Collections

Sets

Unique Collections

You're tracking which pages a user has visited. You don't want duplicates, and you need fast "have they seen this?" checks. Sets automatically ignore duplicates and provide O(1) membership testing with in.

Collect unique tags

Add items to a set - duplicates are ignored.

unique_tags.py
Replay: real traced execution (multi-file project)
def main():
    # Create a set of tags for a blog post
    tags = set()

    print("=== Adding Blog Tags ===")

    # Add tags
    tags.add("python")
    print(f"Added 'python': {tags}")

    tags.add("programming")
    print(f"Added 'programming': {tags}")

    tags.add("tutorial")
    print(f"Added 'tutorial': {tags}")

    # Try to add duplicate
    tags.add("python")
    print(f"Added 'python' again: {tags}")
    print("Duplicate ignored!")

    # Add more tags
    tags.add("beginner")
    tags.add("coding")
    tags.add("python")
    print(f"\nWith more tags: {tags}")

    print("\n=== Blog Post Tags ===")
    print(f"Total unique tags: {len(tags)}")
    print(f"Tags: {tags}")

    # Display as hashtags
    print("\n=== Formatted ===")
    hashtags = " ".join(f"#{tag}" for tag in tags)
    print(hashtags)

    # Alternative creation with literal
    print("\n=== Set Literal ===")
    colors = {"red", "green", "blue"}  # Note: {} is empty dict, not set!
    print(f"Colors: {colors}")

main()

  1. main()

    43main()44#@help create

  2. tags ← set(), hashtags ← #beginner #python #tutorial #programming #coding

    1#@var=default,moreTags2def main():3    # Create a set of tags for a blog post  #?create4    tags→ set() = set()5    6    print("=== Adding Blog Tags ===")7    8    # Add tags  #?add9    tags→ {'python'}.add("python")10    print(f"Added 'python': {tags{'python'}}")11    12    tags→ {'python', 'programming'}.add("programming")13    print(f"Added 'programming': {tags{'python', 'programming'}}")14    15    tags→ {'python', 'tutorial', 'programming'}.add("tutorial")16    print(f"Added 'tutorial': {tags{'python', 'tutorial', 'programming'}}")17    18    # Try to add duplicate  #?duplicate19    tags{'python', 'tutorial', 'programming'}.add("python")20    print(f"Added 'python' again: {tags{'python', 'tutorial', 'programming'}}")21    print("Duplicate ignored!")22    23    # Add more tags24    tags→ {'python', 'beginner', 'tutorial', 'programming'}.add("beginner")  #@var=_,!25    tags→ {'beginner', 'python', 'tutorial', 'programming', 'coding'}.add("coding")    #@var=_,!26    tags{'beginner', 'python', 'tutorial', 'programming', 'coding'}.add("python")    #@var=_,!  # Still ignored27    print(f"\nWith more tags: {tags{'beginner', 'python', 'tutorial', 'programming', 'coding'}}")  #@var=_,!28    29    print("\n=== Blog Post Tags ===")30    print(f"Total unique tags: {len(tags{'beginner', 'python', 'tutorial', 'programming', 'coding'})}")31    print(f"Tags: {tags{'beginner', 'python', 'tutorial', 'programming', 'coding'}}")32    33    # Display as hashtags34    print("\n=== Formatted ===")35    hashtags→ #beginner #python #tutorial #programming #coding = " ".join(f"#{tag}" for tag in tags{'beginner', 'python', 'tutorial', 'programming', 'coding'})36    print(hashtags#beginner #python #tutorial #programming #coding)37    38    # Alternative creation with literal  #?literal39    print("\n=== Set Literal ===")40    colors→ {'green', 'red', 'blue'} = {"red", "green", "blue"}  # Note: {} is empty dict, not set!41    print(f"Colors: {colors{'green', 'red', 'blue'}}")
    output=== Adding Blog Tags ===
Added 'python': {'python'}
Added 'programming': {'python', 'programming'}
Added 'tutorial': {'python', 'tutorial', 'programming'}
Added 'python' again: {'python', 'tutorial', 'programming'}
Duplicate ignored!

