String Methods - Python Programming

Text data often needs cleaning, parsing, and transformation before use. Python strings have built-in methods for splitting CSV data, removing whitespace, validating user input, and searching for patterns. These methods form the foundation of text processing in Python.

Python strings have many built-in methods for common text operations. These methods create new strings (strings are immutable) and cover splitting, joining, searching, replacing, and validation.

Splitting and Joining

                            text
                            text = "apple,banana,cherry,date" (default)text = "red,green,blue"text = "cat,dog,bird"
                        

                            split_join.py
                        
Replay: real traced execution (multi-file project)

                            # Split and join


# Split string into list
text = "apple,banana,cherry,date"
fruits = text.split(",")
print("Split by comma:", fruits)

sentence = "The quick brown fox"
words = sentence.split()  # Default: split by whitespace
print("Split by space:", words)

multiline = "Line1\nLine2\nLine3"
lines = multiline.split("\n")
print("Split by newline:", lines)

# Split with limit
data = "a:b:c:d:e"
parts = data.split(":", 2)  # Split into max 3 parts
print("Limited split:", parts)

# Join list into string
# Join with separator
fruits_joined = ", ".join(fruits)
print("\nJoined:", fruits_joined)

# Join with different separator
path = "/".join(["home", "user", "documents", "file.txt"])
print("Path:", path)

# Join words with space
words_joined = " ".join(words)
print("Sentence:", words_joined)

# Join empty separator
chars = ["H", "e", "l", "l", "o"]
word = "".join(chars)
print("Word:", word)

# Split and join


# Split string into list
text = "red,green,blue"
fruits = text.split(",")
print("Split by comma:", fruits)

sentence = "The quick brown fox"
words = sentence.split()  # Default: split by whitespace
print("Split by space:", words)

multiline = "Line1\nLine2\nLine3"
lines = multiline.split("\n")
print("Split by newline:", lines)

# Split with limit
data = "a:b:c:d:e"
parts = data.split(":", 2)  # Split into max 3 parts
print("Limited split:", parts)

# Join list into string
# Join with separator
fruits_joined = ", ".join(fruits)
print("\nJoined:", fruits_joined)

# Join with different separator
path = "/".join(["home", "user", "documents", "file.txt"])
print("Path:", path)

# Join words with space
words_joined = " ".join(words)
print("Sentence:", words_joined)

# Join empty separator
chars = ["H", "e", "l", "l", "o"]
word = "".join(chars)
print("Word:", word)

# Split and join


# Split string into list
text = "cat,dog,bird"
fruits = text.split(",")
print("Split by comma:", fruits)

sentence = "The quick brown fox"
words = sentence.split()  # Default: split by whitespace
print("Split by space:", words)

multiline = "Line1\nLine2\nLine3"
lines = multiline.split("\n")
print("Split by newline:", lines)

# Split with limit
data = "a:b:c:d:e"
parts = data.split(":", 2)  # Split into max 3 parts
print("Limited split:", parts)

# Join list into string
# Join with separator
fruits_joined = ", ".join(fruits)
print("\nJoined:", fruits_joined)

# Join with different separator
path = "/".join(["home", "user", "documents", "file.txt"])
print("Path:", path)

# Join words with space
words_joined = " ".join(words)
print("Sentence:", words_joined)

# Join empty separator
chars = ["H", "e", "l", "l", "o"]
word = "".join(chars)
print("Word:", word)


                        
1text ← apple,banana,cherry,date, fruits ← ['apple', 'banana', 'cherry', 'date']⋯4# Split string into list5text→ apple,banana,cherry,date = "apple,banana,cherry,date"6#@text="red,green,blue", "cat,dog,bird"7fruits→ ['apple', 'banana', 'cherry', 'date'] = textapple,banana,cherry,date.split(",")8print("Split by comma:", fruits['apple', 'banana', 'cherry', 'date'])910sentence→ The quick brown fox = "The quick brown fox"11words→ ['The', 'quick', 'brown', 'fox'] = sentenceThe quick brown fox.split()  # Default: split by whitespace12print("Split by space:", words['The', 'quick', 'brown', 'fox'])1314multiline→ Line1
Line2
Line3 = "Line1\nLine2\nLine3"15lines→ ['Line1', 'Line2', 'Line3'] = multilineLine1
Line2
Line3.split("\n")16print("Split by newline:", lines['Line1', 'Line2', 'Line3'])1718# Split with limit19data→ a:b:c:d:e = "a:b:c:d:e"20parts→ ['a', 'b', 'c:d:e'] = dataa:b:c:d:e.split(":", 2)  # Split into max 3 parts21print("Limited split:", parts['a', 'b', 'c:d:e'])2223# Join list into string24# Join with separator25fruits_joined→ apple, banana, cherry, date = ", ".join(fruits['apple', 'banana', 'cherry', 'date'])26print("\nJoined:", fruits_joinedapple, banana, cherry, date)2728# Join with different separator29path→ home/user/documents/file.txt = "/".join(["home", "user", "documents", "file.txt"])30print("Path:", pathhome/user/documents/file.txt)3132# Join words with space33words_joined→ The quick brown fox = " ".join(words['The', 'quick', 'brown', 'fox'])34print("Sentence:", words_joinedThe quick brown fox)3536# Join empty separator37chars→ ['H', 'e', 'l', 'l', 'o'] = ["H", "e", "l", "l", "o"]38word→ Hello = "".join(chars['H', 'e', 'l', 'l', 'o'])39print("Word:", wordHello)⋯outputSplit by comma: ['apple', 'banana', 'cherry', 'date']
Split by space: ['The', 'quick', 'brown', 'fox']
Split by newline: ['Line1', 'Line2', 'Line3']
Limited split: ['a', 'b', 'c:d:e']

