String Basics
The most common data type you'll work with isn't numbers — it's text. Usernames, emails, articles, JSON data, HTML... all strings. This lesson systematically covers Python string operations.
1. Three Types of Quotes
Python allows three ways to define strings:
PYTHON
# Single quotes
s1 = 'Hello'
# Double quotes
s2 = "Hello"
# Triple quotes (multi-line strings)
s3 = """This is the first line
This is the second line
This is the third line"""
Single and double quotes are equivalent — pick one and stay consistent:
PYTHON
# These two are equivalent
name = 'Xiao Ming'
name = "Xiao Ming"
But if your string contains quotes, you'll need to mix:
PYTHON
# String has single quotes, wrap with double quotes
text = "It's a beautiful day"
print(text) # It's a beautiful day
# String has double quotes, wrap with single quotes
text = 'He said: "Hello!"'
print(text) # He said: "Hello!"
# Or use an escape character (not recommended, hurts readability)
text = 'It\'s a beautiful day'
💡 Tip: It's recommended to use double quotes consistently. English text often contains apostrophes (don't, it's) — with single quotes you'd need to escape them. Double quotes avoid this hassle.
Triple Quotes: Great for Multi-line Text
Triple quotes (""" or ''') preserve line breaks and formatting:
PYTHON
# Multi-line text with triple quotes
message = """Dear Customer,
Thank you for choosing our service. Your order has been confirmed.
Estimated delivery: 3 business days.
If you have any questions, please contact support.
Have a nice day!"""
print(message)
Output:
TEXT
Dear Customer,
Thank you for choosing our service. Your order has been confirmed.
Estimated delivery: 3 business days.
If you have any questions, please contact support.
Have a nice day!
Triple quotes are commonly used for: long text, documentation strings, and function descriptions.
2. Escape Characters
Some characters can't be written directly in a string (like newlines, tabs, or quotes themselves). Use escape characters — a backslash \ followed by a character.
| Escape | Meaning |
|---|---|
\n |
Newline |
\t |
Tab |
\\ |
Backslash itself |
\' |
Single quote |
\" |
Double quote |
PYTHON
print("First line\nSecond line") # \n newline
print("Name:\tZhang San\tAge:\t25") # \t tab alignment
print("C:\\Users\\XiaoMing\\Desktop") # \\ outputs backslash
print("He said: \"Hello!\"") # \" double quote
Output:
TEXT
First line
Second line
Name: Zhang San Age: 25
C:\Users\XiaoMing\Desktop
He said: "Hello!"
Example: Formatting a Table with Escape Characters (Difficulty ⭐)
PYTHON
# Use \t to align a table
print("Item\tPrice\tQty\tTotal")
print("------------------------")
print("Apple\t5.0\t3\t15.0")
print("Banana\t3.5\t5\t17.5")
print("Milk\t12.0\t2\t24.0")
Output:
TEXT
Item Price Qty Total
------------------------
Apple 5.0 3 15.0
Banana 3.5 5 17.5
Milk 12.0 2 24.0
💡 Tip:
\t is useful for aligning text, but if content lengths vary, alignment may not be perfect. For production, use f-string formatting ({value:10} to occupy 10 character width).
3. Raw Strings
If you've ever written Windows file paths, you've experienced this pain:
PYTHON
# Oops! \n is interpreted as newline
path = "C:\Users\new_folder\documents"
print(path)
# Output: C:\Users
# ew_folder\documents
Solution: prefix the string with r, telling Python "don't process escape characters, output as-is":
PYTHON
# ✅ Raw string — r prefix
path = r"C:\Users\new_folder\documents"
print(path)
# Output: C:\Users\new_folder\documents
In a raw string, each character is itself — \n no longer means newline, \t no longer means tab.
PYTHON
# Regex patterns (we'll use this later)
pattern = r"\d{3}-\d{8}" # Match phone numbers: 010-12345678
# File paths
log_path = r"D:\logs\app\2026\06\error.log"
print(f"Log path: {log_path}")
⚠️ Note: The last character of a raw string cannot be a backslash —
r"abc\" will error. Why? Even though \" isn't treated as an escape, the parser sees the backslash as escaping the closing quote, causing an unterminated string.
