Python String isdigit() Method

The isdigit() method in Python checks whether all characters in a string are digits (0–9). It's a handy way to verify if a string represents a number.

Syntax

string.isdigit()

Parameters:

• This method does not take any parameters.

Returns:

• True – if all characters in the string are digits.
• False – if the string has one or more non-digit characters.

Example 1: Basic Usage

text = "12345"
print(text.isdigit())

True

Example 2: Contains Letters

text = "123abc"
print(text.isdigit())

False

Example 3: Empty String

text = ""
print(text.isdigit())

False

Use Case: Validating User Input

The isdigit() method is commonly used to validate input from users. For example, when asking for age or mobile number:

age = input("Enter your age: ")
if age.isdigit():
    print("Valid age!")
else:
    print("Please enter digits only.")

Output Example:

Enter your age: 25
Valid age!

Common Mistakes

• Does not allow negative numbers (e.g., "-123" → False)
• Decimal numbers (e.g., "3.14") also return False

Related Methods

• isnumeric() – similar to isdigit() but includes Unicode numeric characters
• isdecimal() – even stricter than isdigit()

Interview Tip

Use isdigit() to filter numeric strings during input validation, form parsing, or data cleaning.

Summary

• isdigit() returns True if all characters are digits (0–9)
• Returns False for empty strings, decimals, or non-digit characters
• Commonly used in input validation

Practice Problem

Write a program that asks for a PIN code and prints "Valid PIN" only if all characters are digits.

pin = input("Enter your 4-digit PIN: ")
if pin.isdigit() and len(pin) == 4:
    print("Valid PIN")
else:
    print("Invalid PIN. Please enter a 4-digit number.")