With more tags: {'beginner', 'python', 'tutorial', 'programming', 'coding'}

=== Blog Post Tags ===
Total unique tags: 5
Tags: {'beginner', 'python', 'tutorial', 'programming', 'coding'}

=== Formatted ===
#beginner #python #tutorial #programming #coding

=== Set Literal ===
Colors: {'green', 'red', 'blue'}

  3. main()

    43main()44#@help create

Use add() to add elements. Duplicates are silently ignored.

set Unordered collection of unique elements. O(1) add, remove, `in` check.

Fast membership check

Test if an item is in the set.

current_user
check_membership.py
Replay: real traced execution (multi-file project)
def main():
    # Create an allowlist of premium users
    premium_users = {"alice", "bob", "carol", "david"}

    print("=== Premium Membership ===")
    print(f"Premium users: {premium_users}")

    # Check if user has access
    current_user = "bob"

    print("\n=== Access Check ===")
    print(f"User: {current_user}")

    if current_user in premium_users:
        print("✓ Premium access granted!")
        print("Welcome to exclusive content.")
    else:
        print("✗ Not a premium user.")
        print("Upgrade to access premium features!")

    # Blocked users check
    blocked_users = {"spammer", "troll", "eve"}

    print("\n=== Security Check ===")
    if current_user in blocked_users:
        print("⛔ User is blocked!")
    else:
        print("✓ User is not blocked.")

    # Check multiple users
    print("\n=== Batch Check ===")
    users_to_check = ["alice", "eve", "bob", "frank"]

    for user in users_to_check:
        status = "Premium" if user in premium_users else "Regular"
        print(f"{user}: {status}")

    # Speed comparison note
    print("\n=== Performance Note ===")
    print("'in' check for set: O(1) - instant!")
    print("'in' check for list: O(n) - slower")

main()

def main():
    # Create an allowlist of premium users
    premium_users = {"alice", "bob", "carol", "david"}

    print("=== Premium Membership ===")
    print(f"Premium users: {premium_users}")

    # Check if user has access
    current_user = "eve"

    print("\n=== Access Check ===")
    print(f"User: {current_user}")

    if current_user in premium_users:
        print("✓ Premium access granted!")
        print("Welcome to exclusive content.")
    else:
        print("✗ Not a premium user.")
        print("Upgrade to access premium features!")

    # Blocked users check
    blocked_users = {"spammer", "troll", "eve"}

    print("\n=== Security Check ===")
    if current_user in blocked_users:
        print("⛔ User is blocked!")
    else:
        print("✓ User is not blocked.")

    # Check multiple users
    print("\n=== Batch Check ===")
    users_to_check = ["alice", "eve", "bob", "frank"]

    for user in users_to_check:
        status = "Premium" if user in premium_users else "Regular"
        print(f"{user}: {status}")

    # Speed comparison note
    print("\n=== Performance Note ===")
    print("'in' check for set: O(1) - instant!")
    print("'in' check for list: O(n) - slower")

main()

  1. main()

    44main()45#@help allowlist

  2. premium_users ← {'carol', 'alice', 'bob', 'david'}, current_user ← bob

    1#@var=default,blocked2def main():3    # Create an allowlist of premium users  #?allowlist4    premium_users→ {'carol', 'alice', 'bob', 'david'} = {"alice", "bob", "carol", "david"}5    6    print("=== Premium Membership ===")7    print(f"Premium users: {premium_users{'carol', 'alice', 'bob', 'david'}}")8    9    # Check if user has access  #?in10    current_user→ bob = "bob"  #@var=_,eve11    12    print("\n=== Access Check ===")13    print(f"User: {current_userbob}")
    output=== Premium Membership ===
Premium users: {'carol', 'alice', 'bob', 'david'}

=== Access Check ===
User: bob

  3. if current_user in premium_users:

    15if current_userbob in premium_users{'carol', 'alice', 'bob', 'david'}:16    print("✓ Premium access granted!")17    print("Welcome to exclusive content.")18else:
    output✓ Premium access granted!
Welcome to exclusive content.

