Search and Validation Methods

15 min · Beginner

Validating user input before processing is essential for application stability. Python provides robust string validation predicates and search methods.

Prefixes and suffixes: startswith and endswith

Prefix is the text at the start of a string and suffix is the text at the end. startswith() and endswith() test them and return True or False.

Use .startswith() and .endswith() to inspect filenames, URLs, or protocols.

Python

url = "https://mentornxt.com"print(url.startswith("https://"))print(url.endswith(".com"))

Example 1 · Accepting only web addresses

Python

link = "https://mentornxt.com/java"print(link.startswith("https://"))print(link.endswith((".com", ".org")))print(link.endswith(".pdf"))

Output

True
False
False

What this code does

startswith checks the beginning of the text.
endswith also accepts a tuple, so one call can test several endings.
Each call returns True or False.

Both methods accept a single string or a tuple of prefixes to check: url.startswith(("http://", "https://")).

Searching: find() versus index()

find() is a string method that returns the index of the first match, or -1 when there is none. index() does the same but raises ValueError instead of returning -1.

Both methods locate the first index of a substring, but handle missing elements differently.

Python

text = "python developer"print(text.find("dev"))print(text.find("java"))

Example 2 · Splitting a log line at the colon

Python

1log = "ERROR: disk full"2pos = log.find(":")3print(pos)4print(log[:pos])5print(log.find("warning"))6print("disk" in log)

Output

5
ERROR
-1
True

What this code does

find gives the index of the colon, which is 5.
The slice log[:pos] is the text before it.
A missing text gives -1, and in gives a plain True or False.

.find() returns the starting index (7), or -1 if not found. In contrast, .index("java") would raise a ValueError.

Validation predicates: isdigit, isalpha, isalnum

Validation is checking that input has the expected shape before it is used, for example that a one-time code contains only digits and has the right length.

Test whether text consists purely of digits, alphabet letters, or alphanumeric characters.

Python

otp = "849201"username = "alex99"print(otp.isdigit())print(username.isalnum())

Example 3 · Checking a 6-digit account number

Python

account_no = "894231"is_valid = account_no.isdigit() and len(account_no) == 6
print(f"Account: {account_no}")print(f"Is valid 6-digit account: {is_valid}")

Output

Account: 894231
Is valid 6-digit account: True

What this code does

Checks that input consists strictly of numeric digits and has length 6.
Returns True for valid OTP or account tokens.

Both checks evaluate to True. Notice that "12.5".isdigit() evaluates to False because the dot . is not a digit.

Next step

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9 quick questions

Search and Validation Methods

15 min · Beginner

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Validating user input before processing is essential for application stability. Python provides robust string validation predicates and search methods.

Prefixes and suffixes: startswith and endswith

Prefix is the text at the start of a string and suffix is the text at the end. startswith() and endswith() test them and return True or False.

Use .startswith() and .endswith() to inspect filenames, URLs, or protocols.

Python

url = "https://mentornxt.com"print(url.startswith("https://"))print(url.endswith(".com"))

Example 1 · Accepting only web addresses

Python

link = "https://mentornxt.com/java"print(link.startswith("https://"))print(link.endswith((".com", ".org")))print(link.endswith(".pdf"))

Output

True
False
False

What this code does

startswith checks the beginning of the text.
endswith also accepts a tuple, so one call can test several endings.
Each call returns True or False.

Both methods accept a single string or a tuple of prefixes to check: url.startswith(("http://", "https://")).

Searching: find() versus index()

find() is a string method that returns the index of the first match, or -1 when there is none. index() does the same but raises ValueError instead of returning -1.

Both methods locate the first index of a substring, but handle missing elements differently.

Python

text = "python developer"print(text.find("dev"))print(text.find("java"))

Example 2 · Splitting a log line at the colon

Python

1log = "ERROR: disk full"2pos = log.find(":")3print(pos)4print(log[:pos])5print(log.find("warning"))6print("disk" in log)

Output

5
ERROR
-1
True

What this code does

find gives the index of the colon, which is 5.
The slice log[:pos] is the text before it.
A missing text gives -1, and in gives a plain True or False.

.find() returns the starting index (7), or -1 if not found. In contrast, .index("java") would raise a ValueError.

Validation predicates: isdigit, isalpha, isalnum

Validation is checking that input has the expected shape before it is used, for example that a one-time code contains only digits and has the right length.

Test whether text consists purely of digits, alphabet letters, or alphanumeric characters.

Python

otp = "849201"username = "alex99"print(otp.isdigit())print(username.isalnum())

Example 3 · Checking a 6-digit account number

Python

account_no = "894231"is_valid = account_no.isdigit() and len(account_no) == 6
print(f"Account: {account_no}")print(f"Is valid 6-digit account: {is_valid}")

Output

Account: 894231
Is valid 6-digit account: True

What this code does

Checks that input consists strictly of numeric digits and has length 6.
Returns True for valid OTP or account tokens.

Both checks evaluate to True. Notice that "12.5".isdigit() evaluates to False because the dot . is not a digit.

Next step

Check your understanding

9 quick questions