Python String isascii() Method

The isascii() method in Python checks whether all the characters in a string are ASCII characters (i.e., in the range 0 to 127).

It returns True if all characters are ASCII, otherwise False. This method is available from Python 3.7 and later.

Syntax

string.isascii()

Parameters:

• No parameters are required.

Returns:

• True – If all characters in the string are ASCII.
• False – If at least one character is not an ASCII character.

Example 1: Simple ASCII String

text = "Hello123"
print(text.isascii())

True

Example 2: String with Unicode Characters

text = "Café"
print(text.isascii())

False

The character é is not an ASCII character.

Example 3: Empty String

text = ""
print(text.isascii())

True

An empty string is considered ASCII.

Use Cases

• Validating user input for systems that only support ASCII
• Pre-processing text before encoding or sending to legacy systems
• Detecting presence of special or non-English characters

Common Mistakes

• Assuming isascii() checks if a string contains only letters or digits. It doesn’t—it allows symbols, punctuation, and whitespace too, as long as they’re ASCII.
• Using it in Python versions earlier than 3.7 will raise an AttributeError.

Interview Tip

If you’re asked to filter out non-ASCII characters from a string, isascii() can help quickly detect such cases or be used in list comprehensions to clean strings.

Summary

• string.isascii() checks if all characters are in the ASCII range.
• Returns True for ASCII-only strings, False otherwise.
• Empty strings return True.
• Available in Python 3.7+

Practice Problem

Write a program that checks if user input contains only ASCII characters.

text = input("Enter some text: ")
if text.isascii():
    print("All characters are ASCII.")
else:
    print("Non-ASCII characters found!")