  4. blocked_users ← {'troll', 'spammer', 'eve'}

    22# Blocked users check  #@var=!,_23blocked_users→ {'troll', 'spammer', 'eve'} = {"spammer", "troll", "eve"}2425print("\n=== Security Check ===")  #@var=!,_26if current_user in blocked_users:  #@var=!,_
    output
=== Security Check ===

  5. #@var=!,_ else: #@var=!,_

    26if current_user in blocked_users:  #@var=!,_27    print("⛔ User is blocked!")  #@var=!,_28else:  #@var=!,_29    print("✓ User is not blocked.")  #@var=!,_
    output✓ User is not blocked.

  6. users_to_check ← ['alice', 'eve', 'bob', 'frank']

    31# Check multiple users32print("\n=== Batch Check ===")33users_to_check→ ['alice', 'eve', 'bob', 'frank'] = ["alice", "eve", "bob", "frank"]
    output
=== Batch Check ===

  7. status ← Premium

    pass 1 of 4
    35for useralice in users_to_check['alice', 'eve', 'bob', 'frank']:36    status→ Premium = "Premium" if useralice in premium_users{'carol', 'alice', 'bob', 'david'} else "Regular"37    print(f"{useralice}: {statusPremium}")
    outputalice: Premium
    All 4 passes — pass 1 is the card above
    passuserstatus
    1alicePremium
    2eveRegular
    3bobPremium
    4frankRegular

  8. print(" === Performance Note ===")

    39# Speed comparison note  #?speed40print("\n=== Performance Note ===")41print("'in' check for set: O(1) - instant!")42print("'in' check for list: O(n) - slower")
    output
=== Performance Note ===
'in' check for set: O(1) - instant!
'in' check for list: O(n) - slower

  9. main()

    44main()45#@help allowlist

  1. main()

    43main()

  2. premium_users ← {'david', 'bob', 'alice', 'carol'}, current_user ← eve

    1def main():2    # Create an allowlist of premium users3    premium_users→ {'david', 'bob', 'alice', 'carol'} = {"alice", "bob", "carol", "david"}4    5    print("=== Premium Membership ===")6    print(f"Premium users: {premium_users{'david', 'bob', 'alice', 'carol'}}")7    8    # Check if user has access9    current_user→ eve = "eve"10    11    print("\n=== Access Check ===")12    print(f"User: {current_usereve}")
    output=== Premium Membership ===
Premium users: {'david', 'bob', 'alice', 'carol'}

=== Access Check ===
User: eve

  3. else:

    15    print("✓ Premium access granted!")16    print("Welcome to exclusive content.")17else:18    print("✗ Not a premium user.")19    print("Upgrade to access premium features!")
    output✗ Not a premium user.
Upgrade to access premium features!

  4. blocked_users ← {'troll', 'eve', 'spammer'}

    21# Blocked users check22blocked_users→ {'troll', 'eve', 'spammer'} = {"spammer", "troll", "eve"}2324print("\n=== Security Check ===")25if current_user in blocked_users:
    output
=== Security Check ===

  5. if current_user in blocked_users:

    24print("\n=== Security Check ===")25if current_usereve in blocked_users{'troll', 'eve', 'spammer'}:26    print("⛔ User is blocked!")27else:
    output⛔ User is blocked!

  6. users_to_check ← ['alice', 'eve', 'bob', 'frank']

    30# Check multiple users31print("\n=== Batch Check ===")32users_to_check→ ['alice', 'eve', 'bob', 'frank'] = ["alice", "eve", "bob", "frank"]
    output
=== Batch Check ===

  7. status ← Premium

    pass 1 of 4
    34for useralice in users_to_check['alice', 'eve', 'bob', 'frank']:35    status→ Premium = "Premium" if useralice in premium_users{'david', 'bob', 'alice', 'carol'} else "Regular"36    print(f"{useralice}: {statusPremium}")
    outputalice: Premium
    All 4 passes — pass 1 is the card above
    passuserstatus
    1alicePremium
    2eveRegular
    3bobPremium
    4frankRegular

  8. print(" === Performance Note ===")

    38# Speed comparison note39print("\n=== Performance Note ===")40print("'in' check for set: O(1) - instant!")41print("'in' check for list: O(n) - slower")
    output
=== Performance Note ===
'in' check for set: O(1) - instant!
'in' check for list: O(n) - slower

  9. main()

    43main()

item in s is O(1) - much faster than list's O(n) search.

