通过python分割一个大的csv文件

Question

我有一个csv文件有500万行。 我想将文件拆分成由用户指定的行数。

已开发了以下代码，但其执行时间太长。任何人都可以帮助我优化代码。

import csv 
print "Please delete the previous created files. If any." 

filepath = raw_input("Enter the File path: ") 

line_count = 0 
filenum = 1 
try: 
    in_file = raw_input("Enter Input File name: ") 
    if in_file[-4:] == ".csv": 
     split_size = int(raw_input("Enter size: ")) 
     print "Split Size ---", split_size 
     print in_file, " will split into", split_size, "rows per file named as OutPut-file_*.csv (* = 1,2,3 and so on)" 
     with open (in_file,'r') as file1: 
      row_count = 0 
      reader = csv.reader(file1) 
      for line in file1: 
       #print line 
      with open(filepath + "\\OutPut-file_" +str(filenum) + ".csv", "a") as out_file: 
       if row_count < split_size: 
        out_file.write(line) 
        row_count = row_count +1 
       else: 
        filenum = filenum + 1 
        row_count = 0 
      line_count = line_count+1 
     print "Total Files Written --", filenum 
    else: 
     print "Please enter the Name of the file correctly."   
except IOError as e: 
    print "Oops..! Please Enter correct file path values", e 
except ValueError: 
    print "Oops..! Please Enter correct values" 

我自己也尝试没有 “打开”

Answer 1

Oups！您一再重新打开输出文件的每一行，当它是一个昂贵的操作...你的代码可能会变成：

... 
    with open (in_file,'r') as file1: 
     row_count = 0 
     #reader = csv.reader(file1) # unused here 
     out_file = open(filepath + "\\OutPut-file_" +str(filenum) + ".csv", "a") 
     for line in file1: 
      #print line 
      if row_count >= split_size: 
       out_file.close() 
       filenum = filenum + 1 
       out_file = open(filepath + "\\OutPut-file_" +str(filenum) + ".csv", "a") 
       row_count = 0 
      out_file.write(line) 
      row_count = row_count +1 
      line_count = line_count+1 
     ... 

理想情况下，你甚至应该在try块之前初始化out_file = None，确保干净if out_file is not None: out_file.close()

备注：此代码仅在行数中分开（与您的一样）。这意味着如果csv文件可以在引用字段中包含换行符，那么将会给出错误的输出...

Answer 2

您绝对可以使用python的多处理模块。

这是我获得的结果，当我有一个csv文件，其中有1,000,000行。

import time 
from multiprocessing import Pool 

def saving_csv_normally(start): 
    out_file = open('out_normally/' + str(start/batch_size) + '.csv', 'w') 
    for i in range(start, start+batch_size): 
    out_file.write(arr[i]) 
    out_file.close() 

def saving_csv_multi(start): 
    out_file = open('out_multi/' + str(start/batch_size) + '.csv', 'w') 
    for i in range(start, start+batch_size): 
    out_file.write(arr[i]) 
    out_file.close() 

def saving_csv_multi_async(start): 
    out_file = open('out_multi_async/' + str(start/batch_size) + '.csv', 'w') 
    for i in range(start, start+batch_size): 
    out_file.write(arr[i]) 
    out_file.close() 

with open('files/test.csv') as file: 
    arr = file.readlines() 

print "length of file : ", len(arr) 

batch_size = 100 #split in number of rows 

start = time.time() 
for i in range(0, len(arr), batch_size): 
    saving_csv_normally(i) 
print "time taken normally : ", time.time()-start 

#multiprocessing 
p = Pool() 
start = time.time() 
p.map(saving_csv_multi, range(0, len(arr), batch_size), chunksize=len(arr)/4) #chunksize you can define as much as you want 
print "time taken for multiprocessing : ", time.time()-start 

# it does the same thing aynchronically 
start = time.time() 
for i in p.imap_unordered(saving_csv_multi_async, range(0, len(arr), batch_size), chunksize=len(arr)/4): 
    continue 
print "time taken for multiprocessing async : ", time.time()-start 

输出显示每个所用的时间：

length of file : 1000000 
time taken normally : 0.733881950378 
time taken for multiprocessing : 0.508712053299 
time taken for multiprocessing async : 0.471592903137 

我已经定义三个单独用作在p.map传递只能有一个参数的功能和作为我存储在三个不同的文件夹的CSV文件这就是为什么我写了三个函数。