How Python Handles Big Files

 The Python programming language has become more and more popular in handling data analysis and processing because of its certain unique advantages. It’s easy to read and maintain. pandas, with a rich library of functions and methods packaged in it, is a fast, flexible and easy to use data analysis and manipulation tool built on top of Python. It is one of the big boosters to make Python an efficient and powerful data analysis environment.

  pandas is memory-based. It does a great job when the to-be-manipulated data can fit into the memory. It is inconvenient, even unable, to deal with big data, which can’t be wholly loaded into the memory. Large files, however, like those containing data imported from the database or downloaded from the web, are common in real-world businesses. We need to have ways to manage them. How? That’s what I’d like to say something about.

  By “big data” here, I am not talking about the TB or PB level data that requires distributed processing. I mean the GB level file data that can’t fit into the normal PC memory but can be held on disk. This is the more common type of big file processing scenario.

  Since a big file can’t be loaded into the memory at once, we often need to retrieve it line by line or chunk by chunk for further processing. Both Python and pandas support this way of retrieval, but they don’t have cursors. Because of the absence of a cursor mechanism, we need to write code to implement the chunk-by-chunk retrieval in order to use it in functions and methods; sometimes we even have to write code to implement functions and methods. Here I list the typical scenarios of big file processing and their code examples to make you better understand Python’s way of dealing with them.

I. Aggregation

  A simple aggregation is to traverse values in the target column and to perform calculation according to the specified aggregate operation, such as the sum operation that adds up traversed values; the count operation that records the number of traversed values; and the mean operation that adds up and counts the traversed values and then divides the sum by the number. Here let’s look at how Python does a sum.

  Below is a part of a file:

  

  To calculate the total sales amount, that is, doing sum over the amount column:

  1. Retrieve file line by line

total=0 with open("orders.txt",'r') as f:       line=f.readline()     while   True:           line = f.readline()         if   not line:               break           total += float(line.split("\t")[4]) print(total)

Open the   file Read the header   row Read detail   data line by line Reading   finishes when all lines are traversed Get   cumulated value

  2. Retrieve file chunk by chunk in pandas

  pandas supports data retrieval chunk by chunk. Below is the workflow diagram:

  

import pandas as pd chunk_data =   pd.read_csv("orders.txt",sep="\t",chunksize=100000) total=0 for chunk in chunk_data: total+=chunk['amount'].sum() print(total)

Retrieve   the file chunk by chunk; each contains 100,000 lines Add up   amounts of all chunks

  Pandas is good at retrieval and processing in large chunks. In theory, the bigger the chunk size, the faster the processing. Note that the chunk size should be able to fit into the available memory. If the chunksize is set as 1, it is a line-by-line retrieval, which is extremely slow. So I do not recommend a line-by-line retrieval when handling large files in pandas.

II. Filtering

  The workflow diagram for filtering in pandas:

  

  Similar to the aggregation, pandas will divide a big file into multiple chunks (n), filter each data chunk and concatenate the filtering results.

  To get the sales records in New York state according to the above file:

  1. With small data sets

import pandas as pd chunk_data =   pd.read_csv("orders.txt",sep="\t",chunksize=100000) chunk_list = []   for chunk in chunk_data:       chunk_list.append(chunk[chunk.state=="New York"]) res = pd.concat(chunk_list) print(res)

Define an   empty list for storing the result set Filter   chunk by chunk Concatenate   filtering results

  2. With big data sets

import pandas as pd chunk_data =   pd.read_csv("orders.txt",sep="\t",chunksize=100000) n=0 for chunk in chunk_data:       need_data = chunk[chunk.state=='New York']     if n ==   0:           need_data.to_csv("orders_filter.txt",index=None)           n+=1     else:           need_data.to_csv("orders_filter.txt",index=None,mode='a',header=None)

For the   result set of processing the first chunk, write it to the target file with   headers retained and index removed For the   result sets of processing other chunks, append them to the target file with   both headers and index removed

  The logic of doing aggregates and filters is simple. But as Python doesn’t provide the cursor data type, we need to write a lot of code to get them done.

III. Sorting

  The workflow diagram for sorting in pandas:

  

  Sorting is complicated because you need to:

1. Retrieve one chunk each time;
2. Sort this chunk;
3. Write the sorting result of each chunk to a temporary file;
4. Maintain a list of k elements (k is the number of chunks) into which a row of data in each temporary file is put;
5. Sort records in the list by the sorting field (same as the sort direction in step 2);
6. Write the record with smallest (in ascending order) or largest (in descending order) value to the result file;
7. Put another row from each temporary file to the list;
8. Repeat step 6, 7 until all records are written to the result file.