Remove an item

Remove elements with remove() or discard().

remove_item.py
Replay: real traced execution (multi-file project)
def main():
    cart = {"Laptop", "Mouse", "Keyboard", "Monitor", "Headphones"}

    print("=== Shopping Cart ===")
    print(f"Items: {cart}")
    print(f"Count: {len(cart)}")

    # remove() vs discard()
    print("\n=== Removing Items ===")

    # remove() raises KeyError if not found
    to_remove = "Mouse"
    cart.remove(to_remove)
    print(f"Removed '{to_remove}': {cart}")

    # discard() does nothing if not found (no error!)
    not_in_cart = "Tablet"
    cart.discard(not_in_cart)
    print(f"Discard '{not_in_cart}' (not in cart): no error!")
    print(f"Cart unchanged: {cart}")

    # This would cause error:
    # cart.remove("Tablet")  # KeyError!

    # Remove more items
    cart.discard("Keyboard")
    cart.discard("Monitor")
    print(f"\nAfter more removals: {cart}")

    # Clear all
    print("\n=== Clearing Cart ===")
    print(f"Before clear: {len(cart)} items")
    cart.clear()
    print(f"After clear: {len(cart)} items")
    print(f"Cart is empty: {len(cart) == 0}")

    # pop() - remove arbitrary item
    demo_set = {1, 2, 3, 4, 5}
    print("\n=== pop() Demo ===")
    print(f"Set: {demo_set}")
    popped = demo_set.pop()  # Removes and returns ONE item
    print(f"Popped: {popped}")
    print(f"Remaining: {demo_set}")

main()

  1. main()

    46main()47#@help remove

  2. cart ← {'Mouse', 'Headphones', 'Monitor', 'Laptop', 'Keyboard'}

    1#@var=default,clearAll2def main():3    cart→ {'Mouse', 'Headphones', 'Monitor', 'Laptop', 'Keyboard'} = {"Laptop", "Mouse", "Keyboard", "Monitor", "Headphones"}4    5    print("=== Shopping Cart ===")6    print(f"Items: {cart{'Mouse', 'Headphones', 'Monitor', 'Laptop', 'Keyboard'}}")7    print(f"Count: {len(cart{'Mouse', 'Headphones', 'Monitor', 'Laptop', 'Keyboard'})}")8    9    # remove() vs discard()  #?remove10    print("\n=== Removing Items ===")11    12    # remove() raises KeyError if not found13    to_remove→ Mouse = "Mouse"14    cart→ {'Headphones', 'Monitor', 'Laptop', 'Keyboard'}.remove(to_removeMouse)15    print(f"Removed '{to_removeMouse}': {cart{'Headphones', 'Monitor', 'Laptop', 'Keyboard'}}")16    17    # discard() does nothing if not found (no error!)  #?discard18    not_in_cart→ Tablet = "Tablet"19    cart{'Headphones', 'Monitor', 'Laptop', 'Keyboard'}.discard(not_in_cartTablet)20    print(f"Discard '{not_in_cartTablet}' (not in cart): no error!")21    print(f"Cart unchanged: {cart{'Headphones', 'Monitor', 'Laptop', 'Keyboard'}}")22    23    # This would cause error:24    # cart.remove("Tablet")  # KeyError!25    26    # Remove more items27    cart→ {'Headphones', 'Monitor', 'Laptop'}.discard("Keyboard")  #@var=_,!28    cart→ {'Headphones', 'Laptop'}.discard("Monitor")   #@var=_,!29    print(f"\nAfter more removals: {cart{'Headphones', 'Laptop'}}")  #@var=_,!30    31    # Clear all  #?clear  #@var=!,_32    print("\n=== Clearing Cart ===")  #@var=!,_33    print(f"Before clear: {len(cart{'Headphones', 'Laptop'})} items")  #@var=!,_34    cart→ set().clear()  #@var=!,_35    print(f"After clear: {len(cartset())} items")  #@var=!,_36    print(f"Cart is empty: {len(cartset()) == 0}")  #@var=!,_37    38    # pop() - remove arbitrary item  #?pop39    demo_set→ {1, 2, 3, 4, 5} = {1, 2, 3, 4, 5}40    print("\n=== pop() Demo ===")41    print(f"Set: {demo_set{1, 2, 3, 4, 5}}")42    popped→ 1 = demo_set→ {2, 3, 4, 5}.pop()  # Removes and returns ONE item43    print(f"Popped: {popped1}")44    print(f"Remaining: {demo_set{2, 3, 4, 5}}")
    output=== Shopping Cart ===
Items: {'Mouse', 'Headphones', 'Monitor', 'Laptop', 'Keyboard'}
Count: 5

