python-20-如何读写CSV数据?

我们通过csv库reader函数和writer函数来执行读和写操作。这两个函数都需要传入一个文件对象

python3和python2的写法有点不一样

报错：_csv.Error: iterator should return strings, not bytes (did you open the file in text mode?)

Sorry, folks, we've got an understanding problem here. CSV files are
typically NOT created by text editors. They are created e.g. by "save as
csv" from a spreadsheet program, or as an output option by some database
query program. They can have just about any character in a field,
including \r and \n. Fields containing those characters should be quoted
(just like a comma) by the csv file producer. A csv reader should be
capable of reproducing the original field division. Here for example is
a dump of a little file I just created using Excel :
...
This sentence in the documentation is NOT an error: """If csvfile is a
file object, it must be opened with the ‘b’ flag on platforms where that
makes a difference."""
           

python 3.5: TypeError: a bytes-like object is required, not ‘str’

出现该错误往往是通过open()函数打开文本文件时，使用了‘rb’属性，如：fileHandle=open(filename,’rb’),则此时是通过二进制方式打开文件的，所以在后面处理时如果使用了str()函数，就会出现该错误，该错误不会再python2中出现。

在open()函数中使用‘r’属性，即文本方式读取，而不是‘rb’,以二进制文件方式读取，可以直接解决问题。

import urllib.request

import csv

urllib.request.urlretrieve('http://table.finance.yahoo.com/table.csv?s=000001.sz','pingan.csv')

rf = open('pingan.csv','r')

reader = csv.reader(rf)

print(next(reader))
print('================================')
print(next(reader))
print('================================')
print(next(reader))
print('================================')
           

写入另一个文件：

wf = open('pingan_copy.csv','w')
writer = csv.writer(wf)
writer.writerow(['Date', 'Open', 'High', 'Low', 'Close', 'Volume', 'Adj Close'])
writer.writerow(next(reader))
wf.flush()
           

问题解决：

with open('pingan.csv','r') as rf:
    reader = csv.reader(rf)
    with open('pingan2.csv' ,'w') as wf:
        writer = csv.writer(wf)
        headers = next(reader)
        writer.writerow(headers)
        for row in reader:
            if row[] < '2016-01-01':
                break
            if int(row[]) >= int():
                writer.writerow(row)

print('end')