4.5 文件读取与存储

学习目标

• 目标
  • 了解Pandas的几种文件读取存储操作
  • 应用CSV方式和HDF方式实现文件的读取和存储
• 应用
  • 实现股票数据的读取存储
• 内容预览
  • 4.5.1 CSV
  • 4.5.2 HDF5
  • 4.5.3 JSON
  • 4.5.4 拓展
  • 4.5.5 总结

我们的数据大部分存在于文件当中，所以pandas会支持复杂的IO操作，pandas的API支持众多的文件格式，如CSV、SQL、XLS、JSON、HDF5。

注：最常用的HDF5和CSV文件

4.5.1 CSV

1 读取csv文件-read_csv()

• pandas.read_csv(filepath_or_buffer, sep =',' , delimiter = None)
  • filepath_or_buffer:文件路径
  • usecols:指定读取的列名，列表形式

读取之前的股票的数据

# 读取文件,并且指定只获取'open', 'high', 'close'指标
data = pd.read_csv("./stock_day/stock_day.csv", usecols=['open', 'high', 'close'])

            open    high    close
2018-02-27    23.53    25.88    24.16
2018-02-26    22.80    23.78    23.53
2018-02-23    22.88    23.37    22.82
2018-02-22    22.25    22.76    22.28
2018-02-14    21.49    21.99    21.92

2 写入csv文件-to_csv()

• DataFrame.to_csv(path_or_buf=None, sep=', ’, columns=None, header=True, index=True, index_label=None, mode='w', encoding=None)

  • path_or_buf :string or file handle, default None
  • sep :character, default ‘,’
  • columns :sequence, optional
  • mode:'w'：重写, 'a' 追加
  • index:是否写进行索引
  • header :boolean or list of string, default True,是否写进列索引值

• Series.to_csv(path=None, index=True, sep=', ', na_rep='', float_format=None, header=False, index_label=None, mode='w', encoding=None, compression=None, date_format=None, decimal='.')

  Write Series to a comma-separated values (csv) file

案例

• 保存'open'列的数据

# 选取10行数据保存,便于观察数据
data[:10].to_csv("./test.csv", columns=['open'])

• 读取，查看结果

pd.read_csv("./test.csv")

     Unnamed: 0    open
0    2018-02-27    23.53
1    2018-02-26    22.80
2    2018-02-23    22.88
3    2018-02-22    22.25
4    2018-02-14    21.49
5    2018-02-13    21.40
6    2018-02-12    20.70
7    2018-02-09    21.20
8    2018-02-08    21.79
9    2018-02-07    22.69

会发现将索引存入到文件当中，变成单独的一列数据。如果需要删除，可以指定index参数,删除原来的文件，重新保存一次。

# index:存储不会讲索引值变成一列数据
stock_day[:10].to_csv("./test.csv", columns=['open'], index=False)

• 指定追加方式

stock_day[:10].to_csv("./test.csv", columns=['open'], index=False, mode='a)

    open
0    23.53
...
9    22.69
10    open
11    23.53
...
20    22.69

又存进了一个列名，所以当以追加方式添加数据的时候，一定要去掉列名columns,指定header=False

stock_day[:10].to_csv("./test.csv", columns=['open'], index=False, mode='a', header=False)
open
0    23.53
...
7    21.20
8    21.79
9    22.69
10    23.53
11    22.80
12    22.88
13    22.25
...
19    22.69

4.5.2 HDF5

read_hdf()与to_hdf()

HDF5文件的读取和存储需要指定一个键，值为要存储的DataFrame

• pandas.read_hdf(path_or_buf，key =None，** kwargs)

  从h5文件当中读取数据

  • path_or_buffer:文件路径
  • key:读取的键
  • mode:打开文件的模式
  • return:Theselected object

• DataFrame.to_hdf(path_or_buf, key, *\kwargs*)

案例

• 读取文件

day_high = pd.read_hdf("./stock_data/day/day_high.h5")

如果读取的时候出现以下错误

需要安装安装tables模块避免不能读取HDF5文件

pip install tables

• 存储文件

day_high.to_hdf("./test.h5", key="day_high")

再次读取的时候, 需要指定键的名字

new_high = pd.read_hdf("./test.h5", key="day_high")

4.5.3 JSON

Json是我们常用的一种数据交换格式，前面在前后端的交互经常用到，也会在存储的时候选择这种格式。所以我们需要知道Pandas如何进行读取和存储JSON格式。

1 read_json()

• pandas.read_json(path_or_buf=None, orient=None, typ='frame', lines=False)