4. String Indexing
A string is a sequence of characters, each with a position (index). Python indexes start from 0:
PYTHON
text = "Python"
# Index: 0→P 1→y 2→t 3→h 4→o 5→n
print(text[0]) # P
print(text[1]) # y
print(text[5]) # n
Python also supports negative indexing, counting from right to left. -1 is the last character:
PYTHON
text = "Python"
print(text[-1]) # n (last)
print(text[-2]) # o (second to last)
print(text[-3]) # h
print(text[-6]) # P (first)
Index Diagram
TEXT
Positive: 0 1 2 3 4 5
┌────┬────┬────┬────┬────┬────┐
│ P │ y │ t │ h │ o │ n │
└────┴────┴────┴────┴────┴────┘
Negative: -6 -5 -4 -3 -2 -1
Example: Extract Information from an ID Number (Difficulty ⭐)
PYTHON
# Extract basic info from an ID number
id_number = "110101199001011234"
# First 6 digits: region code
address_code = id_number[:6] # We'll cover slicing next
print(f"Region code: {address_code}")
# Digits 7-14: date of birth
birth = id_number[6:14]
year = birth[:4]
month = birth[4:6]
day = birth[6:8]
print(f"Date of birth: {year}-{month}-{day}")
# Last digit (-1): checksum
last_digit = id_number[-1]
print(f"Checksum: {last_digit}")
Output:
TEXT
Region code: 110101
Date of birth: 1990-01-01
Checksum: 4
💡 Tip:
string[6:14] is called slicing — we'll cover it next.
5. String Slicing
Slicing extracts a substring from a string. Syntax: string[start:end:step]
PYTHON
text = "Hello, Python!"
# Basic slice: start:end (end is exclusive)
print(text[0:5]) # Hello (characters 0 to 4)
print(text[7:13]) # Python
Shortcuts
PYTHON
text = "Hello, Python!"
print(text[:5]) # Hello — omit start, from beginning up to index 5
print(text[7:]) # Python! — omit end, from index 7 to end
print(text[:]) # Hello, Python! — everything
Step
PYTHON
text = "Hello, Python!"
print(text[::2]) # Hlo yhn! — every 2nd character
print(text[1::2]) # el,Pto — start at 1, every 2nd
Negative Step (Reversal)
PYTHON
text = "Hello, Python!"
print(text[::-1]) # !nohtyP ,olleH — reverse the entire string!
print(text[5:0:-1]) # ,olle — from index 5 to 0, reverse direction
Slice Examples
PYTHON
# Practical slicing tricks
url = "https://www.example.com"
# Remove protocol prefix
without_https = url[8:] # www.example.com
print(without_https)
# Get domain
domain = url[8:-4] # www.example
print(domain)
# Reverse a string
name = "racecar"
reversed_name = name[::-1] # racecar (palindrome)
print(name == reversed_name) # True — it's a palindrome!
# Every other character
code = "H1e2l3l4o5"
clean = code[::2] # Hello
print(clean)
Output:
TEXT
www.example.com
www.example
True
Hello
💡 Tip:
[::-1] is the classic Python way to reverse a string — simple and elegant. Note that slicing never raises IndexError — even if the start or end exceeds the string length, Python safely returns what it can.
6. Common String Methods
Strings have many built-in methods. We'll cover the most common ones here. No need to memorize everything — knowing what's available is enough; you'll memorize with practice.
Case Conversion
PYTHON
text = "Hello, Python!"
print(text.upper()) # HELLO, PYTHON!
print(text.lower()) # hello, python!
print(text.title()) # Hello, Python!
print(text.capitalize()) # Hello, python!
Stripping Whitespace
PYTHON
text = " Hello, World! "
print(f"|{text}|") # | Hello, World! |
print(f"|{text.strip()}|") # |Hello, World!| — both sides
print(f"|{text.lstrip()}|") # |Hello, World! | — left only
print(f"|{text.rstrip()}|") # | Hello, World!| — right only
Searching and Replacing
PYTHON
text = "Hello, Python! Python is fun!"
print(text.find("Python")) # 7 — first occurrence position
print(text.find("Java")) # -1 — not found, returns -1
print(text.count("Python")) # 2 — number of occurrences
new_text = text.replace("Python", "Java")
print(new_text) # Hello, Java! Java is fun!
# replace can specify max replacements
partial = text.replace("Python", "Java", 1)
print(partial) # Hello, Java! Python is fun!
Check Methods
PYTHON
print("hello".isalpha()) # True — all letters
print("hello123".isalpha()) # False — has digits
print("12345".isdigit()) # True — all digits
print("hello123".isalnum()) # True — letters and digits
print("hello!".isalnum()) # False — has exclamation mark
print(" ".isspace()) # True — all whitespace
💡 Tip: These check methods are very useful for input validation. For example, checking if user input is all digits:
if user_input.isdigit(): ensures int() conversion won't fail.
