Data types, variables, constants and type conversion

The four primitive types

• int: whole numbers of any size, such as 42 or -7.
• float: numbers with a fractional part, such as 3.14. Stored in binary, so most decimals are approximations.
• str: text, in quotes. Strings are immutable: methods like upper() return a new string.
• bool: True or False.

type(x) tells you a value's type. Python is dynamically typed: a variable's type is whatever it currently holds, so be deliberate about what you store.

Variables and constants

• An identifier must start with a letter or underscore, may contain letters, digits and underscores, is case sensitive, and must not be a keyword such as class or for.
• Use descriptive snake_case names: total_price, not tp.
• Python has no true constants. By convention a value that must not change is written in UPPER_CASE at the top of the program, such as VAT_RATE = 0.2, and is never reassigned. Named constants remove 'magic numbers' and mean a change is made in one place.

Converting between types

int(), float(), str() and bool() convert values. Watch for these traps:

• int() on a float truncates towards zero; it does not round.
• int('3.7') raises ValueError: convert with float() first.
• bool() is False only for 'empty' values such as 0, '' and []; any non-empty string, even 'False', is True.
• round() uses banker's rounding (halves go to the even neighbour), and floats like 2.675 are stored slightly below the half.

print(type(7), type(7 / 7), type("7"), type(7 > 3))
print(int(3.99), int(-3.99), float("2.5"), str(42) + "!")
print(bool(0), bool(""), bool("False"), bool([0]))
print(round(2.5), round(3.5), round(2.675, 2))

Output:

<class 'int'> <class 'float'> <class 'str'> <class 'bool'>
3 -3 2.5 42!
False False True True
2 4 2.67

The PLS and real Python. The PLS describes round(x, n) as using 'the 0.5 rule', but Python 3 itself rounds an exact half to the even neighbour, as the last line shows. Your code is run in real Python, so predict what Python does; if a question needs halves always rounded up, use the decimal module with ROUND_HALF_UP (see the floating-point lesson). The PLS also describes int() as removing the fractional part, which matches Python: int(-3.99) is -3.

Worked example

A user types '19.99' for a price and '3' for a quantity. Write code that prints the total cost to 2 decimal places.

1. input() always returns a str, so convert: float('19.99') and int('3').
2. Multiply, then format with an f-string to 2 decimal places.
3. ```python price = float("19.99") quantity = int("3") print(f"Total: {price * quantity:.2f}") ```
4. Output: ``` Total: 59.97 ```

Answer: Convert input before arithmetic; use a format specifier, not round(), for display.

Check yourself

1. What does this program print? ```python print(int(-7.8), int(7.8)) ```
   Show answer

   -7 7. int() truncates towards zero, so -7.8 becomes -7 and 7.8 becomes 7. Output: `-7 7`

2. What does this program print? ```python print(round(0.5), round(1.5), round(2.5)) ```
   Show answer

   0 2 2. Python rounds halves to the nearest even number: 0, 2, 2. Output: `0 2 2`

3. What does this program print? ```python print(bool("False"), bool(0.0)) ```
   • False False
   • True False
   • True True
   • False True
   Show answer

   True False. Any non-empty string is True, even 'False'. 0.0 is zero, so False. Output: `True False`

4. What does this program print? ```python print("3" * 3, str(3) + "3", int("3") + 3) ```
   Show answer

   333 33 6. '3' * 3 repeats the string: 333. str(3) + '3' joins strings: 33. int('3') + 3 adds numbers: 6. Output: `333 33 6`

5. What does this program print? ```python print(type(8 / 2).__name__) ```
   Show answer

   float. / always produces a float, even when the result is whole (4.0). Output: `float`

6. Which of these is a valid Python identifier?
   • 2nd_total
   • total-score
   • _total
   • class
   Show answer

   _total. _total starts with an underscore and contains only letters and underscores.

7. How should a value that never changes, such as the number of judges (5), be written in a Python program?
   • NUM_JUDGES = 5 at the top of the program, and the name used everywhere
   • The number 5 typed wherever it is needed
   • num_judges = 5 inside each function that uses it
   • A global variable that functions update
   Show answer

   NUM_JUDGES = 5 at the top of the program, and the name used everywhere. Python's convention for constants is an UPPER_CASE name defined once. It removes magic numbers and makes changes safe.

8. What happens when Python runs int("4.0")?
   • It returns 4
   • A ValueError is raised
   • It returns 4.0
   • It returns '4'
   Show answer

   A ValueError is raised. int() can only convert a string that looks like a whole number. Use int(float('4.0')).

Practise with instant feedback Print this lesson

Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Computer Science specification, Issue 1 (March 2026)
• Pearson Edexcel IAL Computer Science Unit 4 Programming Language Subset (PLS)
• Pearson Edexcel IAL Computer Science sample assessment materials (Unit 3 and Unit 4)

Last updated 8 November 2026. Spotted a mistake? Tell us.