Reading and writing CSV data in Python - Step-by-Step Execution

Concept Flow - Reading and writing CSV data

Open CSV file for reading

↓

Create CSV reader object

↓

Loop: Read each row

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More rows?

No→Close file

Open a CSV file, create a reader, loop through rows, process each, then close the file.

Execution Sample

Python

import csv
with open('data.csv', 'r') as file:
    reader = csv.reader(file)
    for row in reader:
        print(row)

Reads each row from 'data.csv' and prints it as a list.

Execution Table

Step	Action	Evaluation	Result
1	Open 'data.csv' for reading	File opened	File object created
2	Create csv.reader object	csv.reader(file)	Reader object ready
3	Read first row	reader.__next__()	Row 1 data as list
4	Print row	print(row)	Row 1 printed
5	Read second row	reader.__next__()	Row 2 data as list
6	Print row	print(row)	Row 2 printed
7	Read next row	reader.__next__()	No more rows, StopIteration raised
8	Exit loop	StopIteration caught	Loop ends
9	Close file	file.close()	File closed

💡 No more rows to read, StopIteration ends the loop

Variable Tracker

Variable	Start	After 1	After 2	Final
file	None	File object	File object	Closed
reader	None	Reader object	Reader object	Reader object
row	None	Row 1 list	Row 2 list	None (loop ended)

Key Moments - 3 Insights

Why does the loop stop reading rows?

The loop stops because the csv.reader raises StopIteration when no more rows exist, as shown in execution_table step 7.

What type of data is each 'row' variable during the loop?

Each 'row' is a list of strings representing the CSV columns, as seen in execution_table steps 3 and 5.

Why do we use 'with open' instead of just open()?

'with open' automatically closes the file after the block ends, ensuring no file remains open, as shown in execution_table step 9.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'row' after step 3?

A None

B Row 1 data as list

C Row 2 data as list

D File object

💡 Hint

Check the 'Result' column in execution_table row with Step 3

At which step does the loop stop reading rows?

A Step 9

B Step 5

C Step 7

D Step 3

💡 Hint

Look for StopIteration in the 'Evaluation' column in execution_table

If the CSV file had 3 rows instead of 2, how would the execution_table change?

A There would be an extra pair of 'Read row' and 'Print row' steps before step 7

B The file would close earlier

C The reader object would be created twice

D The loop would never end

💡 Hint

Check the pattern of reading and printing rows in execution_table steps 3-6

Concept Snapshot

Reading CSV:
- Use 'with open(filename, "r") as file'
- Create reader: csv.reader(file)
- Loop: for row in reader
- Each row is a list of strings
- File auto-closed after block

Writing CSV:
- Use 'with open(filename, "w", newline="") as file'
- Create writer: csv.writer(file)
- Use writer.writerow(list) to write rows

Full Transcript

This visual trace shows how Python reads CSV data step-by-step. First, the file is opened for reading. Then, a csv.reader object is created to parse the file. The program loops over each row, reading and printing it as a list of strings. When no more rows exist, the reader raises StopIteration, ending the loop. Finally, the file is closed automatically by the 'with' statement. Variables like 'file', 'reader', and 'row' change values as the program runs. Key points include understanding the StopIteration that ends the loop, the list structure of each row, and the automatic file closing with 'with open'. The quiz questions help check understanding of these steps.