03-2-python-csv-files.md

Data 3-2: Python - CSV Files

Writing a CSV file

We will be using the csv.writer to write csv files.

import csv

with open('testwrite.csv', 'w') as f:
    writer = csv.writer(f)
    writer.writerow(['col1', 'col2'])
    writer.writerow(['val1', 'val2'])
    writer.writerow(['val1', 'val2'])
    writer.writerow(['val1', 'val2'])

You can read more about the csv module here: https://docs.python.org/3/library/csv.html

Opening a CSV file

This snippet opens a file in read only mode and uses the csv module to instantiate a csv.DictReader.

The DictReader will parse the CSV and return a dictionary for each record where the keys of the dictionary are the header of the csv.

Then we take all of those dictionaries and put them into a list with rows = list(reader). reader is what is referred to as an iterable in python. Running the list function exhausts the iterator and just gives us the contents of the reader as a list.)

import csv

with open('testwrite.csv', 'r') as f:
    reader = csv.DictReader(f)
    rows = list(reader)

print(rows)

Note that since we have loaded the entire CSV into memory in the variable rows we can now put our for loop outside of the context manager since we no longer need access to the file, f.

➡️ Try It

Write a python script called veggies.py that defines a list of dicts named vegetables like so:

vegetables = [
 {"name": "eggplant", "color": "purple"},
 {"name": "tomato", "color": "red"},
 {"name": "corn", "color": "yellow"},
 {"name": "okra", "color": "green"},
 {"name": "arugula", "color": "green"},
 {"name": "broccoli", "color": "green"},
]

Write a python program that

1. Loops through each vegetable

2. In the loop, write the name of each vegetable and the color into a CSV called vegetables.csv

   The CSV should look like this:

   name	color
   eggplant	purple
   tomato	red
   corn	yellow
   okra	green
   arugula	green
   broccoli	green

3. Bonus: add the length of the name of the vegtable as separate column

hints:

• Don't forget to first write a header row to the CSV
• To get the length of any string use the builtin len method. For example, len('dhrumil') is 7.