Python程序中用csv模块来操作csv文件的基本使用教程

CSV全称为"Comma Separated Values",是一种格式化的文件，由行和列组成，分隔符可以根据需要来变化。
如下面为一csv文件：

    Title,Release Date,Director
    And Now For Something Completely Different,1971,Ian MacNaughton
    Monty Python And The Holy Grail,1975,Terry Gilliam and Terry Jones
    Monty Python's Life Of Brian,1979,Terry Jones
    Monty Python Live At The Hollywood Bowl,1982,Terry Hughes
    Monty Python's The Meaning Of Life,1983,Terry Jones

csv可以比较方便的在不同应用之间迁移数据。可以将数据批量导出为csv格式，然后倒入到其他应用程序中。很多应用中需要导出报表，也通常用csv格式导出，然后用Excel工具进行后续编辑。

打印发行日期及标题，逐行处理：

    for line in open("samples/sample.csv"):
      title, year, director = line.split(",")
      print year, title

使用csv模块处理：

    import csv
    reader = csv.reader(open("samples/sample.csv"))
    for title, year, director in reader:
      print year, title

改变分隔符

创建一csv.excel的子类，并修改分隔符为";"

    # File: csv-example-2.py
    import csv
    class SKV(csv.excel):
      # like excel, but uses semicolons
      delimiter = ";"

    csv.register_dialect("SKV", SKV)
    reader = csv.reader(open("samples/sample.skv"), "SKV")
    for title, year, director in reader:
      print year, title

如果仅仅仅是改变一两个参数，则可以直接在reader参数中设置，如下：

    # File: csv-example-3.py

    import csv

    reader = csv.reader(open("samples/sample.skv"), delimiter=";")

    for title, year, director in reader:
      print year, title

将数据存为CSV格式

通过csv.writer来生成一csv文件。

    # File: csv-example-4.py

    import csv
    import sys

    data = [
      ("And Now For Something Completely Different", 1971, "Ian MacNaughton"),
      ("Monty Python And The Holy Grail", 1975, "Terry Gilliam, Terry Jones"),
      ("Monty Python's Life Of Brian", 1979, "Terry Jones"),
      ("Monty Python Live At The Hollywood Bowl", 1982, "Terry Hughes"),
      ("Monty Python's The Meaning Of Life", 1983, "Terry Jones")
    ]

    writer = csv.writer(sys.stdout)

    for item in data:
      writer.writerow(item)

实例

下面我们来看一个比较完整的例子，代码说明在注释中：

    import csv


    # dialect是访问csv文件时需要指定的参数之一，用来确定csv文件的数据格式
    # 下面这个函数列举系统支持的dialect有哪些，默认值是'excel'，用户也可
    # 以从Dialect派生一个类，使用该类的实例作为dialect参数。
    print csv.list_dialects()


    def test_writer():
      # csv文件必须以二进制方式open
      with open('eggs.csv', 'wb') as csvfile:
        spamwriter = csv.writer(csvfile)
        spamwriter.writerow(['Spam'] * 5 + ['Baked Beans'])
        spamwriter.writerow(['Spam', 'Lovely Spam', 'Wonderful Spam'])


    def test_reader():
      with open('eggs.csv', 'rb') as csvfile:
        spamreader = csv.reader(csvfile)
        for row in spamreader:
          print row


    # sniffer 用来推断csv文件的格式，不是很准确
    def test_sniffer():
      with open('eggs.csv', 'wb') as csvfile:
        spamwriter = csv.writer(csvfile, delimiter=' ')

        spamwriter.writerow(['Spam'] * 2 + ['Baked Beans'])
        spamwriter.writerow(['Spam', 'Lovely Spam', 'Wonderful Spam'])

      # 通常你需要指定与写入者相同的文件格式才能正确的读取数据
      with open('eggs.csv', 'rb') as csvfile:
        spamreader = csv.reader(csvfile, delimiter=' ')
        for row in spamreader:
          print ', '.join(row)

      # 如果不知道文件格式，sniffer就可以派上用场了
      with open('eggs.csv', 'rb') as csvfile:
        # 用sniffer推断文件格式，从而得到dialect
        dialect = csv.Sniffer().sniff(csvfile.read(1024))
        print dialect.delimiter, dialect.quotechar

        # 文件重新移动到头部
        csvfile.seek(0)

        # 用推断出来的dialect创建reader
        reader = csv.reader(csvfile, dialect)

        for row in reader:
          print ', '.join(row)