Strings

A string is a sequence of characters. In Python, strings are immutable, meaning they cannot be changed after they’re created. You can define a string using single, double, or triple quotes.

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Raw Strings

Raw strings are useful for handling special characters like backslashes (\). By prefixing a string with r or R, you tell Python to treat backslashes as literal characters instead of escape characters.

# without raw string
print('C:\\nowhere')  # Output: C:\nowhere

# with raw string
print(r'C:\\nowhere')  # Output: C:\\nowhere

Accessing Strings

Strings in Python are arrays of bytes representing Unicode characters. While Python doesn’t have a char data type, a single character is simply a string with a length of 1. You can access individual characters or parts of a string using square brackets [].

1. Strings as Arrays: You can access specific characters by their index.

a = "Hello, World!"
print(a[1])  # Output: e

2. Looping Through a String: Because strings are sequences, you can easily iterate through each character using a for loop.

for x in "banana":
  print(x)

3. String Length: Use the len() function to get the number of characters in a string.

a = "Hello, World!"
print(len(a))  # Output: 13

4. Checking for Presence: The in keyword checks if a substring is present. The not in keyword checks for its absence.

txt = "The best things in life are free!"
print("free" in txt)  # Output: True

if "expensive" not in txt:
  print("No, 'expensive' is NOT present.")

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String Slicing

Slicing lets you extract a substring by specifying a range of indices. The syntax is string[start:end], where the end index is exclusive.

1. Basic Slicing:

b = "Hello, World!"
print(b[2:5])  # Output: llo

2. Slice from the Start: Omitting the start index begins the slice from the first character.

b = "Hello, World!"
print(b[:5])  # Output: Hello

3. Slice to the End: Omitting the end index extends the slice to the last character.

b = "Hello, World!"
print(b[2:])  # Output: llo, World!

4. Negative Indexing: Negative indices slice from the end of the string.

b = "Hello, World!"
print(b[-5:-2])  # Output: orl

5. Slicing with Skip Value: You can add a third parameter, step, to skip characters. string[start:end:step]

word = "amazing"
print(word[1:6:2])  # Output: mzn

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Updating a String

Since strings are immutable, you can’t modify them directly. Instead, you create a new string by combining parts of the original string. This creates a new memory location for the new string.

my_string = "Python"
new_string = my_string[:2] + "thon"
print(new_string)  # Output: Python

Concatenation and Repetition

You can combine strings using the + operator (concatenation) and repeat them using the * operator.

a = "Hello"
b = "World"
c = a + " " + b
print(c)  # Output: Hello World

repeat_string = "Python" * 3
print(repeat_string)  # Output: PythonPythonPython

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Escape Characters

Escape characters let you include special characters in a string. They are preceded by a backslash (\).

Code	Result
\'	Single Quote
\\	Backslash
\n	New Line
\t	Tab

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String Operators

Operator	Description	Example
+	Concatenation	'Hello' + 'Python' gives 'HelloPython'
*	Repetition	'Hello' * 2 gives 'HelloHello'
[]	Slice	'Hello'[1] gives 'e'
[:]	Range Slice	'Hello'[1:4] gives 'ell'
in	Membership	'H' in 'Hello' gives True
not in	Membership	'M' not in 'Hello' gives True
r/R	Raw String	r'\n' prints \n

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String Formatting

You can’t directly combine strings and numbers with the + operator. Python provides several ways to format strings.

1. % Format Operator (Legacy): This operator is similar to printf() in C.

print("My name is %s and weight is %d kg!" % ('Zara', 21))
# Output: My name is Zara and weight is 21 kg!

2. F-Strings (Python 3.6+): F-Strings are the modern, preferred way to format strings. They’re more readable and efficient. Prefix the string with f and use curly braces {} as placeholders for variables or expressions.

age = 36
txt = f"My name is John, I am {age}"
print(txt)  # Output: My name is John, I am 36

You can also apply modifiers inside the placeholders.

price = 59
txt = f"The price is {price:.2f} dollars"
print(txt)  # Output: The price is 59.00 dollars

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String Methods

Python’s string methods are powerful tools for manipulating and inspecting strings.

