How to Process Data in Batches in Python

There are some situations when you have a huge list of items to process but you cannot do them in one go due to some limitations of the systems that process the list.

Some examples:

  • When you need to access an API that supports only 100 items at once in the request, you need to split your original list into lists of 100 items & combine the results.
  • You have a long list of items that you want to process parallely. You can split them into the number of sub processes that you want & process them independently.
long_list = list(range(100))
sub_list_length = 10
sub_lists = [
    long_list[i : i + sub_list_length]
    for i in range(0, len(long_list), sub_list_length)
]

Let us try to break down the code

long_list is a list of 100 numbers. We are splitting this list of numbers into sub lists specified by the sub_list_length of 10. The list comprehension is relying on slices of the original list.

print(long_list)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99]
print(sub_lists)
[[0, 1, 2, 3, 4, 5, 6, 7, 8, 9], [10, 11, 12, 13, 14, 15, 16, 17, 18, 19], [20, 21, 22, 23, 24, 25, 26, 27, 28, 29], [30, 31, 32, 33, 34, 35, 36, 37, 38, 39], [40, 41, 42, 43, 44, 45, 46, 47, 48, 49], [50, 51, 52, 53, 54, 55, 56, 57, 58, 59], [60, 61, 62, 63, 64, 65, 66, 67, 68, 69], [70, 71, 72, 73, 74, 75, 76, 77, 78, 79], [80, 81, 82, 83, 84, 85, 86, 87, 88, 89], [90, 91, 92, 93, 94, 95, 96, 97, 98, 99]]
print(list(range(0, len(long_list), sub_list_length)))
[0, 10, 20, 30, 40, 50, 60, 70, 80, 90]

Processing the Sub Lists

results = []
for sub_list in sub_lists:
    partial_result = process_function(sub_list)
    results.append(partial_result)

Here process_function can be any function that processes the lists. In our example, this would be the call to the API or sub processes that process the lists.

Bonus Tip: Keeping track of Progress

Additionally, you can keep track of the progress of the process by wrapping up the for loop iterable using an open source library, tqdm to display a progress bar that also indicates how long each iteration takes. It works for any iterable as well.

from tqdm import tqdm
results = []
for sub_list in tqdm(sub_lists):
    partial_result = process_function(sub_list)
    results.append(partial_result)

Progress Bar

This has come in quite handy for me in quite a few cases.

Cover Photo from CHUTTERSNAP on Unsplash

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