Module 05 — Interview Prep: Data Handling

Strings, files, JSON/CSV, exceptions, logging

Theory Questions

Q1: Are strings mutable in Python?
No. str is immutable. All string methods return new strings.

Q2: f-string vs .format() vs %?
f-strings (3.6+): fastest, most readable. Industry standard. Support expressions: f"{x=}".

Q3: Raw string r"..."?
Backslashes treated literally. Used for regex patterns and Windows paths.

Q4: re.match vs re.search vs re.fullmatch?
match: start of string. search: anywhere. fullmatch: entire string must match.

Q5: Why use with open()?
Context manager ensures file closed even on exception. Prevents resource leaks.

Q6: Text mode vs binary mode?
Text: str, encoding applied. Binary: bytes, no encoding. Use 'rb'/'wb' for images, etc.

Q7: JSON serializable types?
dict, list, str, int, float, bool, None. NOT: datetime, set, custom objects (need default handler).

Q8: Why encoding='utf-8'?
Explicit encoding prevents platform-dependent defaults. UTF-8 is universal standard.

Q9: pathlib vs os.path?
pathlib: OOP, cross-platform, readable. Preferred in modern Python.

Q10: Exception hierarchy?
BaseException → Exception → (ValueError, TypeError, KeyError, IOError, etc.). Catch specific, not bare except.

Q11: try/except/else/finally order?
try → except (on error) → else (if no error) → finally (always).

Q12: raise from purpose?
Exception chaining — preserves original cause in __cause__.

Q13: Custom exception best practice?
Inherit from Exception. Create hierarchy with base app exception.

Q14: logging levels?
DEBUG < INFO < WARNING < ERROR < CRITICAL. Production typically INFO or WARNING.

Q15: logging vs print?
logging: levels, handlers, rotation, production-ready. print: debugging only.

Q16: CSV newline='' on Windows?
Prevents double newlines when writing CSV files.

Q17: What is assert?
Debug check. Disabled with python -O. Not for production validation.

Q18: Bare except: problem?
Catches everything including KeyboardInterrupt, SystemExit. Always catch specific exceptions.

Coding Questions

Q19: Valid palindrome

def is_palindrome(s):
    s = "".join(c.lower() for c in s if c.isalnum())
    return s == s[::-1]

Q20: Reverse words in sentence

def reverse_words(s):
    return " ".join(s.split()[::-1])

Q21: Count word frequency

from collections import Counter
def word_freq(text):
    return Counter(text.lower().split())

Q22: Read JSON safely

import json
from pathlib import Path
def load_json(path):
    with open(path, encoding="utf-8") as f:
        return json.load(f)

Q23: Write CSV from dicts

import csv
def write_csv(path, rows, fieldnames):
    with open(path, "w", newline="", encoding="utf-8") as f:
        writer = csv.DictWriter(f, fieldnames=fieldnames)
        writer.writeheader()
        writer.writerows(rows)

Q24: Custom exception hierarchy

class AppError(Exception): pass
class ValidationError(AppError):
    def __init__(self, field, msg):
        self.field = field
        super().__init__(f"{field}: {msg}")

Q25: Retry with exception handling

def parse_int(value, default=0):
    try:
        return int(value)
    except (ValueError, TypeError):
        return default

Q26: Extract emails with regex

import re
def find_emails(text):
    return re.findall(r"[\w.+-]+@[\w-]+\.[\w.-]+", text)

Q27: Setup logger

import logging
logging.basicConfig(level=logging.INFO,
    format="%(asctime)s %(levelname)s %(message)s")
logger = logging.getLogger(__name__)
logger.info("Application started")

Common Pitfalls

String concatenation with += in loop (O(n²))
Not closing files (use with)
Bare except swallowing errors
Using print in production
Not handling JSON decode errors

Quick Checklist

Write palindrome check from memory
Explain try/except/else/finally flow
Read/write JSON and CSV
Explain logging levels
Difference re.match vs re.search

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