Python csv模块在writer.writerow时抛出“错误：期望序列”

Question

我用psycopg2这个库来连接PostgreSQL数据库和Python，下面是我的代码：

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()

运行结果很好，

[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]

但是如果我在下面加一些代码来把结果导出到CSV文件，就出现了这个错误：

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

怎么解决这个问题呢？

我现在用的是Python 2.7.6和Windows 8.1 x64下的pyscripter。如果你有任何建议，请随时告诉我，非常感谢！

Answer 1

  import csv

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

ars 只是一个简单的列表。所以你的循环并不是从 ars 中提取一行数据，而是从 ars 列表中取出一个元素，然后试图把它写成一行。

你可以试着把它换成

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

这样会把每个元素写成 csv 文件中的一行。

如果你想要输出成一行的话，那就不要用循环，直接用

   writer.writerow(ars)