Sequentially read huge CSV file in python

Question

I have a 10gb CSV file that contains some information that I need to use.

As I have limited memory on my PC, I can not read all the file in memory in one single batch. Instead, I would like to iteratively read only some rows of this file.

Say that at the first iteration I want to read the first 100, at the second those going to 101 to 200 and so on.

Is there an efficient way to perform this task in Python? May Pandas provide something useful to this? Or are there better (in terms of memory and speed) methods?

Answer 1

Here is the short answer.

chunksize = 10 ** 6
for chunk in pd.read_csv(filename, chunksize=chunksize):
    process(chunk)

Here is the very long answer.

To get started, you’ll need to import pandas and sqlalchemy. The commands below will do that.

import pandas as pd
from sqlalchemy import create_engine

Next, set up a variable that points to your csv file. This isn’t necessary but it does help in re-usability.

file = '/path/to/csv/file'

With these three lines of code, we are ready to start analyzing our data. Let’s take a look at the ‘head’ of the csv file to see what the contents might look like.

print pd.read_csv(file, nrows=5)

This command uses pandas’ “read_csv” command to read in only 5 rows (nrows=5) and then print those rows to the screen. This lets you understand the structure of the csv file and make sure the data is formatted in a way that makes sense for your work.

Before we can actually work with the data, we need to do something with it so we can begin to filter it to work with subsets of the data. This is usually what I would use pandas’ dataframe for but with large data files, we need to store the data somewhere else. In this case, we’ll set up a local sqllite database, read the csv file in chunks and then write those chunks to sqllite.

To do this, we’ll first need to create the sqllite database using the following command.

csv_database = create_engine('sqlite:///csv_database.db')

Next, we need to iterate through the CSV file in chunks and store the data into sqllite.

chunksize = 100000
i = 0
j = 1
for df in pd.read_csv(file, chunksize=chunksize, iterator=True):
      df = df.rename(columns={c: c.replace(' ', '') for c in df.columns}) 
      df.index += j
      i+=1
      df.to_sql('table', csv_database, if_exists='append')
      j = df.index[-1] + 1

With this code, we are setting the chunksize at 100,000 to keep the size of the chunks managable, initializing a couple of iterators (i=0, j=0) and then running a through a for loop. The for loop read a chunk of data from the CSV file, removes space from any of column names, then stores the chunk into the sqllite database (df.to_sql(…)).

This might take a while if your CSV file is sufficiently large, but the time spent waiting is worth it because you can now use pandas ‘sql’ tools to pull data from the database without worrying about memory constraints.

To access the data now, you can run commands like the following:

df = pd.read_sql_query('SELECT * FROM table', csv_database)

Of course, using ‘select *…’ will load all data into memory, which is the problem we are trying to get away from so you should throw from filters into your select statements to filter the data. For example:

df = pd.read_sql_query('SELECT COl1, COL2 FROM table where COL1 = SOMEVALUE', csv_database)

Answer 2

You can use pandas.read_csv() with chuncksize parameter:

http://pandas.pydata.org/pandas-docs/stable/generated/pandas.read_csv.html#pandas.read_csv

for chunck_df in pd.read_csv('yourfile.csv', chunksize=100):
    # each chunck_df contains a part of the whole CSV