Joined: apple, banana, cherry, date
Path: home/user/documents/file.txt
Sentence: The quick brown fox
Word: Hello
1text ← red,green,blue, fruits ← ['red', 'green', 'blue'], sentence ← The quick brown fox⋯4# Split string into list5text→ red,green,blue = "red,green,blue"6fruits→ ['red', 'green', 'blue'] = textred,green,blue.split(",")7print("Split by comma:", fruits['red', 'green', 'blue'])89sentence→ The quick brown fox = "The quick brown fox"10words→ ['The', 'quick', 'brown', 'fox'] = sentenceThe quick brown fox.split()  # Default: split by whitespace11print("Split by space:", words['The', 'quick', 'brown', 'fox'])1213multiline→ Line1
Line2
Line3 = "Line1\nLine2\nLine3"14lines→ ['Line1', 'Line2', 'Line3'] = multilineLine1
Line2
Line3.split("\n")15print("Split by newline:", lines['Line1', 'Line2', 'Line3'])1617# Split with limit18data→ a:b:c:d:e = "a:b:c:d:e"19parts→ ['a', 'b', 'c:d:e'] = dataa:b:c:d:e.split(":", 2)  # Split into max 3 parts20print("Limited split:", parts['a', 'b', 'c:d:e'])2122# Join list into string23# Join with separator24fruits_joined→ red, green, blue = ", ".join(fruits['red', 'green', 'blue'])25print("\nJoined:", fruits_joinedred, green, blue)2627# Join with different separator28path→ home/user/documents/file.txt = "/".join(["home", "user", "documents", "file.txt"])29print("Path:", pathhome/user/documents/file.txt)3031# Join words with space32words_joined→ The quick brown fox = " ".join(words['The', 'quick', 'brown', 'fox'])33print("Sentence:", words_joinedThe quick brown fox)3435# Join empty separator36chars→ ['H', 'e', 'l', 'l', 'o'] = ["H", "e", "l", "l", "o"]37word→ Hello = "".join(chars['H', 'e', 'l', 'l', 'o'])38print("Word:", wordHello)⋯outputSplit by comma: ['red', 'green', 'blue']
Split by space: ['The', 'quick', 'brown', 'fox']
Split by newline: ['Line1', 'Line2', 'Line3']
Limited split: ['a', 'b', 'c:d:e']

Joined: red, green, blue
Path: home/user/documents/file.txt
Sentence: The quick brown fox
Word: Hello
1text ← cat,dog,bird, fruits ← ['cat', 'dog', 'bird'], sentence ← The quick brown fox⋯4# Split string into list5text→ cat,dog,bird = "cat,dog,bird"6fruits→ ['cat', 'dog', 'bird'] = textcat,dog,bird.split(",")7print("Split by comma:", fruits['cat', 'dog', 'bird'])89sentence→ The quick brown fox = "The quick brown fox"10words→ ['The', 'quick', 'brown', 'fox'] = sentenceThe quick brown fox.split()  # Default: split by whitespace11print("Split by space:", words['The', 'quick', 'brown', 'fox'])1213multiline→ Line1
Line2
Line3 = "Line1\nLine2\nLine3"14lines→ ['Line1', 'Line2', 'Line3'] = multilineLine1
Line2
Line3.split("\n")15print("Split by newline:", lines['Line1', 'Line2', 'Line3'])1617# Split with limit18data→ a:b:c:d:e = "a:b:c:d:e"19parts→ ['a', 'b', 'c:d:e'] = dataa:b:c:d:e.split(":", 2)  # Split into max 3 parts20print("Limited split:", parts['a', 'b', 'c:d:e'])2122# Join list into string23# Join with separator24fruits_joined→ cat, dog, bird = ", ".join(fruits['cat', 'dog', 'bird'])25print("\nJoined:", fruits_joinedcat, dog, bird)2627# Join with different separator28path→ home/user/documents/file.txt = "/".join(["home", "user", "documents", "file.txt"])29print("Path:", pathhome/user/documents/file.txt)3031# Join words with space32words_joined→ The quick brown fox = " ".join(words['The', 'quick', 'brown', 'fox'])33print("Sentence:", words_joinedThe quick brown fox)3435# Join empty separator36chars→ ['H', 'e', 'l', 'l', 'o'] = ["H", "e", "l", "l", "o"]37word→ Hello = "".join(chars['H', 'e', 'l', 'l', 'o'])38print("Word:", wordHello)⋯outputSplit by comma: ['cat', 'dog', 'bird']
Split by space: ['The', 'quick', 'brown', 'fox']
Split by newline: ['Line1', 'Line2', 'Line3']
Limited split: ['a', 'b', 'c:d:e']

Joined: cat, dog, bird
Path: home/user/documents/file.txt
Sentence: The quick brown fox
Word: Hello

split_join Breaking strings into lists and combining lists into strings

Trimming and Replacing

                            strip_replace.py
                        
Replay: real traced execution (multi-file project)

                            # Strip and replace


# Strip whitespace
text1 = "   Hello World   "
print("Original:", repr(text1))
print("strip():", repr(text1.strip()))
print("lstrip():", repr(text1.lstrip()))
print("rstrip():", repr(text1.rstrip()))

# Strip specific characters
text2 = "***Hello***"
print("\nStrip asterisks:", text2.strip("*"))

text3 = "...Hello..."
print("Strip dots:", text3.strip("."))

