Python isdigit() Explained: Check if a String Contains Only Digits (with examples)

Hello, this is BlockDMask.

Today, we will explore how to check whether a string contains only digits in Python using the isdigit() method.

This is very useful when you're validating user input, processing files, or scraping web data where numeric checks are needed.

Table of Contents

• 1. What is isdigit() in Python?
• 2. Limitations and common pitfalls of isdigit()
• 3. Practical examples of isdigit()
• 4. Difference between isdigit(), isdecimal(), and isnumeric()

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1. What is isdigit() in Python?

The isdigit() method checks if a string contains only numeric digits (0–9). It belongs to the string class and can be used like this:

str.isdigit()

Basic behavior:

• Returns True if all characters are digits (0-9)
• Returns False if any non-digit character exists
• Signs like - (negative) or . (decimal) make it return False
• Certain Unicode numeric characters may return True (e.g. superscripts, fractions)

Typically used for checking whether user input contains a pure positive integer.

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2. Limitations and common pitfalls of isdigit()

• "3.14".isdigit() → False (cannot detect decimals)
• "-42".isdigit() → False (cannot detect negatives)
• "⅔".isdigit() → True (Unicode fraction accepted)

👉 In real-world input validation, isdigit() is often not enough. A more reliable way to check full integer inputs:

def is_integer(s):
    try:
        int(s)
        return True
    except ValueError:
        return False

This works for both positive and negative integers.

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3. Practical examples of isdigit()

Let’s look at various strings:

samples = [
    "PythonRocks",   # letters only
    "456Data",       # mixed digits & letters
    "789012",        # pure digits
    "-789",          # negative number
    "12.34",         # decimal number
    "5²",            # superscript 2 (unicode)
    "⅕",             # unicode fraction
    "0",             # zero
    "007",           # leading zeros
]

for s in samples:
    print(f"'{s}'.isdigit() : {s.isdigit()}")

Expected Output:

'PythonRocks'.isdigit() : False
'456Data'.isdigit() : False
'789012'.isdigit() : True
'-789'.isdigit() : False
'12.34'.isdigit() : False
'5²'.isdigit() : True
'⅕'.isdigit() : True
'0'.isdigit() : True
'007'.isdigit() : True

🔎 Real-world usage example: validating user input

user_input = input("Enter your age: ")

if user_input.isdigit():
    print("Valid age input.")
else:
    print("Please enter digits only.")

✅ But remember: this only works for positive integers!

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4. Difference between isdigit(), isdecimal(), and isnumeric()

Python provides 3 very similar methods that often confuse beginners. Here’s a simple comparison:

Method	Allowed Characters	Example
isdecimal()	Only decimal digits (0–9)	"123" → True
isdigit()	Decimal digits + some unicode numerics	"⅕" → True
isnumeric()	Widest coverage: numerals, fractions, unicode numerics	"二" (Chinese 2) → True

✅ Usually for most applications, isdecimal() is the strictest and safest for purely numeric inputs.

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In this post, we learned how isdigit() works in Python, where it’s useful, its limitations, and how it differs from similar methods.
Thanks for reading!