String Validation Methods

What you'll learn

Topic	Methods
Case checks	isupper() & islower()
Letters only	isalpha()
Digits only	isdigit()
Alphanumeric & space	isalnum() & isspace()

Important: These methods only return True or False. They never modify the string.

Validation flow (remember this first)

Before choosing a validation method, ask: What am I checking?

1. isupper()

Returns True only when all alphabet letters are uppercase.

print("PYTHON".isupper())   # True
print("PYThon".isupper())   # False
print("python".isupper())   # False

2. islower()

Returns True only when all letters are lowercase.

print("python".islower())   # True
print("Python".islower())   # False
print("PYTHON".islower())   # False

Rule: Every letter must be lowercase.

isupper() vs islower()

String	isupper()	islower()
"PYTHON"	True	False
"python"	False	True
"PyThon"	False	False
"123"	False	False

Why is "123" False for both?

3. isalpha()

Returns True only when the string contains letters only.

print("Apple".isalpha())             # True
print("Apple123".isalpha())          # False
print("Guido Van Rossum".isalpha())  # False

Remember: Spaces are not letters.

4. isdigit()

Returns True only when every character is a digit.

print("123".isdigit())       # True
print("12.34".isdigit())     # False
print("Python123".isdigit()) # False

Why decimals fail:

5. isalnum()

Returns True when the string contains letters and/or digits only.

print("Python312".isalnum())   # True
print("Python".isalnum())      # True
print("312".isalnum())         # True

print("Python 312".isalnum())  # False
print("Python3.12".isalnum()) # False

Decision rule:

6. isspace()

Returns True only when every character is whitespace.

print("   ".isspace())     # True
print("".isspace())        # False
print("python".isspace())  # False

Important: An empty string ("") is not considered whitespace.

Comparison table

String	Alpha	Digit	Alnum
"Apple"	Yes	No	Yes
"123"	No	Yes	Yes
"Python312"	No	No	Yes
"Python 312"	No	No	No
"12.34"	No	No	No

Playground example

tests = [
    "PYTHON",
    "python",
    "PYThon",
    "123",
    "Python312",
    "   "
]

for s in tests:
    print(s)
    print("isupper :", s.isupper())
    print("islower :", s.islower())
    print("isalpha :", s.isalpha())
    print("isdigit :", s.isdigit())
    print("isalnum :", s.isalnum())
    print("isspace :", s.isspace())
    print("-" * 20)

Visual output idea:

Use the lesson playground at the bottom of the page to run this code live.

Quick knowledge check

1. Does "PYTHON311".isalnum() return True?
   Answer: Yes — it contains only letters and digits.

2. Does "123 456".isdigit() return True?
   Answer: No — the space breaks it.

3. When is isspace() True?
   Answer: When every character is whitespace (spaces, tabs, or newlines).

30-second visual revision

Method	Easy memory
isupper()	All BIG letters
islower()	All small letters
isalpha()	Alphabet only
isdigit()	Numbers only
isalnum()	Letters + numbers
isspace()	Whitespace only

Interview shortcut: Alpha → Letters · Digit → Numbers · Alnum → Both · Space → Only whitespace · Upper/Lower → Case validation

Next: Index and Count Methods.