerow(row)
Error: sequence expected

How to fix this？

I am working with python 2.7.6 and pyscripter under Windows 8.1 x64. Feel free to give me any suggestion, thanks a lot!

Why are you opening as bytes (binary) file? A csv file is a text file. — Trimax
– Trimax, Commented Apr 2, 2014 at 9:46
maybe duplication with stackoverflow.com/questions/12396284/… — JonZ
– JonZ, Commented Apr 2, 2014 at 9:47
@Trimax: In Python 2.x, you must use "rb" and "wb" for csv file IO. — Joel Cornett
– Joel Cornett, Commented Apr 2, 2014 at 13:54

Arvind · Accepted Answer · 2014-04-02 14:29:42Z

8

  import csv

  with open('test.csv', 'wb') as f:
     writer = csv.writer(f, delimiter = ',')
     for row in ars:
         writer.writerow(row)

ars is just a single list. So your for loop does not extract a row from ars. It takes an element from the ars list and tries to write it as a row.

Try replacing it with

     for row in ars:
         writer.writerow([row])

This will write each element as a row in csv file.

or if u want to have a single row as in output then just dont use for loop ,instead use

   writer.writerow(ars)

edited Apr 2, 2014 at 14:29

answered Apr 2, 2014 at 13:50

Arvind

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