  • 将JSON格式准换成默认的Pandas DataFrame格式
  • orient : string,Indication of expected JSON string format.
    • 'split' : dict like {index -> [index], columns -> [columns], data -> [values]}
    • 'records' : list like [{column -> value}, ... , {column -> value}]
    • 'index' : dict like {index -> {column -> value}}
    • 'columns' : dict like {column -> {index -> value}},默认该格式
    • 'values' : just the values array
  • lines : boolean, default False
    • 按照每行读取json对象
  • typ : default ‘frame’， 指定转换成的对象类型series或者dataframe

  案例

• 数据介绍

这里使用一个新闻标题讽刺数据集，格式为json。is_sarcastic：1讽刺的，否则为0；headline：新闻报道的标题；article_link：链接到原始新闻文章。存储格式为：

{"article_link": "https://www.huffingtonpost.com/entry/versace-black-code_us_5861fbefe4b0de3a08f600d5", "headline": "former versace store clerk sues over secret 'black code' for minority shoppers", "is_sarcastic": 0}
{"article_link": "https://www.huffingtonpost.com/entry/roseanne-revival-review_us_5ab3a497e4b054d118e04365", "headline": "the 'roseanne' revival catches up to our thorny political mood, for better and worse", "is_sarcastic": 0}

• 读取

orient指定存储的json格式，lines指定按行作为一个样本

json_read = pd.read_json("Sarcasm_Headlines_Dataset.json", orient="records", lines=True)

结果为：

2 to_json()

• DataFrame.to_json(path_or_buf=None, orient=None, lines=False)
  • 将Pandas 对象存储为json格式
  • path_or_buf=None：文件地址
  • orient:存储的json形式，{‘split’,’records’,’index’,’columns’,’values’}
  • lines:一个对象存储为一行

案例

• 存储文件

json_read.to_json("./test.json", orient='records')

结果

[{"article_link":"https:\/\/www.huffingtonpost.com\/entry\/versace-black-code_us_5861fbefe4b0de3a08f600d5","headline":"former versace store clerk sues over secret 'black code' for minority shoppers","is_sarcastic":0},{"article_link":"https:\/\/www.huffingtonpost.com\/entry\/roseanne-revival-review_us_5ab3a497e4b054d118e04365","headline":"the 'roseanne' revival catches up to our thorny political mood, for better and worse","is_sarcastic":0},{"article_link":"https:\/\/local.theonion.com\/mom-starting-to-fear-son-s-web-series-closest-thing-she-1819576697","headline":"mom starting to fear son's web series closest thing she will have to grandchild","is_sarcastic":1},{"article_link":"https:\/\/politics.theonion.com\/boehner-just-wants-wife-to-listen-not-come-up-with-alt-1819574302","headline":"boehner just wants wife to listen, not come up with alternative debt-reduction ideas","is_sarcastic":1},{"article_link":"https:\/\/www.huffingtonpost.com\/entry\/jk-rowling-wishes-snape-happy-birthday_us_569117c4e4b0cad15e64fdcb","headline":"j.k. rowling wishes snape happy birthday in the most magical way","is_sarcastic":0},{"article_link":"https:\/\/www.huffingtonpost.com\/entry\/advancing-the-worlds-women_b_6810038.html","headline":"advancing the world's women","is_sarcastic":0},....]

• 修改lines参数为True

json_read.to_json("./test.json", orient='records', lines=True)

结果

{"article_link":"https:\/\/www.huffingtonpost.com\/entry\/versace-black-code_us_5861fbefe4b0de3a08f600d5","headline":"former versace store clerk sues over secret 'black code' for minority shoppers","is_sarcastic":0}
{"article_link":"https:\/\/www.huffingtonpost.com\/entry\/roseanne-revival-review_us_5ab3a497e4b054d118e04365","headline":"the 'roseanne' revival catches up to our thorny political mood, for better and worse","is_sarcastic":0}
{"article_link":"https:\/\/local.theonion.com\/mom-starting-to-fear-son-s-web-series-closest-thing-she-1819576697","headline":"mom starting to fear son's web series closest thing she will have to grandchild","is_sarcastic":1}
{"article_link":"https:\/\/politics.theonion.com\/boehner-just-wants-wife-to-listen-not-come-up-with-alt-1819574302","headline":"boehner just wants wife to listen, not come up with alternative debt-reduction ideas","is_sarcastic":1}
{"article_link":"https:\/\/www.huffingtonpost.com\/entry\/jk-rowling-wishes-snape-happy-birthday_us_569117c4e4b0cad15e64fdcb","headline":"j.k. rowling wishes snape happy birthday in the most magical way","is_sarcastic":0}...

4.5.4 拓展

优先选择使用HDF5文件存储

• HDF5在存储的是支持压缩，使用的方式是blosc，这个是速度最快的也是pandas默认支持的
• 使用压缩可以提磁盘利用率，节省空间
• HDF5还是跨平台的，可以轻松迁移到hadoop 上面

4.5.5 总结

• pandas的CSV、HDF5、JSON文件的读取