Python provides many string methods for checking the contents of a string. These methods return Boolean values (True or False) based on whether a particular condition is satisfied.

These methods are commonly used for validating user input, checking passwords, verifying data, and making decisions in programs.

In this lesson, we will study the following commonly used string methods:

• startswith()
• endswith()
• isalpha()
• isdigit()
• isalnum()

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Syntax #

string.method(argument)

or

string.method()

Components #

Component	Description
string	The string object.
.	Dot operator used to access the method.
method()	String method being called.
argument	Optional value required by some methods.

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Example 1: Checking the Beginning of a String #

main.py

text = "Python Programming"

print(text.startswith("Python"))

Output #

True

Explanation #

• The startswith() method checks whether the string begins with "Python".
• Since the condition is satisfied, it returns True.
• The comparison is case-sensitive.

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Example 2: Checking the End of a String #

main.py

filename = "report.pdf"

print(filename.endswith(".pdf"))

Output #

True

Explanation #

• endswith() checks whether the string ends with .pdf.
• Since the filename has the required extension, the result is True.
• This method is commonly used when validating file names.

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Example 3: Using isalpha() #

main.py

word = "Python"

print(word.isalpha())

Output #

True

Explanation #

• isalpha() checks whether all characters are alphabetic.
• The string contains only letters.
• Therefore, the result is True.

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Example 4: Using isdigit() #

main.py

number = "12345"

print(number.isdigit())

Output #

True

Explanation #

• isdigit() checks whether every character is a digit.
• Since all characters are numeric, the method returns True.

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Example 5: Using isalnum() #

main.py

text = "Python123"

print(text.isalnum())

Output #

True

Explanation #

• isalnum() checks whether all characters are either letters or digits.
• The string contains only alphabets and numbers.
• Therefore, the result is True.

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Example 6: User Input Validation #

main.py

username = input("Enter username: ")

print(username.isalnum())

Sample Input #

Alice123

Output #

True

Explanation #

• The user enters a username.
• isalnum() checks whether it contains only letters and digits.
• Since no special characters are present, the result is True.

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Example 7: Alphabetic Check with Spaces #

main.py

name = "Alice Johnson"

print(name.isalpha())

Output #

False

Explanation #

• Although the string contains only letters and a space, isalpha() does not allow spaces.
• Every character must be an alphabetic letter.
• Therefore, the result is False.

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Example 8: Checking Numeric Input #

main.py

value = "98A7"

print(value.isdigit())

Output #

False

Explanation #

• The string contains the letter A.
• Since not every character is a digit, isdigit() returns False.

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Example 9: Case-Sensitive Prefix Check #

main.py

text = "Python"

print(text.startswith("python"))

Output #

False

Explanation #

• startswith() is case-sensitive.
• "Python" and "python" are different strings.
• Therefore, the result is False.

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Example 10: Checking the Result Type #

main.py

text = "Python"

result = text.isalpha()

print(result)
print(type(result))

Output #

True
<class 'bool'>

Explanation #

• These string methods return Boolean values.
• The result is either True or False.
• Therefore, the returned type is bool.

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Common Mistakes #

1. Expecting isalpha() to Allow Spaces #

Incorrect

name = "John Doe"

print(name.isalpha())

Incorrect Expectation #

True

Actual Output #

False

Reason

Spaces are not alphabetic characters.

Correct

print(name.replace(" ", "").isalpha())

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2. Assuming isdigit() Accepts Decimal Numbers #

Incorrect

number = "12.5"

print(number.isdigit())

Output #

False

Reason

The decimal point (.) is not a digit.

Correct

number = "125"

print(number.isdigit())

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3. Assuming isalnum() Allows Special Characters #

Incorrect

username = "Alice_123"

print(username.isalnum())

Incorrect Expectation #

True

Actual Output #

False

Reason

The underscore (_) is not an alphabetic character or a digit.

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4. Forgetting That startswith() Is Case-Sensitive #

Incorrect

text = "Python"

print(text.startswith("python"))

Incorrect Expectation #

True

Actual Output #

False

Reason

String comparisons performed by startswith() are case-sensitive.

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Best Practices #

• Use startswith() and endswith() when validating filenames, URLs, and prefixes.
• Use isalpha() to validate alphabetic input.
• Use isdigit() before converting a numeric string using int().
• Use isalnum() when validating usernames or identification codes.
• Remember that these methods return Boolean values.

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Key Points to Remember #

• startswith() checks whether a string begins with a specified substring.
• endswith() checks whether a string ends with a specified substring.
• isalpha() returns True only if all characters are alphabetic.
• isdigit() returns True only if all characters are digits.
• isalnum() returns True only if all characters are letters or digits.
• These methods are case-sensitive where applicable.
• All these methods return values of type bool.