Splitting and Joining
PYTHON
# split(): string → list
sentence = "I love Python"
words = sentence.split()
print(words) # ['I', 'love', 'Python']
# Specify delimiter
csv_line = "Zhang San,25,Beijing"
fields = csv_line.split(",")
print(fields) # ['Zhang San', '25', 'Beijing']
# join(): list → string
parts = ["2026", "06", "23"]
date = "-".join(parts)
print(date) # 2026-06-23
Example: Data Cleaning (Difficulty ⭐⭐)
PYTHON
# Simulate dirty user input
raw_data = " Zhang San , 25 , Beijing \n"
# Cleaning process
# 1. strip() removes leading/trailing whitespace
cleaned = raw_data.strip()
print(f"Step 1: |{cleaned}|")
# 2. split() by comma
fields = cleaned.split(",")
print(f"Step 2: {fields}")
# 3. Strip extra spaces from each field
name = fields[0].strip()
age = fields[1].strip()
city = fields[2].strip()
print(f"Name: {name}")
print(f"Age: {age}")
print(f"City: {city}")
Output:
TEXT
Step 1: |Zhang San , 25 , Beijing|
Step 2: ['Zhang San ', ' 25 ', ' Beijing']
Name: Zhang San
Age: 25
City: Beijing
7. len(): Getting String Length
len() is a built-in Python function that returns the length of a string (or other sequences):
PYTHON
text = "Hello, Python!"
print(len(text)) # 15
# Chinese characters each count as 1
chinese = "你好世界"
print(len(chinese)) # 4
# Empty string has length 0
print(len("")) # 0
# Common use: limit input length
username = "Xiao Ming"
if len(username) < 2:
print("Username too short")
elif len(username) > 10:
print("Username too long")
else:
print(f"Username '{username}' is valid")
💡 Tip: In Python 3, string length counts characters, not bytes.
len("中") returns 1, not 3 (it would return 3 in Python 2). This makes string operations more intuitive.
Common Use Cases
- Input cleaning: Use
strip()to remove leading/trailing spaces,lower()to normalize email addresses. - File path handling: Use
split()to extract filenames,replace()to convert path separators. - Log analysis: Use
find()to locate keywords, slicing to extract specific log sections. - Data formatting: Use
split()to parse CSV rows,join()to reassemble output. - Password validation: Use
isalnum(),isdigit(),isalpha()to check password character types. - Text reversal:
[::-1]for quick string reversal, useful for palindrome checks or simple encoding.
❓ FAQ
Q Single quotes vs double quotes — which should I use? Why does Python allow both?
A Historically, Python's designers believed in giving programmers choice. C/Java programmers tend to use double quotes; JavaScript programmers use both. It's recommended to use double quotes consistently, because English often has single quotes (it's, don't, can't), so double quotes avoid escaping. More importantly, stay consistent within a project — don't mix them.
⚠️ Q: Why is the end index excluded? Like
text[0:5] doesn't include index 5? A: This is Python's design philosophy — half-open intervals [start, end). Two benefits: 1) text[:5] is easy to remember (first 5 characters); 2) text[0:5] and text[5:10] seamlessly connect without overlap or gaps. You'll appreciate this design once you get used to it.
Q: What's the difference between find() and index()? A: They do the same thing — find substring position. Difference: find() returns -1 when not found; index() raises an error (ValueError). Use find() for safety in daily work. Use index() when you're certain the substring exists (e.g., extracting from fixed-format data).
Q: Why don't string methods modify the original string? (e.g., text.upper() doesn't change text) A: Because strings in Python are immutable. Once created, they can't be modified. text.upper() actually returns a new string; the original text stays the same. This ensures data safety — when you pass a string to a function, you can be sure it won't be secretly modified. To "modify" a string, create a new one through slicing and concatenation.📖 Summary
- Strings have three quote forms: single, double, triple; double quotes are recommended
- Escape characters
\n,\t,\\, etc., represent special characters - Raw strings
r"..."prevent backslash escaping, great for paths and regex - Indexing starts at 0, supports negative indexing (
-1is the last character) - Slicing
[start:end:step]extracts substrings; end is exclusive; negative step reverses - Common methods:
strip(),split(),join(),find(),replace() - Check methods:
isdigit(),isalpha(),isalnum(), etc., for input validation - Strings are immutable — all "modifications" create new strings
📝 Exercises
-
Basic (Difficulty ⭐): Given
s = "Python programming is fun!", do the following:- Extract the first 6 characters
- Extract the last 4 characters
- Reverse the entire string
- Convert all letters to uppercase
-
Intermediate (Difficulty ⭐⭐): Write an email validator. Given
email = "user@example.com", check if:- It contains
@ - There's content on both sides of
@(neither side empty) - It ends with
.com,.cn, or.org - Output each check result (True/False) Hint: Use
find()to locate@, slicing to check both sides.
- It contains
-
Challenge (Difficulty ⭐⭐⭐): Write a data cleaner. Given:
TEXTdata = " Zhang San, 25 , Beijing ; Li Si , 30, Shanghai ; Wang Wu , 22, Guangzhou "Split by
;to get each person's data, then split by,to get each field. Usestrip()to clean each field, then output a neatly aligned table:TEXTName Age City ──────────────────── Zhang San 25 Beijing Li Si 30 Shanghai Wang Wu 22 Guangzhou