Combining Strings

Using the "+" Operator

The "+" operator allows concatenation of multiple strings.

str1 = "aaa"
str2 = "bbb"
result = str1 + str2
print(result)
# Output: aaabbb

Note that direct concatenation with non-string types is not allowed:

num = 100
str1 = "hello"
# print(str1 + num)  # This would raise TypeError

To resolve this, convert the number to a string first:

num = 100
str1 = "hello"
result = str1 + str(num)
print(result)
# Output: hello100

Using the join() Method

The join() method combines elements of a sequence into a single string using a specified separator.

separator = "-"
sequence = ("a", "b", "c")
result = separator.join(sequence)
print(result)
# Output: a-b-c

Measuring String Length

Use the built-in len() function to determine string length:

str1 = "hello"
length = len(str1)
print(length)
# Output: 5

str2 = "你好"
length = len(str2)
print(length)
# Output: 2

In UTF-8 encoding, Chinese characters occupy three bytes each:

str1 = "你好"
print(len(str1.encode('utf-8')))
# Output: 6

Extracting Substrings

Syntax: string[start:end:step]

str1 = "hello world!"
print(str1[1])     # Output: e
print(str1[2:])    # Output: llo world!
print(str1[:4])    # Output: hell
print(str1[1:5])   # Output: ello
print(str1[-1])    # Output: !
print(str1[2:-2])  # Output: llo worl

Splitting Strings

Use the split() method to divide a string into a list.

str1 = "i am a good boy!"
parts = str1.split()
print(parts)
# Output: ['i', 'am', 'a', 'good', 'boy!']

parts = str1.split(" ", 3)
print(parts)
# Output: ['i', 'am', 'a', 'good boy!']

Searching Within Strings

count() Method

Counts occurrences of a substring within a string:

str1 = "hello world"
count = str1.count('o')
print(count)
# Output: 2

find() Method

Returns index of first occurrence or -1 if not found:

str1 = "hello world!"
index = str1.find('wo')
print(index)
# Output: 6

index() Method

Similar to find(), but raises a exception when substring is not found:

str1 = "hello world!"
try:
    index = str1.index('m')
except ValueError:
    print("Substring not found")

startswith() and endswith() Methods

Check whether a string starts or ends with a specific prefix or suffix:

str1 = "hello world!"
print(str1.startswith('hello'))  # True
print(str1.endswith('world!'))   # True

Case Conversion

lower() Method

Converts uppercase letters to lowercase:

str1 = "Hello World!"
lowercase = str1.lower()
print(lowercase)
# Output: hello world!

upper() Method

Converts lowercase letters to uppercase:

str1 = "Hello World!"
uppercase = str1.upper()
print(uppercase)
# Output: HELLO WORLD!

Removing Whitespace and Special Characters

strip() Method

Removes leading and trailing whitespace or specified characters:

str1 = " hello world! "
stripped = str1.strip()
print(stripped)
# Output: hello world!

str2 = "#hello world#@#"
stripped = str2.strip('#')
print(stripped)
# Output: hello world#@

lstrip() Method

Removes leading whitespace or specified characetrs:

str1 = " hello world! "
lstripped = str1.lstrip()
print(lstripped)
# Output: hello world! 

rstrip() Method

Removes trailing whitespace or specified characters:

str1 = " hello world! "
rstripped = str1.rstrip()
print(rstripped)
# Output:  hello world!

Replacing Text

The replace() method substitutes one substring with another:

original = "hello world !!! hhh"
replaced = original.replace(" ", "-")
print(replaced)
# Output: hello-world-!!!-hhh

replaced = original.replace(" ", "-", 1)
print(replaced)
# Output: hello-world !!! hhh