Python Keywords and Identifiers Explained — Rules, Reserved Words and Real Mistakes

import keyword  # Python's built-in keyword module — no install needed

# Get the full list of all reserved keywords in your Python version
all_keywords = keyword.kwlist

print(f"Python {__import__('sys').version.split()[0]} has {len(all_keywords)} keywords:\n")

# Print keywords in rows of 5 so they're easy to read
for index, word in enumerate(all_keywords):
    # end='' prevents a newline after each word; we add our own spacing
    print(f"{word:<12}", end='')
    # After every 5th keyword, drop to a new line
    if (index + 1) % 5 == 0:
        print()  # moves the cursor to the next line

print()  # final newline for clean terminal output

# Check whether a specific word is a keyword — returns True or False
word_to_check = "while"
print(f"\nIs '{word_to_check}' a Python keyword? {keyword.iskeyword(word_to_check)}")

word_to_check = "speed"
print(f"Is '{word_to_check}' a Python keyword? {keyword.iskeyword(word_to_check)}")

# ── LEGAL IDENTIFIERS ─────────────────────────────────────────────────────────

player_name = "Alice"          # lowercase_with_underscores — PEP 8 for variables
player_score = 0               # digits are fine anywhere except the first position
MAX_LEVEL = 100                # ALL_CAPS signals a constant — never meant to change
_checkpoint_data = [1, 5, 9]   # single leading underscore = internal/private use

print(f"Player: {player_name}")
print(f"Score:  {player_score}")
print(f"Max level: {MAX_LEVEL}")
print(f"Checkpoints: {_checkpoint_data}")

# ── CASE SENSITIVITY IN ACTION ────────────────────────────────────────────────

total_coins = 50    # variable 'total_coins' — lowercase
Total_Coins = 200   # COMPLETELY different variable — Python sees a new name entirely

print(f"\ntotal_coins  = {total_coins}")   # prints 50
print(f"Total_Coins  = {Total_Coins}")   # prints 200 — different variable!

# ── PEP 8 CLASS NAMING ────────────────────────────────────────────────────────

class GamePlayer:              # CapitalisedWords (PascalCase) for class names
    def __init__(self, name):  # __init__ is a dunder — Python's constructor method
        self.name = name       # 'self.name' stores the player's name on the object

new_player = GamePlayer("Bob")  # create an instance of GamePlayer
print(f"\nNew player name: {new_player.name}")

# ── WHAT HAPPENS WITH ILLEGAL NAMES — commented out to allow file to run ──────
# 1player = "invalid"    # SyntaxError: starts with a digit
# player-name = "invalid" # SyntaxError: hyphens are not allowed
# for = 10               # SyntaxError: 'for' is a keyword

# This single program intentionally demonstrates keywords from every category.
# Read each section's comment to see which keyword group is in play.

# ── VALUE KEYWORDS: True, False, None ────────────────────────────────────────
game_active = True          # True — one of only two boolean values
high_score_set = False      # False — the other boolean value
last_winner = None          # None — represents 'no value yet'

print("=== Game State ===")
print(f"Game active:      {game_active}")
print(f"High score set:   {high_score_set}")
print(f"Last winner:      {last_winner}")

# ── CONTROL FLOW KEYWORDS: if, elif, else, for, while, break, continue, pass ──
player_lives = 3
current_level = 7

print("\n=== Difficulty Check ===")
if current_level >= 10:           # 'if' starts a condition block
    print("Expert territory!")
elif current_level >= 5:          # 'elif' = else-if — checked only if 'if' was False
    print("Mid-game — stay sharp.")  # this branch runs: level is 7
else:                              # 'else' catches everything else
    print("Just getting started.")

print("\n=== Inventory Scan ===")
inventory = ["sword", "shield", "potion", "torch", "map"]
for item in inventory:            # 'for' loops over each item in the list
    if item == "potion":          # skip the potion — keep it for emergencies
        continue                  # 'continue' jumps straight to the next iteration
    if item == "torch":
        break                     # 'break' exits the loop entirely when torch found
    print(f"  Equipped: {item}")

print("\n=== Lives Countdown ===")
while player_lives > 0:           # 'while' keeps looping as long as condition is True
    print(f"  Lives remaining: {player_lives}")
    player_lives -= 1             # subtract 1 each iteration to avoid infinite loop

# ── FUNCTION KEYWORDS: def, return, lambda ────────────────────────────────────
print("\n=== Scoring Functions ===")

def calculate_bonus(base_score, multiplier):   # 'def' defines a reusable function
    """Returns the final score after applying the bonus multiplier."""
    bonus_score = base_score * multiplier
    return bonus_score                         # 'return' sends the result back to caller

double_points = lambda score: score * 2       # 'lambda' — a one-liner function

final_score = calculate_bonus(150, 3)
print(f"Bonus score:   {final_score}")
print(f"Doubled score: {double_points(final_score)}")

# ── ERROR-HANDLING KEYWORDS: try, except, finally, raise ──────────────────────
print("\n=== Safe Division ===")

def safe_divide(total_points, number_of_players):
    try:                                       # 'try' — attempt this risky operation
        result = total_points / number_of_players
    except ZeroDivisionError:                  # 'except' — caught! handle the error
        print("  Can't divide by zero players — game not started yet.")
        return None
    finally:                                   # 'finally' — runs whether error or not
        print("  Division attempt complete.")
    return result

print(safe_divide(900, 3))   # normal case
print(safe_divide(900, 0))   # triggers ZeroDivisionError

# ── LOGICAL / MEMBERSHIP KEYWORDS: and, or, not, in, is ──────────────────────
print("\n=== Win Condition ===")
has_key = True
boss_defeated = True

if has_key and boss_defeated:   # 'and' — both must be True
    print("You win! Both conditions met.")

print(f"'map' in inventory: {'map' in inventory}")    # 'in' checks membership
print(f"last_winner is None: {last_winner is None}")  # 'is' checks identity