  To sort the above file by amount in ascending order, I write a complete Python program of implementing the external sorting algorithm:

import pandas as pd import os import time import shutil import uuid import traceback   def parse_type(s):     if   s.isdigit():           return int(s)     try:         res   = float(s)           return res     except:           return s     def pos_by(by,head,sep):     by_num   = 0     for col   in head.split(sep):         if   col.strip()==by:               break           else:               by_num+=1     return   by_num   def   merge_sort(directory,ofile,by,ascending=True,sep=","):     with open(ofile,'w') as outfile:                   file_list = os.listdir(directory)                   file_chunk = [open(directory+"/"+file,'r') for file in file_list]           k_row = [file_chunk[i].readline()for i in range(len(file_chunk))]         by   = pos_by(by,k_row[0],sep)                   outfile.write(k_row[0])     k_row =   [file_chunk[i].readline()for i in range(len(file_chunk))] k_by = [parse_type(k_row[i].split(sep)[by].strip())  for i in range(len(file_chunk))]   with open(ofile,'a') as outfile:                   while True:               for i in range(len(k_by)):                   if i >= len(k_by):                     break                                     sorted_k_by = sorted(k_by) if ascending else sorted(k_by,reverse=True)                   if k_by[i] == sorted_k_by[0]:                     outfile.write(k_row[i])                     k_row[i] =   file_chunk[i].readline()                     if not k_row[i]:                         file_chunk[i].close()                         del(file_chunk[i])                         del(k_row[i])                         del(k_by[i])                     else:                         k_by[i] =   parse_type(k_row[i].split(sep)[by].strip())               if len(k_by)==0:                   break       def   external_sort(file_path,by,ofile,tmp_dir,ascending=True,chunksize=50000,sep=',', usecols=None,index_col=None): os.makedirs(tmp_dir,exist_ok=True)       try:           data_chunk = pd.read_csv(file_path,sep=sep,usecols=usecols,index_col=index_col,chunksize=chunksize)         for   chunk in data_chunk:               chunk = chunk.sort_values(by,ascending=ascending)               chunk.to_csv(tmp_dir+"/"+"chunk"+str(int(time.time()*10**7))+str(uuid.uuid4())+".csv",index=None,sep=sep)           merge_sort(tmp_dir,ofile=ofile,by=by,ascending=ascending,sep=sep)     except   Exception:           print(traceback.format_exc())       finally:           shutil.rmtree(tmp_dir, ignore_errors=True)     if __name__ == "__main__":     infile   = "D:/python_question_data/orders.txt"     ofile =   "D:/python_question_data/extra_sort_res_py.txt"     tmp =   "D:/python_question_data/tmp"       external_sort(infile,'amount',ofile,tmp,ascending=True,chunksize=1000000,sep='\t')

Function Parse data   type for the string Function Find the   position of the column name by which records are ordered in the headers Function External   merge sort List   temporary files Open a   temporary file Read the   headers Get the   position of column name by which records are ordered among the headers Export the   headers Read the   first line of detail data Maintain a   list of k elements to store k sorting column values Perform   sort in the order of the list Export the   row with the smallest value Read and   process temporary files one by one If the file   traversal isn’t finished, continue reading and update the list Finish   reading the file Function External   sort Create a   directory to store the temporary files Retrieve   the file chunk by chunk Sort the   chunks one by one Write the   sorted file External   merge sort Delete the temporary   directory Main   program Call the   external sort function

  Python handles the external sort using line-by-line merge & write. I didn’t use pandas because it is incredibly slow when doing the line-wise retrieval. Yet it is fast to do the chunk-wise merge in pandas. You can compare their speeds if you want to.

  The code is too complicated compared with that for aggregation and filtering. It’s beyond a non-professional programmer’s ability. The second problem is that it is slow to execute.

  The third problem is that it is only for standard structured files and single column sorting. If the file doesn’t have a header row, or if there are variable number of separators in rows, or if the sorting column contains values of nonstandard date format, or if there are multiple sorting columns, the code will be more complicated.

IV. Grouping

  It’s not easy to group and summarize a big file in Python, too. A convenient way out is to sort the file by the grouping column and then to traverse the ordered file during which neighboring records are put to same group if they have same grouping column values and a record is put to a new group if its grouping column value is different from the previous one. If a result set is too large, we need to write grouping result before the memory lose its hold.

  It’s convenient yet slow because a full-text sorting is needed. Generally databases use the hash grouping to increase speed. It’s effective but much more complicated. It’s almost impossible for non-professionals to do that.

So, it’s inconvenient and difficult to handle big files with Python because of the absence of cursor data type and relevant functions. We have to write all the code ourselves and the code is inefficient.

  If only there was a language that a non-professional programmer can handle to process large files. Luckily, we have esProc SPL.

  It’s convenient and easy to use. Because SPL is designed to process structured data and equipped with a richer library of functions than pandas and the built-in cursor data type. It handles large files concisely, effortlessly and efficiently.

  1. Aggregation

	A
1	=file(file_path).cursor@tc()
2	=A1.total(sum(col))

  2. Filtering

	A	B
1	=file(file_path).cursor@tc()
2	=A1.select(key==condition)
3	=A2.fetch()	/ Fetch data from a small result set
4	=file(out_file).export@tc(A2)	/ Write a large result set to a target   file

  3. Sorting

	A
1	=file(file_path).cursor@tc()
2	=A1.sortx(key)
3	=file(out_file).export@tc(A2)

  4. Grouping

	A	B
1	=file(file_path).cursor@tc()
2	=A1.groups(key;sum(coli):total)	/ Return a small result set directly
3	=A1.groupx(key;sum(coli):total)
4	=file(out_file).export@tc(A3)	/ Write a large result set to a target   file

  SPL also employs the above-mentioned HASH algorithm to effectively increase performance.

  SPL has the embedded parallel processing ability to be able to make the most use of the multi-core CPU to boost performance. A @m option only enables a function to perform parallel computing.

	A
1	=file(file_path).cursor@mtc()
2	=A1.groups(key;sum(coli):total)

  There are a lot of Python-version parallel programs, but none is simple enough.