# Replace substring
sentence = "I love Java"
updated = sentence.replace("Java", "Python")
print("\nReplace:", updated)

# Replace all occurrences
text4 = "Hello Hello Hello"
replaced = text4.replace("Hello", "Hi")
print("Replace all:", replaced)

# Replace with count limit
text5 = "one one one one"
replaced_limit = text5.replace("one", "two", 2)
print("Replace 2:", replaced_limit)

# Remove substring (replace with empty)
text6 = "Hello, World!"
no_comma = text6.replace(",", "")
print("Remove comma:", no_comma)

# Case conversion
text = "Hello World"
print("\nUpper:", text.upper())
print("Lower:", text.lower())
print("Title:", text.title())
print("Capitalize:", text.capitalize())
print("Swapcase:", text.swapcase())


                        
1text1 ←    Hello World   , text2 ← ***Hello***, text3 ← ...Hello...⋯4# Strip whitespace5text1→    Hello World    = "   Hello World   "6print("Original:", repr(text1   Hello World   ))7print("strip():", repr(text1   Hello World   .strip()))8print("lstrip():", repr(text1   Hello World   .lstrip()))9print("rstrip():", repr(text1   Hello World   .rstrip()))1011# Strip specific characters12text2→ ***Hello*** = "***Hello***"13print("\nStrip asterisks:", text2***Hello***.strip("*"))1415text3→ ...Hello... = "...Hello..."16print("Strip dots:", text3...Hello....strip("."))1718# Replace substring19sentence→ I love Java = "I love Java"20updated→ I love Python = sentenceI love Java.replace("Java", "Python")21print("\nReplace:", updatedI love Python)2223# Replace all occurrences24text4→ Hello Hello Hello = "Hello Hello Hello"25replaced→ Hi Hi Hi = text4Hello Hello Hello.replace("Hello", "Hi")26print("Replace all:", replacedHi Hi Hi)2728# Replace with count limit29text5→ one one one one = "one one one one"30replaced_limit→ two two one one = text5one one one one.replace("one", "two", 2)31print("Replace 2:", replaced_limittwo two one one)3233# Remove substring (replace with empty)34text6→ Hello, World! = "Hello, World!"35no_comma→ Hello World! = text6Hello, World!.replace(",", "")36print("Remove comma:", no_commaHello World!)3738# Case conversion39text→ Hello World = "Hello World"40print("\nUpper:", textHello World.upper())41print("Lower:", textHello World.lower())42print("Title:", textHello World.title())43print("Capitalize:", textHello World.capitalize())44print("Swapcase:", textHello World.swapcase())⋯outputOriginal: '   Hello World   '
strip(): 'Hello World'
lstrip(): 'Hello World   '
rstrip(): '   Hello World'

Strip asterisks: Hello
Strip dots: Hello

Replace: I love Python
Replace all: Hi Hi Hi
Replace 2: two two one one
Remove comma: Hello World!

Upper: HELLO WORLD
Lower: hello world
Title: Hello World
Capitalize: Hello world
Swapcase: hELLO wORLD

strip_replace Removing whitespace and substituting substrings

Searching

                            find_index.py
                        
Replay: real traced execution (multi-file project)

                            # Find and index


# Find substring
text = "Hello World Hello"

# find() returns index or -1
pos1 = text.find("World")
print(f"find('World'): {pos1}")

pos2 = text.find("Python")
print(f"find('Python'): {pos2}")  # -1 not found

# Find from specific position
pos3 = text.find("Hello", 1)  # Start search at index 1
print(f"find('Hello', 1): {pos3}")  # Finds second occurrence

# rfind() searches from right
pos4 = text.rfind("Hello")
print(f"rfind('Hello'): {pos4}")  # Last occurrence

# Index (raises exception if not found)
try:
    pos5 = text.index("World")
    print(f"\nindex('World'): {pos5}")

    # This will raise ValueError
    pos6 = text.index("Python")
except ValueError as e:
    print(f"Error: {e}")

# Count occurrences
count1 = text.count("Hello")
print(f"\ncount('Hello'): {count1}")

count2 = text.count("o")
print(f"count('o'): {count2}")

count3 = text.count("xyz")
print(f"count('xyz'): {count3}")

# Practical: check if substring exists
email = "user@example.com"

if "@" in email and "." in email:
    print(f"\n'{email}' looks like an email")

# Using find
if email.find("@") != -1:
    at_pos = email.find("@")
    domain = email[at_pos + 1:]
    print(f"Domain: {domain}")


