使用Python分割基于特定列的csv文件

Question

我是一名Python初学者，并且制作了一些基本脚本。我最近面临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分割成多个较小的文件。

例如，该文件可能是这样的：

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

而且我希望将文件分割成单独的文件： Books.csv，Series.csv，Movie.csv

在现实中会有数百个类别，而且他们不会被排序。在这种情况下，他们在第一列，但将来他们可能不会。

我已经在网上找到了一些解决方案，但没有在Python中。有一个非常简单的AWK命令可以在一行中完成，但是我无法在工作中访问AWK。

我写了下面的代码，它可以工作，但我认为它可能是非常低效的。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！

Answer 1

一种高效的内存方式，避免在这里追加重新打开的文件（只要不打算生成大量的打开文件句柄）就是使用dict将类别映射到fileobj 。当该文件尚未打开，然后创建它，写标题，然后总是写的所有行到相应的文件，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()