=== Removing Items ===
Removed 'Mouse': {'Headphones', 'Monitor', 'Laptop', 'Keyboard'}
Discard 'Tablet' (not in cart): no error!
Cart unchanged: {'Headphones', 'Monitor', 'Laptop', 'Keyboard'}

After more removals: {'Headphones', 'Laptop'}

=== Clearing Cart ===
Before clear: 2 items
After clear: 0 items
Cart is empty: True

=== pop() Demo ===
Set: {1, 2, 3, 4, 5}
Popped: 1
Remaining: {2, 3, 4, 5}

  3. main()

    46main()47#@help remove

remove() raises KeyError if missing. discard() doesn't.

discard Safe remove: `s.discard(item)` - no error if item not present.

Combine two sets

Create a union of two sets.

set_union.py
Replay: real traced execution (multi-file project)
def main():
    # Two teams' skill sets
    team_a = {"Java", "Python", "SQL", "Git"}
    team_b = {"JavaScript", "Python", "CSS", "Git"}

    print("=== Team Skills ===")
    print(f"Team A: {team_a}")
    print(f"Team B: {team_b}")

    # Union: All skills combined
    all_skills = team_a | team_b  # Or: team_a.union(team_b)

    print("\n=== Union (All Skills) ===")
    print(f"Combined: {all_skills}")
    print("(Duplicates automatically removed)")

    # Alternative syntax
    all_skills_method = team_a.union(team_b)
    print(f"Using .union(): {all_skills_method}")

    # Count unique skills
    print("\n=== Statistics ===")
    print(f"Team A skills: {len(team_a)}")
    print(f"Team B skills: {len(team_b)}")
    print(f"Unique combined: {len(all_skills)}")
    overlap = len(team_a) + len(team_b) - len(all_skills)
    print(f"Overlap count: {overlap}")

    # Practical example: Merge user permissions
    print("\n=== Permission Merge ===")

    basic_perms = {"read", "comment"}
    editor_perms = {"read", "write", "edit", "comment"}
    admin_perms = {"read", "write", "edit", "delete", "manage"}

    # Build editor permissions (basic + editor)
    full_editor_perms = basic_perms | editor_perms
    print(f"Editor has: {full_editor_perms}")

    # Build admin permissions (all combined)
    full_admin_perms = basic_perms | editor_perms | admin_perms
    print(f"Admin has: {full_admin_perms}")

    # Update in place with |=
    user_perms = {"read"}
    user_perms |= {"comment", "vote"}  # Add more
    print(f"\nUser perms after |=: {user_perms}")

main()

  1. main()

    49main()50#@help union

  2. team_a ← {'Python', 'Java', 'Git', 'SQL'}, team_b ← {'JavaScript', 'CSS', 'Python', 'Git'}

