利用python做表格數據處理
技術背景
數據處理是一個當下非常熱門的研究方向，通過對於大型實際場景中的數據進行建模，可以用於預測下一階段可能出現的情況。比如我們有過去的2002年-2018年的黃金價格的數據：
該數據來源於Gitee上的一個開源項目。其中包含有：時間、開盤價、收盤價、最高價、最低價、交易數以及成交額這麼幾個參數。假如我們使用一個機器學習的模型去分析這個數據，也許我們可以預測在這個數據中並不存在的金價數據。如果預測的契合度較好，那麼對於一些人的投資策略來說有重大意義。但是這種實際場景下的數據，往往數據量是非常大的。雖然這裡我們使用到的數據隻有300多KB，但是我們更多的時候不得不考慮10個GB甚至是1個TB以上的數據的處理。如果處理都無法處理，那我們如何對這些數據進行建模呢？
python對Excel表格的處理
首先我們看一個最簡單的情況，我們先不考慮性能的問題，那麼我們可以使用xlrd這個工具來在python中打開和加載一個Excel表格：
# table.py
def read_table_by_xlrd():
import xlrd
workbook = xlrd.open_workbook(r'data.xls')
sheet_name = workbook.sheet_names()
print ('All sheets in the file data.xls are: {}'.format(sheet_name))
sheet = workbook.sheet_by_index(0)
print ('The cell value of row index 0 and col index 1 is: {}'.format(sheet.cell_value(0, 1)))
print ('The elements of row index 0 are: {}'.format(sheet.row_values(0)))
print ('The length of col index 1 are: {}'.format(len(sheet.col_values(1))))
if __name__ == '__main__':
read_table_by_xlrd()
上述代碼的輸出如下：
[dechin@dechin-manjaro gold]$ python3 table.py All sheets in the file data.xls are: ['Sheet1', 'Sheet2', 'Sheet3'] The cell value of row index 0 and col index 1 is: 開 The elements of row index 0 are: ['時間', '開', '高', '低', '收', '量', '額'] The length of col index 1 are: 3923
我們這裡成功的將一個xls格式的表格加載到瞭python的內存中，我們可以對這些數據進行分析。如果需要對這些數據修改，可以使用openpyxl這個倉庫，但是這裡我們不做過多的贅述。
在python中還有另外一個非常常用且非常強大的庫可以用來處理表格數據，那就是pandas，這裡我們利用ipython這個工具簡單展示一下使用pandas處理表格數據的方法：
[dechin@dechin-manjaro gold]$ ipython
Python 3.8.5 (default, Sep 4 2020, 07:30:14)
Type 'copyright', 'credits' or 'license' for more information
IPython 7.19.0 -- An enhanced Interactive Python. Type '?' for help.
In [1]: import pandas as pd
In [2]: !ls -l
總用量 368
-rw-r--r-- 1 dechin dechin 372736 3月 27 21:31 data.xls
-rw-r--r-- 1 dechin dechin 563 3月 27 21:42 table.py
In [3]: data = pd.read_excel('data.xls', 'Sheet1') # 讀取excel格式的文件
In [4]: data.to_csv('data.csv', encoding='utf-8') # 轉成csv格式的文件
In [7]: !ls -l
總用量 588
-rw-r--r-- 1 dechin dechin 221872 3月 27 21:52 data.csv
-rw-r--r-- 1 dechin dechin 372736 3月 27 21:31 data.xls
-rw-r--r-- 1 dechin dechin 563 3月 27 21:42 table.py
In [8]: !head -n 10 data.csv # 讀取csv文件的頭10行
,時間,開,高,低,收,量,額
0,2002-10-30,83.98,92.38,82.0,83.52,352,29373370
1,2002-10-31,83.9,83.92,83.9,83.91,66,5537480
2,2002-11-01,84.5,84.65,84.0,84.51,77,6502510
3,2002-11-04,84.9,85.06,84.9,84.99,95,8076330
4,2002-11-05,85.1,85.2,85.1,85.13,61,5193650
5,2002-11-06,84.9,84.9,84.9,84.9,1,84900
6,2002-11-07,85.0,85.15,85.0,85.14,26,2212310
7,2002-11-08,85.25,85.28,85.1,85.16,35,2981780
8,2002-11-11,85.18,85.19,85.18,85.19,65,5537050
在ipython中我們不僅可以執行python指令，還可以在前面加一個!就能夠執行一些系統命令，非常的方便。csv格式的文件，其實就是用逗號跟換行符來替代常用的\t字符串進行數據的分隔。
