← VCE Data Analytics U4O1

Type checking in Python

A type check method: returns True if a string holds only digits, so text can be rejected where a number is needed. The alternative is to put int(value) inside try/except.

Part of: VCE Data Analytics - U4O1 - Development and Evaluation — Module 78, VCE Applied Computing U1O2 Programming: Validating data — Module 4

Understanding Type checking in Python in VCE Data Analytics U4O1

What it does: The isdigit() check is False for text like 'abc', so the program rejects it and shows an error message. If int(value) fails, Python raises a ValueError and the except block handles the invalid type. isdigit() is a method, so it needs brackets. It returns True if the string holds only digits. A type check asks whether the input is a number at all. A range check asks whether that number is within limits. Checking with isdigit() (or wrapping int() in try/except) stops bad input from crashing the program. Calling isdigit(age) causes a NameError. The correct type check is if age.isdigit():.

Common mistakes: The else keyword must be followed by a colon. The line should be else:. A try block starts with a colon, not a semicolon. The line should be try:.

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This module's key terms are also in a printable crossword (Modules 63–83) and word search (Modules 63–105).

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Learning

Duo

Which check confirms that the string age is made up of digits only?

age.isdigit()age.isdigits

Matches

if isdigit(age):

↓

isdigit() is a string method, so it must be called on the string as age.isdigit(), not as a stand-alone function.

Applying

Meanings

Term

isdigit()

Meaning

A type check method: returns True if a string holds only digits, so text can be rejected where a number is needed. The alternative is to put int(value) inside try/except.

Practice

age = "15"

Testing

Gaps

if age.():

Check Answer

Spots

3 print("Valid number")

4else

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Quick Questions

1. Which check confirms that the string age is made up of digits only?

A. age.isdigits

B. age.isdigit() ✓

isdigit() is a method, so it needs brackets. It returns True if the string holds only digits.

2. What does "abc".isdigit() return?

A. True

B. False ✓

Letters are not digits, so isdigit() returns False. That is how text gets rejected where a number is needed.

3. Which code is a type check using try/except?

A. try: n = int(value) except ValueError: print("Not a number") ✓

B. try: n = int(value) except NameError: print("Not a number")

int("abc") raises a ValueError, so that is the error the except block must catch.

4. What happens when int("hello") runs without try/except?

A. The program crashes with a ValueError ✓

B. It returns 0 and carries on

int() cannot convert text to a number. Without try/except, Python stops with a ValueError.

5. Which input would "25".isdigit() accept as a valid number entry?

A. "25" (returns True) ✓

B. "2.5" (returns True)

The decimal point is not a digit, so "2.5".isdigit() returns False. Only "25" passes.

Match the Pairs

age.isdigit() → Returns True only if the string age contains digits only, so it passes as a whole number

int(value) → Tries to convert text to a whole number and raises ValueError if it cannot

Reject text where a number is needed by testing the string before using it → if not score.isdigit(): print("Enter a number")

Use try/except to catch input that cannot be converted to an integer → try: n = int(value) except ValueError: print("Not a number")

if isdigit(age): → isdigit() is a string method, so it must be called on the string as age.isdigit(), not as a stand-alone function.

n = int(value) except ValueError: → The except block has no try before it, so int(value) is not protected and the code is a syntax error.

Key Term

isdigit()

A type check method: returns True if a string holds only digits, so text can be rejected where a number is needed. The alternative is to put int(value) inside try/except.

Fill in the Blanks

1. age = input("Age: ") if age.isdigit(): print("Age accepted")

Hint: Type the string method that returns True only when every character is a digit.

2. try: age = int(input("Age: ")) except ValueError:

Hint: Type the function that tries to convert text into a whole number.

3. try: score = int(entry) except ValueError:

Hint: Type the name of the error Python raises when int() is given text that is not a number.

4. if not age.isdigit(): print("Numbers only!")

Hint: Type the keyword that reverses True/False so that text is rejected.

5. try: num = int(value) except ValueError:

Hint: Type the keyword that catches the error when the type check fails.

Spot the Error

Question 1

A age = input("Age: ")

B if age.isdigit():

C print("Valid number")

D else

E print("Not a number")

Line D — The else keyword must be followed by a colon. The line should be else:

Question 2

A try;

B qty = int(input("Qty: "))

C except ValueError:

D print("Not a number")

Line A — A try block starts with a colon, not a semicolon. The line should be try:

Question 3

A value = int(input("Enter score: "))

B if value.isdigit():

C print("Accepted")

D else:

E print("Rejected")

Line A — isdigit() only works on strings. Converting the input to an int first would crash on the next line. It should be value = input("Enter score: ") so the text can be checked first.

Question 4

A text = input("Quantity: ")

B if text.isdigit():

C qty = int(text)

D print("Total:", text * 2)

E else:

F print("Digits only")

Line D — text is still a string, so text * 2 repeats the text instead of doubling the number. It should use the converted number: qty * 2.

Question 5

A text = input("Age: ")

B if text.isdigit:

C print("Valid")

D else:

E print("Invalid")

Line B — isdigit is a method and needs brackets to run. Without them the condition is always true, so text is never rejected. It should be if text.isdigit():

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