Working with CSV files

• CSV (Comma Separated Values) is a plain-text tabular format where each line is a row and fields are delimited (commonly by commas).
• Widely used for spreadsheets, database exports, DevOps reports or inventories.
• Python’s built-in csv module handles reading, writing, quoting, delimiters, headers, and dialects.
• Always open files with newline='' and encoding='utf-8' for cross-platform consistency.

CSV Format Basics

• Each row represents a record; fields separated by a delimiter (comma by default).
• Optional header row defines column names.
• Fields containing delimiters, quotes, or newlines must be quoted (usually with double quotes).
• Alternative delimiters (tabs, semicolons) and quoting conventions are supported via dialects and parameters.

Reading CSV files with csv.reader

• Iterates over rows, returning each as a list of strings.
• Use next(reader) to skip or extract the header.
• Accepts delimiter, quotechar, and other formatting parameters.

import csv
from pathlib import Path

csv_path = Path("servers.csv")

with csv_path.open("r", encoding="utf-8", newline="") as file:
    reader = csv.reader(file)
    header = next(reader)
    print(f"Header: {header}")

    for idx, row in enumerate(reader, start=1):
        print(f"Row {idx}: {row}")

Reading with csv.DictReader

• Reads rows into dictionaries using the header row as keys.
• Access fields by column name instead of index.
• Optional fieldnames argument overrides header names.

import csv
from pathlib import Path

csv_path = Path("servers.csv")

with csv_path.open("r", encoding="utf-8", newline="") as file:
    dict_reader = csv.DictReader(file)
    print(f"Fieldnames: {dict_reader.fieldnames}")

    for idx, record in enumerate(dict_reader, start=1):
        print(f"Record {idx}: {record}")

Example of servers.csv

hostname,ip_address,role,status,tags
web01,10.0.1.5,webserver,running,"frontend,prod"
db01,10.0.2.10,database,maintenance,"backend,staging"

Writing with csv.writer

• Write rows from lists using .writerow() or .writerows().
• Open file with newline='' to avoid blank lines.
• Control delimiter and quoting via parameters.

import csv
from pathlib import Path

data = [
    ["hostname", "ip_address", "role"],
    ["web02", "10.0.1.6", "webserver"],
    ["app01", "10.0.3.15", "application"],
]

out_path = Path("output_basic.csv")

with out_path.open("w", encoding="utf-8", newline="") as file:
    writer = csv.writer(file)
    writer.writerows(data)

Writing with csv.DictWriter

• Write dictionaries using fieldnames to define header and column order.
• Call .writeheader() before .writerows().

import csv
from pathlib import Path

records = [
    {
        "host": "web01",
        "port": "80",
        "status": "running"
    },
    {
        "host": "db02",
        "status": "maintenance",
        "tags": "prod,finance"
    }
]

out_dict_path = Path("output_dict.csv")
fieldnames = set()

for record in records:
    fieldnames = fieldnames | record.keys()

with out_dict_path.open("w", encoding="utf-8", newline="") as file:
    writer = csv.DictWriter(
        file,
        fieldnames=fieldnames,
        restval="undefined",
        extrasaction="ignore"
    )
    writer.writeheader()
    writer.writerows(records)