    1def main():2    # Two teams' skill sets3    team_a→ {'Python', 'Java', 'Git', 'SQL'} = {"Java", "Python", "SQL", "Git"}4    team_b→ {'JavaScript', 'CSS', 'Python', 'Git'} = {"JavaScript", "Python", "CSS", "Git"}5    6    print("=== Team Skills ===")7    print(f"Team A: {team_a{'Python', 'Java', 'Git', 'SQL'}}")8    print(f"Team B: {team_b{'JavaScript', 'CSS', 'Python', 'Git'}}")9    10    # Union: All skills combined  #?union11    all_skills→ {'SQL', 'CSS', 'Java', 'Python', 'JavaScript', 'Git'} = team_a{'Python', 'Java', 'Git', 'SQL'} | team_b{'JavaScript', 'CSS', 'Python', 'Git'}  # Or: team_a.union(team_b)12    13    print("\n=== Union (All Skills) ===")14    print(f"Combined: {all_skills{'SQL', 'CSS', 'Java', 'Python', 'JavaScript', 'Git'}}")15    print("(Duplicates automatically removed)")16    17    # Alternative syntax18    all_skills_method→ {'SQL', 'CSS', 'Java', 'Python', 'JavaScript', 'Git'} = team_a{'Python', 'Java', 'Git', 'SQL'}.union(team_b{'JavaScript', 'CSS', 'Python', 'Git'})19    print(f"Using .union(): {all_skills_method{'SQL', 'CSS', 'Java', 'Python', 'JavaScript', 'Git'}}")20    21    # Count unique skills22    print("\n=== Statistics ===")23    print(f"Team A skills: {len(team_a{'Python', 'Java', 'Git', 'SQL'})}")24    print(f"Team B skills: {len(team_b{'JavaScript', 'CSS', 'Python', 'Git'})}")25    print(f"Unique combined: {len(all_skills{'SQL', 'CSS', 'Java', 'Python', 'JavaScript', 'Git'})}")26    overlap→ 2 = len(team_a{'Python', 'Java', 'Git', 'SQL'}) + len(team_b{'JavaScript', 'CSS', 'Python', 'Git'}) - len(all_skills{'SQL', 'CSS', 'Java', 'Python', 'JavaScript', 'Git'})27    print(f"Overlap count: {overlap2}")28    29    # Practical example: Merge user permissions  #?perms30    print("\n=== Permission Merge ===")31    32    basic_perms→ {'comment', 'read'} = {"read", "comment"}33    editor_perms→ {'write', 'edit', 'comment', 'read'} = {"read", "write", "edit", "comment"}34    admin_perms→ {'edit', 'write', 'delete', 'manage', 'read'} = {"read", "write", "edit", "delete", "manage"}35    36    # Build editor permissions (basic + editor)37    full_editor_perms→ {'edit', 'comment', 'write', 'read'} = basic_perms{'comment', 'read'} | editor_perms{'write', 'edit', 'comment', 'read'}38    print(f"Editor has: {full_editor_perms{'edit', 'comment', 'write', 'read'}}")39    40    # Build admin permissions (all combined)41    full_admin_perms→ {'comment', 'write', 'delete', 'manage', 'read', 'edit'} = basic_perms{'comment', 'read'} | editor_perms{'write', 'edit', 'comment', 'read'} | admin_perms{'edit', 'write', 'delete', 'manage', 'read'}42    print(f"Admin has: {full_admin_perms{'comment', 'write', 'delete', 'manage', 'read', 'edit'}}")43    44    # Update in place with |=  #?inplace45    user_perms→ {'read'} = {"read"}46    user_perms→ {'vote', 'comment', 'read'} |= {"comment", "vote"}  # Add more47    print(f"\nUser perms after |=: {user_perms{'vote', 'comment', 'read'}}")
    output=== Team Skills ===
Team A: {'Python', 'Java', 'Git', 'SQL'}
Team B: {'JavaScript', 'CSS', 'Python', 'Git'}

=== Union (All Skills) ===
Combined: {'SQL', 'CSS', 'Java', 'Python', 'JavaScript', 'Git'}
(Duplicates automatically removed)
Using .union(): {'SQL', 'CSS', 'Java', 'Python', 'JavaScript', 'Git'}

=== Statistics ===
Team A skills: 4
Team B skills: 4
Unique combined: 6
Overlap count: 2

=== Permission Merge ===
Editor has: {'edit', 'comment', 'write', 'read'}
Admin has: {'comment', 'write', 'delete', 'manage', 'read', 'edit'}

User perms after |=: {'vote', 'comment', 'read'}

  3. main()

    49main()50#@help union

Use | operator or union() method. Result has all unique elements.

union Combine sets: `s1 | s2` or `s1.union(s2)`. All unique elements.