                        
1text ← Hello World Hello, pos1 ← 6, pos2 ← -1, pos3 ← 12, pos4 ← 12⋯4# Find substring5text→ Hello World Hello = "Hello World Hello"67# find() returns index or -18pos1→ 6 = textHello World Hello.find("World")9print(f"find('World'): {pos16}")1011pos2→ -1 = textHello World Hello.find("Python")12print(f"find('Python'): {pos2-1}")  # -1 not found1314# Find from specific position15pos3→ 12 = textHello World Hello.find("Hello", 1)  # Start search at index 116print(f"find('Hello', 1): {pos312}")  # Finds second occurrence1718# rfind() searches from right19pos4→ 12 = textHello World Hello.rfind("Hello")20print(f"rfind('Hello'): {pos412}")  # Last occurrence⋯outputfind('World'): 6
find('Python'): -1
find('Hello', 1): 12
rfind('Hello'): 12
2pos5 ← 6⋯22# Index (raises exception if not found)23try:24    pos5→ 6 = textHello World Hello.index("World")25    print(f"\nindex('World'): {pos56}")2627    # This will raise ValueError28    pos6 = textHello World Hello.index("Python")29except ValueError as e:⋯output
index('World'): 6
3except ValueError as e:⋯28    pos6 = text.index("Python")29except ValueError as e:30    print(f"Error: {esubstring not found}")⋯outputError: substring not found
4count1 ← 2, count2 ← 3, count3 ← 0, email ← user@example.com⋯32# Count occurrences33count1→ 2 = textHello World Hello.count("Hello")34print(f"\ncount('Hello'): {count12}")3536count2→ 3 = textHello World Hello.count("o")37print(f"count('o'): {count23}")3839count3→ 0 = textHello World Hello.count("xyz")40print(f"count('xyz'): {count30}")4142# Practical: check if substring exists43email→ user@example.com = "user@example.com"⋯output
count('Hello'): 2
count('o'): 3
count('xyz'): 0
5if "@" in email and "." in email:⋯45if "@" in emailuser@example.com and "." in email:46    print(f"\n'{emailuser@example.com}' looks like an email")⋯output
'user@example.com' looks like an email
6at_pos ← 4, domain ← example.com⋯48# Using find49if emailuser@example.com.find("@") != -1:50    at_pos→ 4 = emailuser@example.com.find("@")51    domain→ example.com = email[at_pos + 1:]example.com52    print(f"Domain: {domainexample.com}")⋯outputDomain: example.com

find_search Finding substrings and their positions in strings

Validation

                            validation.py
                        
Replay: real traced execution (multi-file project)

                            # String validation methods


# Digit checking
age_str = "25"
invalid_age = "25.5"

print(f"'{age_str}'.isdigit(): {age_str.isdigit()}")
print(f"'{invalid_age}'.isdigit(): {invalid_age.isdigit()}")

# Validate numeric input
inputs = ["123", "abc", "12.34", "-5", ""]

print("\nDigit validation:")
for inp in inputs:
    if inp.isdigit():
        print(f"  '{inp}' is valid positive integer")
    else:
        print(f"  '{inp}' is NOT a digit string")

# Alpha checking
name = "John"
mixed = "John123"

print(f"\n'{name}'.isalpha(): {name.isalpha()}")
print(f"'{mixed}'.isalpha(): {mixed.isalpha()}")

# Check if valid name (letters only)
names = ["Alice", "Bob123", "Charlie-", "Diana"]

print("\nValid names (letters only):")
for n in names:
    if n.isalpha():
        print(f"  {n}")

# Alphanumeric checking
username = "user123"
invalid_user = "user@123"

print(f"\n'{username}'.isalnum(): {username.isalnum()}")
print(f"'{invalid_user}'.isalnum(): {invalid_user.isalnum()}")

# Case checking
text1 = "HELLO"
text2 = "hello"
text3 = "Hello"

print(f"\n'{text1}'.isupper(): {text1.isupper()}")
print(f"'{text2}'.islower(): {text2.islower()}")
print(f"'{text3}'.istitle(): {text3.istitle()}")

# Space checking
spaces = "   "
empty = ""
text = "Hello"

print(f"\n'{spaces}'.isspace(): {spaces.isspace()}")
print(f"'{empty}'.isspace(): {empty.isspace()}")
print(f"'{text}'.isspace(): {text.isspace()}")

# Practical: validate username
def validate_username(username):
    """Validate username: 3-20 chars, alphanumeric"""
    if len(username) < 3 or len(username) > 20:
        return False, "Length must be 3-20 characters"

    if not username.isalnum():
        return False, "Only letters and numbers allowed"

    if username.isdigit():
        return False, "Cannot be all digits"

    return True, "Valid username"


test_users = ["alice", "bob123", "a", "user@name", "12345", "VeryLongUsernameThatExceedsLimit"]

print("\nUsername validation:")
for user in test_users:
    valid, message = validate_username(user)
    status = "✓" if valid else "✗"
    print(f"  {status} '{user}': {message}")


