Check if a String Contains Only Uppercase Letters

Python offers several elegant ways to determine whether a string comprises solely uppercase letters. This capability is frequently needed in data validation, text processing, and various other programming scenarios. This post will explore different methods, ranging from simple built-in functions to more nuanced approaches. We’ll cover the pros and cons of each, ensuring you choose the best technique for your specific needs.

Method 1: Using isupper()

The most straightforward approach utilizes Python’s built-in string method isupper(). This method directly checks if all characters in a string are uppercase.

def is_all_uppercase(input_string):
  """Checks if a string contains only uppercase letters using isupper().

  Args:
    input_string: The string to check.

  Returns:
    True if the string contains only uppercase letters, False otherwise.
  """
  return input_string.isupper()

string1 = "HELLO"
string2 = "Hello World"
string3 = "123ABC"

print(f"'{string1}' is all uppercase: {is_all_uppercase(string1)}") # Output: True
print(f"'{string2}' is all uppercase: {is_all_uppercase(string2)}") # Output: False
print(f"'{string3}' is all uppercase: {is_all_uppercase(string3)}") # Output: False

This method is concise and efficient, making it ideal for most situations. However, it’s important to note that isupper() returns False if the string is empty or contains any non-alphabetic characters.

Method 2: Using a Loop and isupper() on individual characters

For a more granular control, you can iterate through each character and check it individually using isupper(). This allows for more complex logic if needed.

def is_all_uppercase_loop(input_string):
  """Checks if a string contains only uppercase letters using a loop and isupper().

  Args:
    input_string: The string to check.

  Returns:
    True if the string contains only uppercase letters, False otherwise.  Returns True for empty strings.
  """
  for char in input_string:
    if not char.isupper():
      return False
  return True

string1 = "HELLO"
string2 = "Hello World"
string3 = "123ABC"
string4 = ""

print(f"'{string1}' is all uppercase: {is_all_uppercase_loop(string1)}") # Output: True
print(f"'{string2}' is all uppercase: {is_all_uppercase_loop(string2)}") # Output: False
print(f"'{string3}' is all uppercase: {is_all_uppercase_loop(string3)}") # Output: False
print(f"'{string4}' is all uppercase: {is_all_uppercase_loop(string4)}") # Output: True

This approach offers greater flexibility but is slightly less efficient than the direct isupper() method. Note that this example handles empty strings differently than isupper().

Method 3: Using Regular Expressions

For more complex pattern matching beyond simple uppercase checks, regular expressions provide a powerful solution.

import re

def is_all_uppercase_regex(input_string):
  """Checks if a string contains only uppercase letters using regular expressions.

  Args:
    input_string: The string to check.

  Returns:
    True if the string contains only uppercase letters, False otherwise. Returns True for empty strings.
  """
  return bool(re.fullmatch(r"[A-Z]*", input_string))

string1 = "HELLO"
string2 = "Hello World"
string3 = "123ABC"
string4 = ""

print(f"'{string1}' is all uppercase: {is_all_uppercase_regex(string1)}") # Output: True
print(f"'{string2}' is all uppercase: {is_all_uppercase_regex(string2)}") # Output: False
print(f"'{string3}' is all uppercase: {is_all_uppercase_regex(string3)}") # Output: False
print(f"'{string4}' is all uppercase: {is_all_uppercase_regex(string4)}") # Output: True

This method uses re.fullmatch() to ensure the entire string matches the pattern [A-Z]*, which represents zero or more uppercase letters. This approach is more powerful but can be less readable for simple cases. Again, note the handling of empty strings.

Each method offers a different trade-off between readability, efficiency, and flexibility. The best choice depends on your specific requirements and coding style.