Remove duplicates from list

Convert a list to set to eliminate duplicates.

remove_duplicates.py
Replay: real traced execution (multi-file project)
def main():
    # List with duplicate entries
    emails = [
        "alice@example.com",
        "bob@example.com",
        "alice@example.com",  # duplicate
        "carol@example.com",
        "bob@example.com",    # duplicate
        "david@example.com",
        "alice@example.com"   # duplicate
    ]

    print("=== Email List (with duplicates) ===")
    print(f"Emails: {emails}")
    print(f"Count: {len(emails)}")

    # Remove duplicates using set
    unique_emails = set(emails)

    print("\n=== After Removing Duplicates ===")
    print(f"Unique: {unique_emails}")
    print(f"Count: {len(unique_emails)}")
    print(f"Removed {len(emails) - len(unique_emails)} duplicates")

    # Convert back to list if needed
    cleaned_list = list(unique_emails)
    print(f"\nAs list: {cleaned_list}")

    # PRESERVE ORDER: use dict.fromkeys()
    print("\n=== Preserving Order ===")
    ordered_unique = list(dict.fromkeys(emails))
    print(f"Order preserved: {ordered_unique}")
    print("(dict maintains insertion order in Python 3.7+)")

    # One-liner pattern
    print("\n=== One-liner Dedup ===")
    nums = [3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5]
    unique_nums = list(set(nums))
    print(f"Original: {nums}")
    print(f"Unique (order lost): {unique_nums}")

    # Duplicate analysis
    print("\n=== Duplicate Analysis ===")
    from collections import Counter
    counts = Counter(emails)

    for email, count in counts.items():
        if count > 1:
            print(f"{email} appeared {count} times")

main()

  1. main()

    52main()53#@help dedup

  2. emails ← ['alice@example.com', 'bob@example.com', 'alice@example.com', 'carol@example.com', 'bob@example.com', 'david@example.com', 'alice@example.com']