                        
1age_str ← 25, invalid_age ← 25.5, inputs ← ['123', 'abc', '12.34', '-5', '']⋯4# Digit checking5age_str→ 25 = "25"6invalid_age→ 25.5 = "25.5"78print(f"'{age_str25}'.isdigit(): {age_str.isdigit()}")9print(f"'{invalid_age25.5}'.isdigit(): {invalid_age.isdigit()}")1011# Validate numeric input12inputs→ ['123', 'abc', '12.34', '-5', ''] = ["123", "abc", "12.34", "-5", ""]1314print("\nDigit validation:")15for inp in inputs:⋯output'25'.isdigit(): True
'25.5'.isdigit(): False

Digit validation:
2for inp in inputs:pass 1 of 5⋯14print("\nDigit validation:")15for inp123 in inputs['123', 'abc', '12.34', '-5', '']:16    if inp.isdigit():17        print(f"  '{inp}' is valid positive integer")⋯All 5 passes — pass 1 is the card abovepassinp
1123
2abc
312.34
4-5
5(empty)
3if inp.isdigit():⋯15for inp in inputs:16    if inp123.isdigit():17        print(f"  '{inp123}' is valid positive integer")18    else:⋯output  '123' is valid positive integer
4else:pass 1 of 4⋯16if inp.isdigit():17    print(f"  '{inp}' is valid positive integer")18else:19    print(f"  '{inpabc}' is NOT a digit string")⋯output  'abc' is NOT a digit stringAll 4 passes — pass 1 is the card abovepassinp
1abc
212.34
3-5
4(empty)
5name ← John, mixed ← John123, names ← ['Alice', 'Bob123', 'Charlie-', 'Diana']⋯21# Alpha checking22name→ John = "John"23mixed→ John123 = "John123"2425print(f"\n'{nameJohn}'.isalpha(): {name.isalpha()}")26print(f"'{mixedJohn123}'.isalpha(): {mixed.isalpha()}")2728# Check if valid name (letters only)29names→ ['Alice', 'Bob123', 'Charlie-', 'Diana'] = ["Alice", "Bob123", "Charlie-", "Diana"]3031print("\nValid names (letters only):")32for n in names:⋯output
'John'.isalpha(): True
'John123'.isalpha(): False

Valid names (letters only):
6for n in names:pass 1 of 4⋯31print("\nValid names (letters only):")32for nAlice in names['Alice', 'Bob123', 'Charlie-', 'Diana']:33    if n.isalpha():34        print(f"  {n}")⋯All 4 passes — pass 1 is the card abovepassn
1Alice
2Bob123
3Charlie-
4Diana
7if n.isalpha():pass 1 of 2⋯32for n in names:33    if nAlice.isalpha():34        print(f"  {nAlice}")⋯output  Alice
8if n.isalpha():pass 2 of 2⋯32for n in names:33    if nDiana.isalpha():34        print(f"  {nDiana}")⋯output  Diana
9username ← user123, invalid_user ← user@123, text1 ← HELLO, text2 ← hello⋯36# Alphanumeric checking37username→ user123 = "user123"38invalid_user→ user@123 = "user@123"3940print(f"\n'{usernameuser123}'.isalnum(): {username.isalnum()}")41print(f"'{invalid_useruser@123}'.isalnum(): {invalid_user.isalnum()}")4243# Case checking44text1→ HELLO = "HELLO"45text2→ hello = "hello"46text3→ Hello = "Hello"4748print(f"\n'{text1HELLO}'.isupper(): {text1.isupper()}")49print(f"'{text2hello}'.islower(): {text2.islower()}")50print(f"'{text3Hello}'.istitle(): {text3.istitle()}")5152# Space checking53spaces→     = "   "54empty→ (empty) = ""55text→ Hello = "Hello"5657print(f"\n'{spaces   }'.isspace(): {spaces.isspace()}")58print(f"'{empty(empty)}'.isspace(): {empty.isspace()}")59print(f"'{textHello}'.isspace(): {text.isspace()}")6061# Practical: validate username62def validate_username(username):63    """Validate username: 3-20 chars, alphanumeric"""64    if len(username) < 3 or len(username) > 20:65        return False, "Length must be 3-20 characters"6667    if not username.isalnum():68        return False, "Only letters and numbers allowed"6970    if username.isdigit():71        return False, "Cannot be all digits"7273    return True, "Valid username"747576test_users→ ['alice', 'bob123', 'a', 'user@name', '12345', 'VeryLongUsernameThatExceedsLimit'] = ["alice", "bob123", "a", "user@name", "12345", "VeryLongUsernameThatExceedsLimit"]7778print("\nUsername validation:")79for user in test_users:⋯output
'user123'.isalnum(): True
'user@123'.isalnum(): False

