Chunked reading for large files in Pandas - Step-by-Step Execution

Concept Flow - Chunked reading for large files

Start reading file

↓

Read chunk of rows

↓

Process chunk

↓

More data?

No→Stop reading

Yes↓

Read chunk of rows

The file is read in small parts called chunks. Each chunk is processed before reading the next. This repeats until the whole file is done.

Execution Sample

Pandas

import pandas as pd
chunk_size = 3
for chunk in pd.read_csv('data.csv', chunksize=chunk_size):
    print(chunk)

Reads a CSV file in chunks of 3 rows and prints each chunk.

Execution Table

Step	Action	Chunk Data (rows)	Output
1	Read first chunk	[Row 0, Row 1, Row 2]	Prints first 3 rows
2	Process first chunk	Same as above	Displayed on screen
3	Read second chunk	[Row 3, Row 4, Row 5]	Prints next 3 rows
4	Process second chunk	Same as above	Displayed on screen
5	Read third chunk	[Row 6, Row 7]	Prints last 2 rows (less than chunk size)
6	Process third chunk	Same as above	Displayed on screen
7	Check for more data	No more rows	Stop reading file

💡 No more rows to read, chunked reading ends

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
chunk	None	[Row 0, Row 1, Row 2]	[Row 3, Row 4, Row 5]	[Row 6, Row 7]	None (loop ends)

Key Moments - 3 Insights

Why does the last chunk have fewer rows than the chunk size?

Because the file's total rows may not be a multiple of the chunk size. The last chunk reads only the remaining rows, as shown in execution_table step 5.

Does chunked reading load the whole file into memory at once?

No, chunked reading loads only one chunk at a time into memory, making it efficient for large files. See execution_table steps 1, 3, and 5 where chunks are read separately.

How do we know when to stop reading chunks?

When the read chunk is empty or smaller than the chunk size and no more data is available, the loop stops. This is shown in execution_table step 7.

Visual Quiz - 3 Questions

Look at the execution_table, what rows does the second chunk contain?

A [Row 3, Row 4, Row 5]

B [Row 0, Row 1, Row 2]

C [Row 6, Row 7]

D No rows, chunk is empty

💡 Hint

Check execution_table row 3 under 'Chunk Data (rows)'

At which step does the chunked reading stop?

A Step 5

B Step 7

C Step 3

D Step 1

💡 Hint

Look at execution_table step 7 where it says 'Stop reading file'

If the chunk size was 2 instead of 3, how would the first chunk change?

A It would contain all rows at once

B It would contain 3 rows as before

C It would contain 2 rows instead of 3

D It would be empty

💡 Hint

Chunk size controls how many rows are read each time, see variable_tracker for chunk sizes

Concept Snapshot

Chunked reading reads large files in small parts called chunks.
Use pandas read_csv with chunksize to set chunk size.
Process each chunk separately to save memory.
Loop ends when no more data is left.
Last chunk may be smaller than chunk size.

Full Transcript

Chunked reading is a way to read big files in small pieces called chunks. We set a chunk size, for example 3 rows. The program reads the first 3 rows, processes them, then reads the next 3 rows, and so on. This continues until the whole file is read. The last chunk may have fewer rows if the total number of rows is not a multiple of the chunk size. This method helps avoid loading the entire file into memory at once, which is useful for very large files. The reading stops when no more rows are left to read.