机器学习之数据预处理，Pandas读取excel数据

Python读写excel的工具库很多，比如最耳熟能详的xlrd、xlwt，xlutils，openpyxl等。其中xlrd和xlwt库通常配合使用，一个用于读，一个用于写excel。xlutils结合xlrd可以达到修改excel文件目的。openpyxl可以对excel文件同时进行读写操作。

而说到数据预处理，pandas就体现除了它的强大之处，并且它还支持可读写多种文档格式，其中就包括对excel的读写。本文重点就是介绍pandas对excel数据集的预处理。

机器学习常用的模型对数据输入都是有要求的，多数机器学习算法最基本的要求是训练数据要转换成数值格式。当然，也有像决策树算法这种不需要转换为数值的算法，这里不做特例讨论。

pandas读取excel文件的函数是pandas.read_excel()，主要参数包括：

io : 读取的excel文档地址，

        string, path object (pathlib.Path or py._path.local.LocalPath),

file-like object, pandas ExcelFile, or xlrd workbook. The string could be a URL. Valid URL schemes include http, ftp, s3, and file. For file URLs, a host is expected. For instance, a local file could be file://localhost/path/to/workbook.xlsx

sheet_name : 读取的excel指定的sheet页

        string, int, mixed list of strings/ints, or None, default 0

Strings are used for sheet names, Integers are used in zero-indexed sheet positions.

Lists of strings/integers are used to request multiple sheets.

Specify None to get all sheets.

str|int -> DataFrame is returned. list|None -> Dict of DataFrames is returned, with keys representing sheets.

Available Cases

• Defaults to 0 -> 1st sheet as a DataFrame
• 1 -> 2nd sheet as a DataFrame
• “Sheet1” -> 1st sheet as a DataFrame
• [0,1,”Sheet5”] -> 1st, 2nd & 5th sheet as a dictionary of DataFrames
• None -> All sheets as a dictionary of DataFrames

header : 设置读取的excel第一行是否作为列名称

        int, list of ints, default 0

Row (0-indexed) to use for the column labels of the parsed DataFrame. If a list of integers is passed those row positions will be combined into a MultiIndex. Use None if there is no header.

names : array-like, default None

List of column names to use. If file contains no header row, then you should explicitly pass header=None

index_col : int, list of ints, default None

Column (0-indexed) to use as the row labels of the DataFrame. Pass None if there is no such column. If a list is passed, those columns will be combined into a MultiIndex. If a subset of data is selected with usecols, index_col is based on the subset.

usecols : int or list, default None

• If None then parse all columns,
• If int then indicates last column to be parsed
• If list of ints then indicates list of column numbers to be parsed
• If string then indicates comma separated list of Excel column letters and column ranges (e.g. “A:E” or “A,C,E:F”). Ranges are inclusive of both sides.

下满是一些pandas读取excel数据的示例：

将数据集写入excel文件：

>>> df_out = pd.DataFrame([('string1', 1),
...                        ('string2', 2),
...                        ('string3', 3)],
...                       columns=['Name', 'Value'])
>>> df_out
      Name  Value
0  string1      1
1  string2      2
2  string3      3
>>> df_out.to_excel('tmp.xlsx')

读取excel文件：

>>> pd.read_excel('tmp.xlsx')
      Name  Value
0  string1      1
1  string2      2
2  string3      3

参数index_col and header 都设置为None表示不读取excel的第一行和第一列作为标题和默认索引：

>>> pd.read_excel('tmp.xlsx', index_col=None, header=None)
     0        1      2
0  NaN     Name  Value
1  0.0  string1      1
2  1.0  string2      2
3  2.0  string3      3

甚至可以专门制定列的格式：

>>> pd.read_excel('tmp.xlsx', dtype={'Name':str, 'Value':float})
      Name  Value
0  string1    1.0
1  string2    2.0
2  string3    3.0

下面是综合示例：读取text.xlsx文件的sheet1页，仅载入D:F列的数据。这里F列是类别标签，需要类别1和类别2转换为数字，应用于机器学习的输入建模。

import pandas as pd

def reader(path,sheet):
	return pd.read_excel(path, sheet_name=sheet, usecols='D:F')
	
trainrd = reader('text.xlsx','sheet1')
trainrd.head(5)  #查看前5行数据
trainrd['x']=0  #新建一列x
trainrd.loc[trainrd['类别']=='类别1','x']=0 #将类别列的文字转换为数字
trainrd.loc[trainrd['类别']=='类别2','x']=1