5 Mistakes That Kill Your Python Developer Job Chances

# Classic interview trap: mutable default argument
def add_item(item, basket=[]):      # BUG: same list reused every call
    basket.append(item)
    return basket

print(add_item("apple"))   # ['apple']
print(add_item("mango"))   # ['apple', 'mango']  <- surprise!

# Fix: use None and create a new list inside
def add_item_fixed(item, basket=None):
    if basket is None:
        basket = []
    basket.append(item)
    return basket

print(add_item_fixed("apple"), add_item_fixed("mango"))

import copy

# Shallow vs deep copy
original = {"name": "Asha", "skills": ["python"]}
shallow = original.copy()
deep = copy.deepcopy(original)
original["skills"].append("sql")
print(shallow["skills"])   # ['python', 'sql'] - shares the inner list
print(deep["skills"])      # ['python']        - independent

# Scope: global vs local
count = 0
def increment():
    global count          # without this line, Python raises UnboundLocalError
    count += 1
increment()
print(count)              # 1

# pricing.py - small, named, testable functions
GST_RATE = 0.18

def apply_discount(amount: float, percent: float) -> float:
    """Return amount after a percentage discount."""
    if not 0 <= percent <= 100:
        raise ValueError("percent must be between 0 and 100")
    return round(amount * (1 - percent / 100), 2)

def with_gst(amount: float) -> float:
    """Return amount including GST."""
    return round(amount * (1 + GST_RATE), 2)

# test_pricing.py - run with: pytest -q
import pytest
from pricing import apply_discount, with_gst

def test_discount_normal():
    assert apply_discount(1000, 10) == 900

def test_discount_zero_and_full():
    assert apply_discount(500, 0) == 500
    assert apply_discount(500, 100) == 0

def test_discount_invalid():
    with pytest.raises(ValueError):
        apply_discount(500, 120)

def test_gst():
    assert with_gst(100) == 118

# Problem: return the first non-repeating character.
# Said aloud before coding:
# 1. Clarify: empty string? case-sensitive? return None if none.
# 2. Plan: count characters, then scan in order - O(n) time.
from collections import Counter

def first_unique(s: str):
    counts = Counter(s)
    for ch in s:
        if counts[ch] == 1:
            return ch
    return None

# 3. Test aloud
assert first_unique("swiss") == "w"
assert first_unique("aabb") is None
assert first_unique("") is None
print("all checks passed")

How to describe a project in 90 seconds

1. Problem: who needed it and why
2. What I built: main features in one sentence
3. My choices: libraries, structure, database - and why
4. Hardest bug: what broke and how I fixed it
5. Quality: tests, error handling, deployment
6. Next step: what I would improve

Only mention work you can explain in detail if asked.

Week 1  Fundamentals: data types, mutability, scope,
        comprehensions, exceptions, OOP. 20 small programs.
Week 2  One original project for a real user.
        Clean modules, README, screenshots.
Week 3  Quality: add pytest tests, ruff/black, small commits,
        deploy or package the project.
Week 4  Interviews: 3 mock interviews out loud,
        rehearse project story, revise weak topics.

Audit every repo on your GitHub:

[ ] README: problem, features, setup, screenshots
[ ] requirements.txt or pyproject.toml
[ ] Code split into functions and modules
[ ] Tests folder with passing tests
[ ] Consistent formatting
[ ] Meaningful commit history
[ ] No secrets or API keys committed
[ ] Pinned: your 3 best projects only

Before writing any interview code:
[ ] Restate the problem
[ ] Ask about input size and edge cases
[ ] Say the approach and its complexity

While coding:
[ ] Use clear names
[ ] Explain each block briefly

After coding:
[ ] Run 2-3 examples by hand, including an edge case
[ ] State time and space complexity