• capitalize(): Converts the first character to uppercase.

  s = "python fundamentals: a complete guide for beginners"
  print(f"capitalize(): '{s.capitalize()}'")
  
  # Output:
  # capitalize(): 'Python fundamentals: a complete guide for beginners'

• upper(): Converts all characters to uppercase.

  s = "python fundamentals: a complete guide for beginners"
  print(f"upper(): '{s.upper()}'")
  
  # Output:
  # upper(): 'PYTHON FUNDAMENTALS: A COMPLETE GUIDE FOR BEGINNERS'

• lower(): Converts all characters to lowercase.

  s = "PYTHON FUNDAMENTALS: A COMPLETE GUIDE FOR BEGINNERS"
  print(f"lower(): '{s.lower()}'")
  
  # Output:
  # lower(): 'python fundamentals: a complete guide for beginners'

• strip(): Removes leading and trailing whitespace.

  s = "   python fundamentals: a complete guide for beginners   "
  print(f"strip(): '{s.strip()}'")
  
  # Output:
  # strip(): 'python fundamentals: a complete guide for beginners'

• split(separator): Splits a string into a list of substrings.

  s = "python,fundamentals,a,complete,guide,for,beginners"
  print(f"split(','): {s.split(',')}")
  
  # Output:
  # split(','): ['python', 'fundamentals', 'a', 'complete', 'guide', 'for', 'beginners']

• join(iterable): Joins elements of an iterable into a single string.

  s = ['python', 'fundamentals', 'a', 'complete', 'guide', 'for', 'beginners']
  print(f"join(): ' '.join(s)")
  
  # Output:
  # join(): 'python fundamentals a complete guide for beginners'

• replace(old, new): Replaces a substring with a new one.

  s = "a guide for python beginners"
  print(f"replace(): '{s.replace('python', 'Java')}'")
  
  # Output:
  # replace(): 'a guide for Java beginners'

• count(substring): Counts occurrences of a substring.

  s = "python is fun, python is easy"
  print(f"count('python'): {s.count('python')}")
  
  # Output:
  # count('python'): 2

• find(substring): Finds the index of a substring, returns -1 if not found.

  s = "python is fun"
  print(f"find('fun'): {s.find('fun')}")
  
  # Output:
  # find('fun'): 10

• startswith(prefix): Checks if the string begins with a specific prefix.

  s = "python is fun"
  print(f"startswith('python'): {s.startswith('python')}")
  
  # Output:
  # startswith('python'): True

• endswith(suffix): Checks if the string ends with a specific suffix.

  s = "python is fun"
  print(f"endswith('fun'): {s.endswith('fun')}")
  
  # Output:
  # endswith('fun'): True

• isalnum(): Checks if all characters are alphanumeric.

  s1 = "python123"
  s2 = "python!"
  print(f"isalnum() for '{s1}': {s1.isalnum()}")
  print(f"isalnum() for '{s2}': {s2.isalnum()}")
  
  # Output:
  # isalnum() for 'python123': True
  # isalnum() for 'python!': False

• isalpha(): Checks if all characters are alphabetic.

  s1 = "python"
  s2 = "python123"
  print(f"isalpha() for '{s1}': {s1.isalpha()}")
  print(f"isalpha() for '{s2}': {s2.isalpha()}")
  
  # Output:
  # isalpha() for 'python': True
  # isalpha() for 'python123': False

• isdigit(): Checks if all characters are digits.

  s1 = "12345"
  s2 = "123a"
  print(f"isdigit() for '{s1}': {s1.isdigit()}")
  print(f"isdigit() for '{s2}': {s2.isdigit()}")
  
  # Output:
  # isdigit() for '12345': True
  # isdigit() for '123a': False

• isupper()/islower(): Checks the case of characters.

  s1 = "PYTHON"
  s2 = "python"
  print(f"isupper() for '{s1}': {s1.isupper()}")
  print(f"islower() for '{s2}': {s2.islower()}")
  
  # Output:
  # isupper() for 'PYTHON': True
  # islower() for 'python': True

• title(): Converts the first letter of each word to uppercase.

  s = "python is fun"
  print(f"title(): '{s.title()}'")
  
  # Output:
  # title(): 'Python Is Fun'

• swapcase(): Swaps the case of all characters.

  s = "Python is FUN"
  print(f"swapcase(): '{s.swapcase()}'")
  
  # Output:
  # swapcase(): 'pYTHON IS fun'

The above string methods are the methods which are used mostly by python programmers.

There are a whole lot of other methods that python provides which can always be referred from the python docs: Python String Methods