'HELLO'.isupper(): True
'hello'.islower(): True
'Hello'.istitle(): True

'   '.isspace(): True
''.isspace(): False
'Hello'.isspace(): False

Username validation:
10for user in test_users:pass 1 of 6⋯78print("\nUsername validation:")79for useralice in test_users['alice', 'bob123', 'a', 'user@name', '12345', 'VeryLongUsernameThatExceedsLimit']:80    valid, message = validate_username(useralice)81    status = "✓" if valid else "✗"⋯All 6 passes — pass 1 is the card abovepassuserusername
1alice—
2bob123—
3aa
4user@nameuser@name
51234512345
6VeryLongUsernameThatExceedsLimitVeryLongUsernameThatExceedsLimit
11def validate_username(username):pass 1 of 6⋯61# Practical: validate username62def validate_username(usernamealice):63    """Validate username: 3-20 chars, alphanumeric"""64    if len(username) < 3 or len(username) > 20:65        return False, "Length must be 3-20 characters"6667    if not username.isalnum():68        return False, "Only letters and numbers allowed"6970    if username.isdigit():71        return False, "Cannot be all digits"7273    return True, "Valid username"⋯All 6 passes — pass 1 is the card abovepassusername
1alice
2bob123
3a
4user@name
512345
6VeryLongUsernameThatExceedsLimit
12valid ← True, message ← Valid username, status ← ✓⋯79for user in test_users:80    valid→ True, message→ Valid username = validate_username(useralice)81    status→ ✓ = "✓" if validTrue else "✗"82    print(f"  {status✓} '{useralice}': {messageValid username}")⋯output  ✓ 'alice': Valid username
13valid ← True, message ← Valid username, status ← ✓⋯79for user in test_users:80    valid→ True, message→ Valid username = validate_username(userbob123)81    status→ ✓ = "✓" if validTrue else "✗"82    print(f"  {status✓} '{userbob123}': {messageValid username}")⋯output  ✓ 'bob123': Valid username
14if len(username) < 3 or len(username) > 20:pass 1 of 2⋯63"""Validate username: 3-20 chars, alphanumeric"""64if len(usernamea) < 3 or len(username) > 20:65    return False, "Length must be 3-20 characters"⋯
15valid ← False, message ← Length must be 3-20 characters, status ← ✗⋯79for user in test_users:80    valid→ False, message→ Length must be 3-20 characters = validate_username(usera)81    status→ ✗ = "✓" if validFalse else "✗"82    print(f"  {status✗} '{usera}': {messageLength must be 3-20 characters}")⋯output  ✗ 'a': Length must be 3-20 characters
16if not username.isalnum():⋯67if not usernameuser@name.isalnum():68    return False, "Only letters and numbers allowed"⋯
17valid ← False, message ← Only letters and numbers allowed, status ← ✗⋯79for user in test_users:80    valid→ False, message→ Only letters and numbers allowed = validate_username(useruser@name)81    status→ ✗ = "✓" if validFalse else "✗"82    print(f"  {status✗} '{useruser@name}': {messageOnly letters and numbers allowed}")⋯output  ✗ 'user@name': Only letters and numbers allowed
18if username.isdigit():⋯70if username12345.isdigit():71    return False, "Cannot be all digits"⋯
19valid ← False, message ← Cannot be all digits, status ← ✗⋯79for user in test_users:80    valid→ False, message→ Cannot be all digits = validate_username(user12345)81    status→ ✗ = "✓" if validFalse else "✗"82    print(f"  {status✗} '{user12345}': {messageCannot be all digits}")⋯output  ✗ '12345': Cannot be all digits
20if len(username) < 3 or len(username) > 20:pass 2 of 2⋯63"""Validate username: 3-20 chars, alphanumeric"""64if len(usernameVeryLongUsernameThatExceedsLimit) < 3 or len(username) > 20:65    return False, "Length must be 3-20 characters"⋯
21valid ← False, message ← Length must be 3-20 characters, status ← ✗⋯79for user in test_users:80    valid→ False, message→ Length must be 3-20 characters = validate_username(userVeryLongUsernameThatExceedsLimit)81    status→ ✗ = "✓" if validFalse else "✗"82    print(f"  {status✗} '{userVeryLongUsernameThatExceedsLimit}': {messageLength must be 3-20 characters}")⋯output  ✗ 'VeryLongUsernameThatExceedsLimit': Length must be 3-20 characters

All 5 passes — pass 1 is the card above
pass	`inp`
1	123
2	abc
3	12.34
4	-5
5	(empty)

All 4 passes — pass 1 is the card above
pass	`inp`
1	abc
2	12.34
3	-5
4	(empty)

All 4 passes — pass 1 is the card above
pass	`n`
1	Alice
2	Bob123
3	Charlie-
4	Diana

All 6 passes — pass 1 is the card above
pass	`user`	`username`
1	alice	—
2	bob123	—
3	a	a
4	user@name	user@name
5	12345	12345
6	VeryLongUsernameThatExceedsLimit	VeryLongUsernameThatExceedsLimit

All 6 passes — pass 1 is the card above
pass	`username`
1	alice
2	bob123
3	a
4	user@name
5	12345
6	VeryLongUsernameThatExceedsLimit

validation Checking string content with isdigit(), isalpha(), and related methods

Prefix and Suffix Checking

                            startswith_endswith.py
                        
Replay: real traced execution (multi-file project)

                            # Startswith and endswith


# Check prefix
filename = "document.txt"

if filename.startswith("doc"):
    print(f"'{filename}' starts with 'doc'")

if filename.startswith("image"):
    print("This won't print")
else:
    print(f"'{filename}' doesn't start with 'image'")

# Check multiple prefixes
url = "https://example.com"
if url.startswith(("http://", "https://")):
    print(f"'{url}' is a valid URL")

