Types and Conversions

Note

Source: Drawn from the SE4ML Python chapter (chapter_python.rst, lines 1760–1801) and the C# edition (data/types-and-conversions.rst). bool truthiness table and None discussion are original additions.

Every value in Python has a type that determines what operations can be performed on it and how it is stored.

Built-in Types

The four types used most often in early programs are:

int

Whole numbers: 0, 42, -17, 1_000_000.

float

Approximate real numbers: 3.14, -0.5, 2.0, 1.5e3.

str

Strings of characters: "Hello", 'Python', "42".

bool

Boolean truth values: True or False.

Use type() to see the type of any value:

>>> type(42)
<class 'int'>
>>> type(3.14)
<class 'float'>
>>> type("Hello")
<class 'str'>
>>> type(True)
<class 'bool'>

Type Conversion

Use the type name as a function to convert between types:

Function	Description	Example
int(x)	Convert to integer (truncates floats)	int(3.9) → 3
float(x)	Convert to float	float(7) → 7.0
str(x)	Convert to string	str(42) → "42"
bool(x)	Convert to bool	bool(0) → False

>>> int("42")
42
>>> float("3.14")
3.14
>>> int(3.99)
3
>>> str(100)
'100'

Note that int() truncates floats toward zero — it does not round:

>>> int(3.9)
3
>>> int(-3.9)
-3

Use round() if you need rounding.

Boolean Values and Truthiness

True and False are the two boolean values. Every Python value can be interpreted as boolean in a condition:

• Falsy values: 0, 0.0, "" (empty string), [] (empty list), {} (empty dict), None

• Truthy values: everything else

>>> bool(0)
False
>>> bool(1)
True
>>> bool("")
False
>>> bool("hello")
True

None

None is a special value that represents the absence of a value. It is the Python equivalent of null in Java or C#.

>>> x = None
>>> x is None
True
>>> print(x)
None

Functions that do not explicitly return a value return None automatically. None is also used as a placeholder when a variable exists but has not been given a meaningful value yet.

Test for None with is None, not with == None:

if result is None:
    print("No result yet.")