但是，不論是使用xlrd還是pandas，我們都會面臨一個同樣的問題：需要把所有的數據加載到內存中進行處理。我們一般的個人電腦隻有8GB-16GB的內存，就算是比較大的64GB的內存，我們也隻能夠在內存中對64GB以下內存大小的文件進行處理，這對於大數據場景來說遠遠不夠。所以，下一章節中介紹的vaex就是一個很好的解決方案。另外，關於Linux下查看本地內存以及使用情況的方法如下：
[dechin@dechin-manjaro gold]$ vmstat procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu----- r b 交換 空閑 緩沖 緩存 si so bi bo in cs us sy id wa st 0 0 0 35812168 328340 2904872 0 0 20 27 362 365 8 4 88 0 0 [dechin@dechin-manjaro gold]$ vmstat 2 3 procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu----- r b 交換 空閑 緩沖 緩存 si so bi bo in cs us sy id wa st 1 0 0 35810916 328356 2905844 0 0 20 27 362 365 8 4 88 0 0 0 0 0 35811916 328364 2904952 0 0 0 6 613 688 1 1 99 0 0 0 0 0 35812168 328364 2904856 0 0 0 0 672 642 0 1 99 0 0
我們可以看到空閑內存大約有36GB的內存，這裡我們本機一共有40GB的內存，算是比較大的瞭。
vaex的安裝與使用
vaex提供瞭一種內存映射的數據處理方案，我們不需要將整個的數據文件加載到內存中進行處理，我們可以直接對硬盤存儲進行操作。換句話說，我們所能夠處理的文件大小不再受到內存大小的限制，隻要在磁盤存儲空間允許的范圍內，我們都可以對這麼大小的文件進行處理。
一般現在個人PC的磁盤最小也有128GB，遠遠大於內存可以承受的范圍。當然，由於分區的不同，不一定能夠保障所有的內存資源都能夠被使用到，這裡附上查看當前目錄分區的可用磁盤空間大小查詢的方法：
[dechin@dechin-manjaro gold]$ df -hl . 文件系統 容量 已用 可用 已用% 掛載點 /dev/nvme0n1p9 144G 57G 80G 42% /
這裡可以看到我們還有80GB的可用磁盤空間，也就是說，如果我們在當前目錄放一個80GB大小的表格文件，那麼用pandas和xlrd都是沒辦法處理的，因為這已經遠遠超出瞭內存可支持的空間。但是用vaex，我們依然可以對這個文件進行處理。
在vaex的官方文檔鏈接中也介紹有vaex的原理和優勢：
vaex的安裝
與大多數的python第三方包類似的，我們可以使用pip來進行下載和管理。當然由於下載的文件會比較多，中間的過程也會較為緩慢，我們隻需安靜等待即可：
[dechin@dechin-manjaro gold]$ python3 -m pip install vaex
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Building wheels for collected packages: frozendict, aplus
Building wheel for frozendict (setup.py) ... done
Created wheel for frozendict: filename=frozendict-1.2-py3-none-any.whl size=3148 sha256=1ae5d8fe0d670f73bf3ee88453978246919197a616f0e08e601c84cc244cb238
Stored in directory: /home/dechin/.cache/pip/wheels/9b/9b/56/5713233cf7226423ab6c58c08081551a301b5863e343ba053c
Building wheel for aplus (setup.py) ... done
Created wheel for aplus: filename=aplus-0.11.0-py3-none-any.whl size=4412 sha256=9762d51c5ece813b0c5a27ff6ebc1a86e709d55edb7003dcc11272c954dd39c7
Stored in directory: /home/dechin/.cache/pip/wheels/de/93/23/3db69e1003030a764c9827dc02137119ec5e6e439afd64eebb
Successfully built frozendict aplus
Installing collected packages: pyarrow, tabulate, frozendict, aplus, python-utils, progressbar2, vaex-core, vaex-ml, vaex-viz, vaex-astro, vaex-hdf5, cachetools, vaex-server, xarray, jupyterlab-widgets, ipywidgets, ipympl, branca, shapely, traittypes, ipyleaflet, ipyvue, ipyvuetify, ipywebrtc, ipydatawidgets, pythreejs, ipyvolume, bqplot, vaex-jupyter, vaex
Attempting uninstall: ipywidgets
Found existing installation: ipywidgets 7.5.1
Uninstalling ipywidgets-7.5.1:
Successfully uninstalled ipywidgets-7.5.1
