Files & Data Formats

Python makes it easy to read and write files. This page covers text files, JSON, and CSV.

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Reading and Writing Text Files

Reading a File

from pathlib import Path

content = Path("data.txt").read_text(encoding="utf-8")
print(content)

Writing a File

from pathlib import Path

Path("output.txt").write_text("Hello, World!", encoding="utf-8")

Using Context Manager (Traditional Way)

with open("data.txt", "r", encoding="utf-8") as f:
    content = f.read()

with open("output.txt", "w", encoding="utf-8") as f:
    f.write("Hello, World!")

Always use context managers

The with statement automatically closes the file, even if an error happens.

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File Modes

Mode	Meaning
"r"	Read (default)
"w"	Write (overwrites file)
"a"	Append (adds to end)
"x"	Create (fails if file exists)
"rb"	Read binary
"wb"	Write binary

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Working with Paths

Use pathlib — it is the modern way to work with file paths:

from pathlib import Path

project_dir = Path("project")
config_file = project_dir / "config" / "settings.json"

print(config_file.exists())    # True/False
print(config_file.name)        # "settings.json"
print(config_file.suffix)      # ".json"
print(config_file.parent)      # project/config
print(config_file.is_file())   # True/False

List Files in a Directory

from pathlib import Path

for file in Path("tests").glob("*.py"):
    print(file.name)

for file in Path("project").rglob("*.py"):
    print(file)  # recursive search

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JSON

JSON is the most common data format for APIs and configuration.

Reading JSON

import json
from pathlib import Path

data = json.loads(Path("data.json").read_text(encoding="utf-8"))
print(data["name"])

Writing JSON

import json
from pathlib import Path

data = {"name": "Alice", "scores": [90, 85, 92]}
Path("output.json").write_text(
    json.dumps(data, indent=2, ensure_ascii=False),
    encoding="utf-8",
)

JSON with Custom Types

import json
from datetime import datetime

def json_serializer(obj):
    if isinstance(obj, datetime):
        return obj.isoformat()
    raise TypeError(f"Type {type(obj)} is not serializable")

data = {"timestamp": datetime.now()}
json_text = json.dumps(data, default=json_serializer)

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CSV

CSV files store tabular data (rows and columns).

Reading CSV

import csv
from pathlib import Path

with Path("users.csv").open(encoding="utf-8") as f:
    reader = csv.DictReader(f)
    for row in reader:
        print(row["name"], row["email"])

Writing CSV

import csv
from pathlib import Path

users = [
    {"name": "Alice", "email": "alice@example.com"},
    {"name": "Bob", "email": "bob@example.com"},
]

with Path("output.csv").open("w", newline="", encoding="utf-8") as f:
    writer = csv.DictWriter(f, fieldnames=["name", "email"])
    writer.writeheader()
    writer.writerows(users)

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YAML

YAML is used for configuration files. It is more readable than JSON for complex settings.

Install

uv add pyyaml

Reading YAML

import yaml
from pathlib import Path

data = yaml.safe_load(Path("config.yaml").read_text(encoding="utf-8"))
print(data["database"]["host"])

Writing YAML

import yaml
from pathlib import Path

config = {
    "database": {
        "host": "localhost",
        "port": 5432,
        "name": "testdb",
    },
    "debug": True,
}
Path("config.yaml").write_text(
    yaml.dump(config, default_flow_style=False),
    encoding="utf-8",
)

Example YAML File

database:
  host: localhost
  port: 5432
  name: testdb

api:
  base_url: https://api.example.com
  timeout: 30

debug: true

Always use yaml.safe_load()

Never use yaml.load() — it can execute arbitrary Python code. Use yaml.safe_load() instead.

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Format Comparison

Format	Best For	Human Readable	Comments
JSON	API responses, data exchange	Medium	No
CSV	Tabular data, spreadsheets	Yes	No
YAML	Configuration files	Yes	Yes

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Best Practices

• Always specify encoding="utf-8" when opening files
• Always use with (context managers) for file operations
• Use pathlib.Path instead of os.path for path handling
• Validate data from external files before using it
• Handle FileNotFoundError when reading files
• Use json.dumps(indent=2) for readable output
• Never hardcode file paths — use configuration or parameters