I have a csv large file that I cannot handle in memory with python. I am splitting it into multiple chunks after grouping by the value of a specific column, using the following logic:
def splitDataFile(self, data_file):
self.list_of_chunk_names = []
csv_reader = csv.reader(open(data_file, "rb"), delimiter="|")
columns = csv_reader.next()
for key,rows in groupby(csv_reader, lambda row: (row[1])):
file_name = "data_chunk"+str(key)+".csv"
self.list_of_chunk_names.append(file_name)
with open(file_name, "w") as output:
output.write("|".join(columns)+"\n")
for row in rows:
output.write("|".join(row)+"\n")
print "message: list of chunks ", self.list_of_chunk_names
return
The logic is working but it's slow. I am wondering how can I optimize this? For instance with pandas?
Edit
Further explanation: I am not looking for a simple splitting to same size chunks (like each one having 1000 rows), I want to split by the value of a column, that's why I am using groupby.
Use this Python 3 program:
#!/usr/bin/env python3
import binascii
import csv
import os.path
import sys
from tkinter.filedialog import askopenfilename, askdirectory
from tkinter.simpledialog import askinteger
def split_csv_file(f, dst_dir, keyfunc):
csv_reader = csv.reader(f)
csv_writers = {}
for row in csv_reader:
k = keyfunc(row)
if k not in csv_writers:
csv_writers[k] = csv.writer(open(os.path.join(dst_dir, k),
mode='w', newline=''))
csv_writers[k].writerow(row)
def get_args_from_cli():
input_filename = sys.argv[1]
column = int(sys.argv[2])
dst_dir = sys.argv[3]
return (input_filename, column, dst_dir)
def get_args_from_gui():
input_filename = askopenfilename(
filetypes=(('CSV', '.csv'),),
title='Select CSV Input File')
column = askinteger('Choose Table Column', 'Table column')
dst_dir = askdirectory(title='Select Destination Directory')
return (input_filename, column, dst_dir)
if __name__ == '__main__':
if len(sys.argv) == 1:
input_filename, column, dst_dir = get_args_from_gui()
elif len(sys.argv) == 4:
input_filename, column, dst_dir = get_args_from_cli()
else:
raise Exception("Invalid number of arguments")
with open(input_filename, mode='r', newline='') as f:
split_csv_file(f, dst_dir, lambda r: r[column-1]+'.csv')
# if the column has funky values resulting in invalid filenames
# replace the line from above with:
# split_csv_file(f, dst_dir, lambda r: binascii.b2a_hex(r[column-1].encode('utf-8')).decode('utf-8')+'.csv')
Save it as split-csv.py
and run it from Explorer or from the command line.
For example to split superuser.csv
based off column 1 and write the output files under dstdir
use:
python split-csv.py data.csv 1 dstdir
If you run it without arguments, a Tkinter based GUI will prompt you to choose the input file, the column (1 based index) and the destination directory.
I am going with something like the following, where I am iterating over the unique values of the column to split by, to filter the data chunks.
def splitWithPandas(data_file, split_by_column):
values_to_split_by = pd.read_csv(data_file, delimiter="|", usecols=[split_by_column])
values_to_split_by.drop_duplicates()
values_to_split_by = pd.unique(values_to_split_by.values.ravel())
for i in values_to_split_by:
iter_csv = pd.read_csv(data_file, delimiter="|", chunksize=100000)
df = pd.concat([chunk[chunk[split_by_column] == i] for chunk in iter_csv])
df.to_csv("data_chunk_"+i, sep="|", index=False)
You will probably get the best performance by using the builtin chunking features of pandas (the chunksize
keyword arg to read_csv
),
http://pandas.pydata.org/pandas-docs/version/0.16.2/generated/pandas.read_csv.html
For example,
reader = pd.read_table('my_data.csv', chunksize=4)
for chunk in reader:
print(chunk)
EDIT:
This might get you somewhere,
import pandas as pd
group_col_indx = 1
group_col = pd.read_csv('test.csv', usecols=[group_col_indx])
keys = group_col.iloc[:,0].unique()
for key in keys:
df_list = []
reader = pd.read_csv('test.csv', chunksize=2)
for chunk in reader:
good_rows = chunk[chunk.iloc[:,group_col_indx] == key]
df_list.append(good_rows)
df_key = pd.concat(df_list)
I suspect that your biggest bottleneck is opening and closing a file handle every time you process a new block of rows. A better approach, as long as the number of files you write to is not too large, is to keep all the files open. Here's an outline:
def splitDataFile(self, data_file):
open_files = dict()
input_file = open(data_file, "rb")
try:
...
csv_reader = csv.reader(input_file, ...)
...
for key, rows in groupby(csv_reader, lambda row: (row[1])):
...
try:
output = open_files[key]
except KeyError:
output = open(file_name, "w")
output.write(...)
...
finally:
for open_file in open_files.itervalues():
open_file.close()
input_file.close()
Of course, if you only have one group with any given key, this will not help. (Actually it may make things worse, because you wind up holding a bunch of files open unnecessarily.) The more often you wind up writing to a single file, the more of a benefit you'll get from this change.
You can combine this with pandas, if you want, and use the chunking features of read_csv
or read_table
to handle the input processing.
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