Successfully installed aplus-0.11.0 bqplot-0.12.23 branca-0.4.2 cachetools-4.2.1 frozendict-1.2 ipydatawidgets-4.2.0 ipyleaflet-0.13.6 ipympl-0.7.0 ipyvolume-0.5.2 ipyvue-1.5.0 ipyvuetify-1.6.2 ipywebrtc-0.5.0 ipywidgets-7.6.3 jupyterlab-widgets-1.0.0 progressbar2-3.53.1 pyarrow-3.0.0 python-utils-2.5.6 pythreejs-2.3.0 shapely-1.7.1 tabulate-0.8.9 traittypes-0.2.1 vaex-4.1.0 vaex-astro-0.8.0 vaex-core-4.1.0 vaex-hdf5-0.7.0 vaex-jupyter-0.6.0 vaex-ml-0.11.1 vaex-server-0.4.0 vaex-viz-0.5.0 xarray-0.17.0
在出現Successfully installed的字樣之後，就代表我們已經安裝成功，可以開始使用瞭。
性能對比
由於使用其他的工具我們也可以正常的打開和讀取表格文件，為瞭體現出使用vaex的優勢，這裡我們直接用ipython來對比一下兩者的打開時間：
[dechin@dechin-manjaro gold]$ ipython
Python 3.8.5 (default, Sep 4 2020, 07:30:14)
Type 'copyright', 'credits' or 'license' for more information
IPython 7.19.0 -- An enhanced Interactive Python. Type '?' for help.
In [1]: import vaex
In [2]: import xlrd
In [3]: %timeit xlrd.open_workbook(r'data.xls')
46.4 ms ± 76.2 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)
In [4]: %timeit vaex.open('data.csv')
4.95 ms ± 48.5 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)
In [7]: %timeit vaex.open('data.hdf5')
1.34 ms ± 1.84 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)
我們從結果中發現，打開同樣的一份文件，使用xlrd需要將近50ms的時間，而vaex最低隻需要1ms的時間，如此巨大的性能優勢使得我們不得不對vaex給予更多的關註。關於跟其他庫的對比，在這個鏈接中已經有人做過瞭，即使是對比pandas，vaex在讀取速度上也有1000多倍的加速，而計算速度的加速效果在數倍，總體來說表現非常的優秀。
數據格式轉換
在上一章節的測試中，我們用到瞭1個沒有提到過的文件：data.hdf5，這個文件其實是從data.csv轉換而來的。這一章節我們主要就介紹如何將數據格式進行轉換，以適配vaex可以打開和識別的格式。第一個方案是使用pandas將csv格式的文件直接轉換為hdf5格式，操作類似於在python對表格數據處理的章節中將xls格式的文件轉換成csv格式：
[dechin@dechin-manjaro gold]$ ipython
Python 3.8.5 (default, Sep 4 2020, 07:30:14)
Type 'copyright', 'credits' or 'license' for more information
IPython 7.19.0 -- An enhanced Interactive Python. Type '?' for help.
In [1]: import pandas as pd
In [4]: data = pd.read_csv('data.csv')
In [10]: data.to_hdf('data.hdf5','data',mode='w',format='table')
In [11]: !ls -l
總用量 932
-rw-r--r-- 1 dechin dechin 221872 3月 27 21:52 data.csv
-rw-r--r-- 1 dechin dechin 348524 3月 27 22:17 data.hdf5
-rw-r--r-- 1 dechin dechin 372736 3月 27 21:31 data.xls
-rw-r--r-- 1 dechin dechin 563 3月 27 21:42 table.py
操作完成之後在當前目錄下生成瞭一個hdf5文件。但是這種操作方式有個弊端，就是生成的hdf5文件跟vaex不是直接適配的關系，如果直接用df = vaex.open('data.hdf5')的方法進行讀取的話，輸出內容如下所示：
In [3]: df Out[3]: # table 0 '(0, [83.98, 92.38, 82. , 83.52], [ 0, ... 1 '(1, [83.9 , 83.92, 83.9 , 83.91], [ 1, ... 2 '(2, [84.5 , 84.65, 84. , 84.51], [ 2, ... 3 '(3, [84.9 , 85.06, 84.9 , 84.99], [ 3, ... 4 '(4, [85.1 , 85.2 , 85.1 , 85.13], [ 4, ... ... ... 3,917 '(3917, [274.65, 275.35, 274.6 , 274.61], [ ... 3,918 '(3918, [274.4, 275.2, 274.1, 275. ], [ 391... 3,919 '(3919, [275. , 275.01, 274. , 274.19], [ ... 3,920 '(3920, [275.2, 275.2, 272.6, 272.9], [ 392... 3,921 '(3921, [272.96, 273.73, 272.5 , 272.93], [ ...
