07 - Strings

Strings represent text. They are ordered sequences of characters and are immutable.

A simple picture

A string is like a train made of character carriages:

"Python"
 0 1 2 3 4 5   <- positions called indexes
 P y t h o n

You can inspect a carriage or build a new train, but you cannot replace one carriage inside the existing string.

Creating strings 

single = 'Python'
double = "Python"
multi_line = """This text
uses more than one line."""

Choose consistent quotes. Use escaping when a string contains the same quote character:

message = "He said, \"Learn Python.\""

Indexing

An index is the numbered position of one character. Indexes start at zero:

word = "Python"
print(word[0])   # P
print(word[2])   # t
print(word[-1])  # n

word[6] raises IndexError because the last valid index is 5.

Slicing

A slice copies a selected part of a string. text[start:stop:step] stops before stop:

word = "Python"
print(word[0:2])  # Py
print(word[2:])   # thon
print(word[:4])   # Pyth
print(word[::2])  # Pto
print(word[::-1]) # nohtyP

Immutability

You cannot replace one character in place:

word = "cat"
# word[0] = "b"  # TypeError
word = "b" + word[1:]
print(word)       # bat

String methods return a new string:

name = "  ravi sharma  "
clean_name = name.strip().title()
print(clean_name)  # Ravi Sharma

Important fact: string methods do not change the original string. Save the returned string when you need it.

name = "  maya  "
name.strip()
print(name)  # The spaces are still present.

clean_name = name.strip()
print(clean_name)  # maya

Useful string methods

Method Purpose Example
.lower() lowercase text.lower()
.upper() uppercase text.upper()
.title() title case name.title()
.strip() remove outer whitespace text.strip()
.replace(a, b) replace text text.replace("old", "new")
.split() create a list of words sentence.split()
.join(items) combine strings ", ".join(names)
.startswith() check beginning code.startswith("PY")
.endswith() check ending file.endswith(".py")
.find() find index or -1 text.find("py")

.split() returns a group called a list, and .join() combines a group of strings. Lists are taught fully in the next chapter.

Searching and counting 

sentence = "Python makes problem solving easier"
print("Python" in sentence)       # True
print(sentence.lower().count("p"))

Use .casefold() instead of .lower() when building more language-aware case-insensitive comparisons.

Real-world example: clean a username 

raw_username = input("Username: ")
username = raw_username.strip().lower()
if len(username) >= 5 and username.isalnum():
    print("Username accepted")
else:
    print("Use at least five letters or digits")

Common mistakes

  • Forgetting that indexes begin at zero.
  • Assuming the slice stop index is included.
  • Calling .strip() and expecting it to remove all internal spaces.
  • Trying to modify a character directly.
  • Comparing text without deciding whether case should matter.

Bug Hunter

Bug 1 — index outside the string 

word = "cat"
print(word[3])

Bug 2 — immutable string 

word = "cat"
word[0] = "b"
print(word)

Bug 3 — returned value ignored 

name = "  maya  "
name.strip()
print(name)
Show Bug Hunter fixes 
# Bug 1: valid indexes are 0, 1, and 2.
print(word[2])

# Bug 2: build a new string.
word = "b" + word[1:]

# Bug 3: save the new string.
name = name.strip()
Optional deeper look: text and memory

Python strings store Unicode text and are immutable. Unicode allows one program to represent many writing systems and symbols. Because a string cannot change in place, operations such as slicing, replacing, or joining produce a new string object. Repeatedly joining thousands of pieces with + can be slower than collecting them and using .join() once.

Practice

Try these problems on this page. Decide first whether spaces and letter case should matter.

Problems

  1. Count characters in a sentence, excluding spaces.
  2. Reverse a string using slicing.
  3. Check whether a word is a palindrome.
  4. Count vowels and consonants.
  5. Normalize a person’s name.
  6. Replace repeated spaces with one space.
  7. Count the words in a sentence.
  8. Find the first occurrence of a target word.
  9. Challenge: check whether two words are anagrams after learning sorted() and lists in Chapter 08.
  10. Validate a username using length and allowed characters.
Show hints
  1. Remove spaces or count only characters that are not spaces.
  2. Use a slice with a step of -1.
  3. Compare the word with its reverse.
  4. Normalize to lowercase and check membership in "aeiou".
  5. Use strip() and title().
  6. Use split() and join().
  7. split() creates a list of words.
  8. Use find() or in.
  9. Normalize both words and compare their sorted characters.
  10. Combine length and allowed-character checks.
Show solution ideas
  1. len(sentence.replace(" ", "")) handles ordinary spaces.
  2. text[::-1].
  3. word == word[::-1] after applying the chosen normalization.
  4. Loop through the text and increment the correct counter.
  5. name.strip().title().
  6. ' '.join(text.split()).
  7. len(sentence.split()).
  8. sentence.lower().find(target.lower()).
  9. Compare sorted(left.lower()) and sorted(right.lower()).
  10. Check length, letters/digits, and any extra username rules.

Homework

Build a text analyzer that reports character count, word count, vowel count, and the longest word in a sentence.

Checkpoint

Write a program that normalizes a sentence by removing outer spaces, converting it to lowercase, and counting its non-space characters.