    1#@var=default,preserveOrder2def main():3    # List with duplicate entries4    emails→ ['alice@example.com', 'bob@example.com', 'alice@example.com', 'carol@example.com', 'bob@example.com', 'david@example.com', 'alice@example.com'] = [5        "alice@example.com",6        "bob@example.com",7        "alice@example.com",  # duplicate8        "carol@example.com",9        "bob@example.com",    # duplicate10        "david@example.com",11        "alice@example.com"   # duplicate12    ]13    14    print("=== Email List (with duplicates) ===")15    print(f"Emails: {emails['alice@example.com', 'bob@example.com', 'alice@example.com', 'carol@example.com', 'bob@example.com', 'david@example.com', 'alice@example.com']}")16    print(f"Count: {len(emails['alice@example.com', 'bob@example.com', 'alice@example.com', 'carol@example.com', 'bob@example.com', 'david@example.com', 'alice@example.com'])}")17    18    # Remove duplicates using set  #?dedup19    unique_emails→ {'alice@example.com', 'bob@example.com', 'carol@example.com', 'david@example.com'} = set(emails['alice@example.com', 'bob@example.com', 'alice@example.com', 'carol@example.com', 'bob@example.com', 'david@example.com', 'alice@example.com'])20    21    print("\n=== After Removing Duplicates ===")22    print(f"Unique: {unique_emails{'alice@example.com', 'bob@example.com', 'carol@example.com', 'david@example.com'}}")23    print(f"Count: {len(unique_emails{'alice@example.com', 'bob@example.com', 'carol@example.com', 'david@example.com'})}")24    print(f"Removed {len(emails['alice@example.com', 'bob@example.com', 'alice@example.com', 'carol@example.com', 'bob@example.com', 'david@example.com', 'alice@example.com']) - len(unique_emails{'alice@example.com', 'bob@example.com', 'carol@example.com', 'david@example.com'})} duplicates")25    26    # Convert back to list if needed  #?convert27    cleaned_list→ ['alice@example.com', 'bob@example.com', 'carol@example.com', 'david@example.com'] = list(unique_emails{'alice@example.com', 'bob@example.com', 'carol@example.com', 'david@example.com'})28    print(f"\nAs list: {cleaned_list['alice@example.com', 'bob@example.com', 'carol@example.com', 'david@example.com']}")29    30    # PRESERVE ORDER: use dict.fromkeys()  #?order  #@var=!,_31    print("\n=== Preserving Order ===")32    ordered_unique→ ['alice@example.com', 'bob@example.com', 'carol@example.com', 'david@example.com'] = list(dict.fromkeys(emails['alice@example.com', 'bob@example.com', 'alice@example.com', 'carol@example.com', 'bob@example.com', 'david@example.com', 'alice@example.com']))33    print(f"Order preserved: {ordered_unique['alice@example.com', 'bob@example.com', 'carol@example.com', 'david@example.com']}")34    print("(dict maintains insertion order in Python 3.7+)")35    36    # One-liner pattern37    print("\n=== One-liner Dedup ===")38    nums→ [3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5] = [3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5]39    unique_nums→ [1, 2, 3, 4, 5, 6, 9] = list(set(nums[3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5]))40    print(f"Original: {nums[3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5]}")41    print(f"Unique (order lost): {unique_nums[1, 2, 3, 4, 5, 6, 9]}")42    43    # Duplicate analysis  #?analysis44    print("\n=== Duplicate Analysis ===")45    from collections import Counter46    counts→ Counter({'alice@example.com': 3, 'bob@example.com': 2, 'carol@example.com': 1, 'david@example.com': 1}) = Counter(emails['alice@example.com', 'bob@example.com', 'alice@example.com', 'carol@example.com', 'bob@example.com', 'david@example.com', 'alice@example.com'])
    output=== Email List (with duplicates) ===
Emails: ['alice@example.com', 'bob@example.com', 'alice@example.com', 'carol@example.com', 'bob@example.com', 'david@example.com', 'alice@example.com']
Count: 7

=== After Removing Duplicates ===
Unique: {'alice@example.com', 'bob@example.com', 'carol@example.com', 'david@example.com'}
Count: 4
Removed 3 duplicates

As list: ['alice@example.com', 'bob@example.com', 'carol@example.com', 'david@example.com']

=== Preserving Order ===
Order preserved: ['alice@example.com', 'bob@example.com', 'carol@example.com', 'david@example.com']
(dict maintains insertion order in Python 3.7+)

=== One-liner Dedup ===
Original: [3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5]
Unique (order lost): [1, 2, 3, 4, 5, 6, 9]

=== Duplicate Analysis ===

  3. for email, count in counts.items():

    pass 1 of 4
    48for emailalice@example.com, count3 in countsCounter({'alice@example.com': 3, 'bob@example.com': 2, 'carol@example.com': 1, 'david@example.com': 1}).items():49    if count > 1:50        print(f"{email} appeared {count} times")
    All 4 passes — pass 1 is the card above
    passemailcount
    1alice@example.com3
    2bob@example.com2
    3carol@example.com1
    4david@example.com1

  4. if count > 1:

    pass 1 of 2
    48for email, count in counts.items():49    if count3 > 1:50        print(f"{emailalice@example.com} appeared {count3} times")
    outputalice@example.com appeared 3 times

  5. if count > 1:

    pass 2 of 2
    48for email, count in counts.items():49    if count2 > 1:50        print(f"{emailbob@example.com} appeared {count2} times")
    outputbob@example.com appeared 2 times

  6. main()

    52main()53#@help dedup

set(list) creates a set, automatically removing duplicates.

Exercise: set_operations.py

Explore intersection, difference, and symmetric difference