在這個數據中，丟失瞭最關鍵的索引信息，雖然數據都被正確的保留瞭下來，但是在讀取上有非常大的不便。因此我們更加推薦第二種數據轉換的方法，直接用vaex進行數據格式的轉換：
[dechin@dechin-manjaro gold]$ ipython
Python 3.8.5 (default, Sep 4 2020, 07:30:14)
Type 'copyright', 'credits' or 'license' for more information
IPython 7.19.0 -- An enhanced Interactive Python. Type '?' for help.
In [1]: import vaex
In [2]: df = vaex.from_csv('data.csv')
In [3]: df.export_hdf5('vaex_data.hdf5')
In [4]: !ls -l
總用量 1220
-rw-r--r-- 1 dechin dechin 221856 3月 27 22:34 data.csv
-rw-r--r-- 1 dechin dechin 348436 3月 27 22:34 data.hdf5
-rw-r--r-- 1 dechin dechin 372736 3月 27 21:31 data.xls
-rw-r--r-- 1 dechin dechin 563 3月 27 21:42 table.py
-rw-r--r-- 1 dechin dechin 293512 3月 27 22:52 vaex_data.hdf5
執行完畢後在當前目錄下生成瞭一個vaex_data.hdf5文件，讓我們再試試讀取這個新的hdf5文件：
[dechin@dechin-manjaro gold]$ ipython
Python 3.8.5 (default, Sep 4 2020, 07:30:14)
Type 'copyright', 'credits' or 'license' for more information
IPython 7.19.0 -- An enhanced Interactive Python. Type '?' for help.
In [1]: import vaex
In [2]: df = vaex.open('vaex_data.hdf5')
In [3]: df
Out[3]:
# i t s h l e n a
0 0 '2002-10-30' 83.98 92.38 82.0 83.52 352 29373370
1 1 '2002-10-31' 83.9 83.92 83.9 83.91 66 5537480
2 2 '2002-11-01' 84.5 84.65 84.0 84.51 77 6502510
3 3 '2002-11-04' 84.9 85.06 84.9 84.99 95 8076330
4 4 '2002-11-05' 85.1 85.2 85.1 85.13 61 5193650
... ... ... ... ... ... ... ... ...
3,917 3917 '2018-11-23' 274.65 275.35 274.6 274.61 13478 3708580608
3,918 3918 '2018-11-26' 274.4 275.2 274.1 275.0 13738 3773763584
3,919 3919 '2018-11-27' 275.0 275.01 274.0 274.19 13984 3836845568
3,920 3920 '2018-11-28' 275.2 275.2 272.6 272.9 15592 4258130688
3,921 3921 '2018-11-28' 272.96 273.73 272.5 272.93 592 161576336
In [4]: df.s
Out[4]:
Expression = s
Length: 3,922 dtype: float64 (column)
-------------------------------------
0 83.98
1 83.9
2 84.5
3 84.9
4 85.1
...
3917 274.65
3918 274.4
3919 275
3920 275.2
3921 272.96
In [11]: df.plot(df.i, df.s, show=True) # 作圖
/home/dechin/anaconda3/lib/python3.8/site-packages/vaex/viz/mpl.py:311: UserWarning: `plot` is deprecated and it will be removed in version 5.x. Please `df.viz.heatmap` instead.
warnings.warn('`plot` is deprecated and it will be removed in version 5.x. Please `df.viz.heatmap` instead.')
這裡我們也需要提一下，在新的hdf5文件中，索引從高、低等中文變成瞭h、l等英文，這是為瞭方便數據的操作，我們在csv文件中將索引手動的修改成瞭英文，再轉換成hdf5的格式。最後我們使用vaex自帶的畫圖功能，繪制瞭這十幾年期間黃金的價格變動：
由於vaex自帶的繪圖方法比較少，總結如下：
最常用的還是熱度圖，因此這裡繪制出來的黃金價格圖的效果也是熱度圖的效果，但是基本上功能是比較完備的，而且性能異常的強大。
總結概要
在這篇文章中我們介紹瞭三種不同的python庫對表格數據進行處理，分別是xlrd、pandas和vaex，其中特別著重的強調瞭一下vaex的優越性能以及在大數據中的應用價值。配合一些簡單的示例，我們可以初步的瞭解到這些庫各自的特點，在實際場景中可以斟酌使用。
以上就是利用python做表格數據處理的詳細內容，更多關於python 表格數據處理的資料請關註WalkonNet其它相關文章！
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