# Check suffix
files = ["report.pdf", "data.csv", "image.png", "script.py"]

print("\nPython files:")
for file in files:
    if file.endswith(".py"):
        print(f"  {file}")

print("\nData files:")
for file in files:
    if file.endswith((".csv", ".json", ".xml")):
        print(f"  {file}")

# Practical: file type detection
def get_file_type(filename):
    """Determine file type by extension"""
    if filename.endswith((".jpg", ".png", ".gif")):
        return "Image"
    elif filename.endswith((".txt", ".md", ".log")):
        return "Text"
    elif filename.endswith((".py", ".java", ".js")):
        return "Code"
    elif filename.endswith((".pdf", ".doc", ".docx")):
        return "Document"
    else:
        return "Unknown"


test_files = ["photo.jpg", "script.py", "README.md", "report.pdf", "data.db"]

print("\nFile types:")
for file in test_files:
    print(f"  {file}: {get_file_type(file)}")

# Remove extension
def remove_extension(filename):
    """Remove file extension"""
    if "." in filename:
        dot_pos = filename.rfind(".")
        return filename[:dot_pos]
    return filename


print("\nWithout extensions:")
for file in test_files:
    print(f"  {file} → {remove_extension(file)}")


                        
1filename ← document.txt⋯4# Check prefix5filename→ document.txt = "document.txt"⋯
2if filename.startswith("doc"):⋯7if filenamedocument.txt.startswith("doc"):8    print(f"'{filenamedocument.txt}' starts with 'doc'")⋯output'document.txt' starts with 'doc'
3else:⋯10if filename.startswith("image"):11    print("This won't print")12else:13    print(f"'{filenamedocument.txt}' doesn't start with 'image'")⋯output'document.txt' doesn't start with 'image'
4url ← https://example.com⋯15# Check multiple prefixes16url→ https://example.com = "https://example.com"17if url.startswith(("http://", "https://")):⋯
5if url.startswith(("http://", "https://")):⋯16url = "https://example.com"17if urlhttps://example.com.startswith(("http://", "https://")):18    print(f"'{urlhttps://example.com}' is a valid URL")⋯output'https://example.com' is a valid URL
6files ← ['report.pdf', 'data.csv', 'image.png', 'script.py']⋯20# Check suffix21files→ ['report.pdf', 'data.csv', 'image.png', 'script.py'] = ["report.pdf", "data.csv", "image.png", "script.py"]2223print("\nPython files:")24for file in files:⋯output
Python files:
7for file in files:pass 1 of 4⋯23print("\nPython files:")24for filereport.pdf in files['report.pdf', 'data.csv', 'image.png', 'script.py']:25    if file.endswith(".py"):26        print(f"  {file}")⋯All 4 passes — pass 1 is the card abovepassfile
1report.pdf
2data.csv
3image.png
4script.py
8if file.endswith(".py"):⋯24for file in files:25    if filescript.py.endswith(".py"):26        print(f"  {filescript.py}")⋯output  script.py
9print(" Data files:")⋯28print("\nData files:")29for file in files:⋯output
Data files:
10for file in files:pass 1 of 4⋯28print("\nData files:")29for filereport.pdf in files['report.pdf', 'data.csv', 'image.png', 'script.py']:30    if file.endswith((".csv", ".json", ".xml")):31        print(f"  {file}")⋯All 4 passes — pass 1 is the card abovepassfile
1report.pdf
2data.csv
3image.png
4script.py
11if file.endswith((".csv", ".json", ".xml")):⋯29for file in files:30    if filedata.csv.endswith((".csv", ".json", ".xml")):31        print(f"  {filedata.csv}")⋯output  data.csv
12test_files ← ['photo.jpg', 'script.py', 'README.md', 'report.pdf', 'data.db']⋯48test_files→ ['photo.jpg', 'script.py', 'README.md', 'report.pdf', 'data.db'] = ["photo.jpg", "script.py", "README.md", "report.pdf", "data.db"]4950print("\nFile types:")51for file in test_files:⋯output
File types:
13for file in test_files:pass 1 of 5⋯50print("\nFile types:")51for filephoto.jpg in test_files['photo.jpg', 'script.py', 'README.md', 'report.pdf', 'data.db']:52    print(f"  {filephoto.jpg}: {get_file_type(file)}")⋯All 5 passes — pass 1 is the card abovepassfilefilename
1photo.jpgphoto.jpg
2script.pyscript.py
3README.mdREADME.md
4report.pdfreport.pdf
5data.db—
14def get_file_type(filename):pass 1 of 5⋯33# Practical: file type detection34def get_file_type(filenamephoto.jpg):35    """Determine file type by extension"""36    if filename.endswith((".jpg", ".png", ".gif")):⋯All 5 passes — pass 1 is the card abovepassfilename
1photo.jpg
2script.py
3README.md
4report.pdf
5data.db
15if filename.endswith((".jpg", ".png", ".gif")):⋯35"""Determine file type by extension"""36if filenamephoto.jpg.endswith((".jpg", ".png", ".gif")):37    return "Image"38elif filename.endswith((".txt", ".md", ".log")):⋯
16print(f"  {file}: {get_file_type(file)}")⋯51for file in test_files:52    print(f"  {filephoto.jpg}: {get_file_type(file)}")⋯output  photo.jpg: Image
17elif filename.endswith((".py", ".java", ".js")):⋯39    return "Text"40elif filenamescript.py.endswith((".py", ".java", ".js")):41    return "Code"42elif filename.endswith((".pdf", ".doc", ".docx")):⋯
18print(f"  {file}: {get_file_type(file)}")⋯51for file in test_files:52    print(f"  {filescript.py}: {get_file_type(file)}")⋯output  script.py: Code
19elif filename.endswith((".txt", ".md", ".log")):⋯37    return "Image"38elif filenameREADME.md.endswith((".txt", ".md", ".log")):39    return "Text"40elif filename.endswith((".py", ".java", ".js")):⋯
20print(f"  {file}: {get_file_type(file)}")⋯51for file in test_files:52    print(f"  {fileREADME.md}: {get_file_type(file)}")⋯output  README.md: Text
21elif filename.endswith((".pdf", ".doc", ".docx")):⋯41    return "Code"42elif filenamereport.pdf.endswith((".pdf", ".doc", ".docx")):43    return "Document"44else:⋯
22print(f"  {file}: {get_file_type(file)}")⋯51for file in test_files:52    print(f"  {filereport.pdf}: {get_file_type(file)}")⋯output  report.pdf: Document
23print(f"  {file}: {get_file_type(file)}")⋯51for file in test_files:52    print(f"  {filedata.db}: {get_file_type(file)}")⋯output  data.db: Unknown
24print(" Without extensions:")⋯63print("\nWithout extensions:")64for file in test_files:⋯output
Without extensions:
25for file in test_files:pass 1 of 5⋯63print("\nWithout extensions:")64for filephoto.jpg in test_files['photo.jpg', 'script.py', 'README.md', 'report.pdf', 'data.db']:65    print(f"  {filephoto.jpg} → {remove_extension(file)}")⋯All 5 passes — pass 1 is the card abovepassfile
1photo.jpg
2script.py
3README.md
4report.pdf
5data.db
26def remove_extension(filename):pass 1 of 5⋯54# Remove extension55def remove_extension(filenamephoto.jpg):56    """Remove file extension"""57    if "." in filename:⋯All 5 passes — pass 1 is the card abovepassfilename
1photo.jpg
2script.py
3README.md
4report.pdf
5data.db
27dot_pos ← 5pass 1 of 5⋯56"""Remove file extension"""57if "." in filenamephoto.jpg:58    dot_pos→ 5 = filenamephoto.jpg.rfind(".")59    return filename[:dot_pos]photo60return filename⋯All 5 passes — pass 1 is the card abovepassfilenamefilename[:dot_pos]dot_pos
1photo.jpgphoto5
2script.pyscript6
3README.mdREADME6
4report.pdfreport6
5data.dbdata4
28print(f"  {file} → {remove_extension(file)}")⋯64for file in test_files:65    print(f"  {filephoto.jpg} → {remove_extension(file)}")⋯output  photo.jpg → photo
29print(f"  {file} → {remove_extension(file)}")⋯64for file in test_files:65    print(f"  {filescript.py} → {remove_extension(file)}")⋯output  script.py → script
30print(f"  {file} → {remove_extension(file)}")⋯64for file in test_files:65    print(f"  {fileREADME.md} → {remove_extension(file)}")⋯output  README.md → README
31print(f"  {file} → {remove_extension(file)}")⋯64for file in test_files:65    print(f"  {filereport.pdf} → {remove_extension(file)}")⋯output  report.pdf → report
32print(f"  {file} → {remove_extension(file)}")⋯64for file in test_files:65    print(f"  {filedata.db} → {remove_extension(file)}")⋯output  data.db → data

