使用Python基于特定列拆分csv文件

2019年7月28日 260次阅读

我是一名
Python初学者,并制作了一些基本的脚本.我最近的挑战是采用一个非常大的csv文件(10gb)并根据每行中特定变量的值将其拆分为许多较小的文件.

例如,文件可能如下所示：

Category,Title,Sales
"Books","Harry Potter",1441556
"Books","Lord of the Rings",14251154
"Series", "Breaking Bad",6246234
"Books","The Alchemist",12562166
"Movie","Inception",1573437

我想将文件拆分为单独的文件：
Books.csv,Series.csv,Movie.csv

实际上将有数百个类别,并且它们不会被分类.在这种情况下,它们位于第一列,但将来它们可能不是.

我在网上找到了一些解决方案,但在Python中没有.有一个非常简单的AWK命令可以在一行中执行此操作,但我无法在工作中访问AWK.

我编写了以下代码,但我认为这可能效率很低.任何人都可以建议如何加快速度？

import csv

#Creates empty set - this will be used to store the values that have already been used
filelist = set()

#Opens the large csv file in "read" mode
with open('//directory/largefile', 'r') as csvfile:

    #Read the first row of the large file and store the whole row as a string (headerstring)
    read_rows = csv.reader(csvfile)
    headerrow = next(read_rows)
    headerstring=','.join(headerrow) 

    for row in read_rows:

        #Store the whole row as a string (rowstring)
        rowstring=','.join(row)

        #Defines filename as the first entry in the row - This could be made dynamic so that the user inputs a column name to use
        filename = (row[0])

        #This basically makes sure it is not looking at the header row.
        if filename != "Category":

            #If the filename is not in the filelist set, add it to the list and create new csv file with header row.
            if filename not in filelist:    
                filelist.add(filename)
                with open('//directory/subfiles/' +str(filename)+'.csv','a') as f:
                    f.write(headerstring)
                    f.write("\n")
                    f.close()    
            #If the filename is in the filelist set, append the current row to the existing csv file.     
            else:
                with open('//directory/subfiles/' +str(filename)+'.csv','a') as f:
                    f.write(rowstring)
                    f.write("\n")
                    f.close()

谢谢！

最佳答案 一种内存有效的方法和避免重新打开文件在这里附加的方法(只要你不会生成大量的打开文件句柄)就是使用dict将类别映射到fileobj.如果该文件尚未打开,则创建它并编写标题,然后始终将所有行写入相应的文件,例如：

import csv

with open('somefile.csv') as fin:    
    csvin = csv.DictReader(fin)
    # Category -> open file lookup
    outputs = {}
    for row in csvin:
        cat = row['Category']
        # Open a new file and write the header
        if cat not in outputs:
            fout = open('{}.csv'.format(cat), 'w')
            dw = csv.DictWriter(fout, fieldnames=csvin.fieldnames)
            dw.writeheader()
            outputs[cat] = fout, dw
        # Always write the row
        outputs[cat][1].writerow(row)
    # Close all the files
    for fout, _ in outputs.values():
        fout.close()
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