Python isdecimal() Method

What You’ll Learn

The isdecimal() method returns True when every character in a string is a decimal digit and the string is not empty. It is ideal for validating whole-number input like PIN codes, ages, and quantities before you convert strings to integers with int().

Definition and Usage

In Python, isdecimal() inspects each character in a string. If every character is a decimal digit (characters in Unicode category Nd that can form base-10 numbers) and the string has at least one character, it returns True. Otherwise it returns False. For example, "12345".isdecimal() is True, but "Hello123".isdecimal() is False because of the letters.

📝 Syntax

The isdecimal() method takes no parameters:

string.isdecimal()

Syntax Rules

• string — any valid Python str object.
• Return value — True if all characters are decimal digits and the string is non-empty; otherwise False.
• No arguments — passing arguments raises TypeError.
• Read-only — the original string is never modified.
• Unicode — includes decimal digits from other scripts (like Arabic-Indic ٠١٢), not just ASCII 0–9.

↩ Return Value

isdecimal() always returns a boolean. It returns True only when the string contains one or more characters and every character is a decimal digit. It returns False for empty strings, strings with letters, and strings with punctuation such as ., -, or spaces.

print("12345".isdecimal())   # True
print("Hello123".isdecimal()) # False (letters)
print("".isdecimal())          # False (empty)
print("3.14".isdecimal())      # False (decimal point)
print("-42".isdecimal())       # False (minus sign)

Examples Gallery

Run these examples in any Python 3 interpreter. Each one shows a common numeric validation scenario.

num_str = "12345"
result = num_str.isdecimal()

print("Text:  ", num_str)
print("Result:", result)

Text:   12345
Result: True

text = "Hello123"
result = text.isdecimal()

print("Text:  ", text)
print("Result:", result)

Text:   Hello123
Result: False

samples = ["", "007", "3.14", "-42", "12 34"]

for text in samples:
    print(repr(text), "→", text.isdecimal())

'' → False
'007' → True
'3.14' → False
'-42' → False
'12 34' → False

def validate_pin(pin):
    if not pin.isdecimal():
        return False, "PIN must contain digits only."
    if len(pin) != 4:
        return False, "PIN must be exactly 4 digits."
    return True, "PIN accepted."

for candidate in ["1234", "12a4", "12345"]:
    ok, message = validate_pin(candidate)
    print(repr(candidate), "→", message)

'1234' → PIN accepted.
'12a4' → PIN must contain digits only.
'12345' → PIN must be exactly 4 digits.

samples = ["12345", "\u0661\u0662\u0663", "\u2460"]

for text in samples:
    label = repr(text)
    print(label)
    print("  isdecimal():", text.isdecimal())
    print("  isdigit():  ", text.isdigit())
    print()

'12345'
  isdecimal(): True
  isdigit():   True

'١٢٣'
  isdecimal(): True
  isdigit():   True

'①'
  isdecimal(): False
  isdigit():   True

🚀 Common Use Cases

• PIN and OTP validation — ensure codes contain digits only before processing.
• Form fields — verify age, quantity, or ID fields are whole numbers.
• Pre-conversion checks — call isdecimal() before int(text) to avoid ValueError.
• Data cleaning — flag non-numeric values in imported CSV columns.
• Menu choices — validate that user input is a numeric option index.

📝 Notes

• Decimal points, minus signs, plus signs, and spaces all cause False.
• An empty string "" always returns False—there must be at least one digit.
• Unicode decimal digits from other scripts (like Arabic-Indic) are accepted, not just ASCII 0–9.
• Superscript and other compatibility digits pass isdigit() but often fail isdecimal().
• isdecimal() does not check length—combine it with len() for fixed-width codes.

💡 Best Practices