All 4 passes — pass 1 is the card above
pass	`file`
1	report.pdf
2	data.csv
3	image.png
4	script.py

All 4 passes — pass 1 is the card above
pass	`file`
1	report.pdf
2	data.csv
3	image.png
4	script.py

All 5 passes — pass 1 is the card above
pass	`file`	`filename`
1	photo.jpg	photo.jpg
2	script.py	script.py
3	README.md	README.md
4	report.pdf	report.pdf
5	data.db	—

All 5 passes — pass 1 is the card above
pass	`filename`
1	photo.jpg
2	script.py
3	README.md
4	report.pdf
5	data.db

All 5 passes — pass 1 is the card above
pass	`file`
1	photo.jpg
2	script.py
3	README.md
4	report.pdf
5	data.db

All 5 passes — pass 1 is the card above
pass	`filename`
1	photo.jpg
2	script.py
3	README.md
4	report.pdf
5	data.db

All 5 passes — pass 1 is the card above
pass	`filename`	`filename[:dot_pos]`	`dot_pos`
1	photo.jpg	photo	5
2	script.py	script	6
3	README.md	README	6
4	report.pdf	report	6
5	data.db	data	4

prefix_suffix Checking if strings start or end with specific substrings

Characteristics

Immutable: Methods return new strings
Chainable: Can chain method calls
Unicode-aware: Work with international text
Many methods: Rich standard library

Exercise: practical.py

Build a text processing pipeline that cleans user input, parses CSV data, and formats phone numbers