Reading and Writing CSV and JSON in Python

import pandas as pd

# Wrong encoding: common error for European or Latin American data
try:
    df = pd.read_csv('data.csv')
except UnicodeDecodeError:
    df = pd.read_csv('data.csv', encoding='latin-1')

# Custom delimiter (common: semicolon in German/French Excel)
df = pd.read_csv('data.csv', sep=';')

# Keep leading zeros on ID-like columns
df = pd.read_csv('data.csv', dtype={'zip_code': str, 'phone': str})

# Parse dates at load time
df = pd.read_csv('data.csv', parse_dates=['signup_date'])

# Treat sentinel values as NaN
df = pd.read_csv('data.csv', na_values=['', 'N/A', '-999', 'unknown'])

# Large files: stream in chunks
for chunk in pd.read_csv('huge.csv', chunksize=10_000):
    process(chunk)

# index=False is almost always what you want
df.to_csv('output.csv', index=False)

# Use UTF-8 (default but be explicit)
df.to_csv('output.csv', encoding='utf-8', index=False)

# Keep floats readable
df.to_csv('output.csv', float_format='%.3f', index=False)

# Compress for big files
df.to_csv('output.csv.gz', compression='gzip', index=False)

import json
import pandas as pd

# Simple read
with open('data.json') as f:
    data = json.load(f)

# Line-delimited JSON (common for big data)
df = pd.read_json('data.jsonl', lines=True)

# Deeply nested JSON into a flat table
records = [
    {'user': {'id': 1, 'name': 'Alice'}, 'events': [{'type': 'click'}]}
]
flat = pd.json_normalize(
    records,
    record_path='events',
    meta=[['user', 'id'], ['user', 'name']]
)
print(flat)

import json
import numpy as np

data = {'scores': [0.1, 0.2, 0.3], 'name': 'test'}

# Pretty-printed, human-readable
with open('out.json', 'w') as f:
    json.dump(data, f, indent=2, ensure_ascii=False)

# numpy types crash default json; convert them
class NPEncoder(json.JSONEncoder):
    def default(self, o):
        if isinstance(o, np.integer): return int(o)
        if isinstance(o, np.floating): return float(o)
        if isinstance(o, np.ndarray): return o.tolist()
        return super().default(o)

with open('out.json', 'w') as f:
    json.dump(data, f, cls=NPEncoder, indent=2)