Built-in Functions in Python

Some tools are so commonly needed that Python does not make you import anything — they are always available the moment you start Python. These are called built-in functions in Python. Unlike modules like math and random, you never write an import line to use them.

This page rounds up the built-ins you'll use constantly: checking and converting types, working with numbers, measuring things, and generating sequences.

What you will learn

• How type() and isinstance() differ
• Converting between types with int(), float(), str()
• Number built-ins: abs(), round(), pow(), max(), min(), sum()
• Measuring things with len()
• Generating sequences of numbers with range()

Checking and converting types

type(42)             # <class 'int'>     tells you what type something is
type(3.14)           # <class 'float'>
type('hello')        # <class 'str'>

isinstance(42, int)          # True    is 42 an integer? yes
isinstance(3.14, int)        # False   is 3.14 an integer? no
isinstance('hello', str)     # True    is 'hello' a string? yes

type() tells you what kind of thing something is. isinstance() asks a yes-or-no question — is this thing a particular type? — which makes it handy for guarding functions against bad input, as covered in Recursion in Python.

You can also convert between types (full details in Type Casting in Python):

int(3.9)       # 3         cuts off the decimal (does NOT round)
float(5)       # 5.0       turns a whole number into a decimal
str(42)        # '42'      turns a number into text

These are useful when, for example, you want to join a number into a sentence — Python will not let you stick a number and a string together directly, but if you convert the number to a string first, it works fine.

Numbers

abs(-7)            # 7       the absolute value (removes the minus sign)
round(3.7)          # 4       rounds to nearest whole number
round(3.14159, 2)   # 3.14    rounds to 2 decimal places
pow(2, 8)           # 256     2 to the power of 8
max(3, 7, 2)        # 7       the largest value
min(3, 7, 2)        # 2       the smallest value
sum([1, 2, 3, 4])   # 10      adds everything in a list

round() is the one to reach for when you actually want rounding behavior — unlike int(), which always truncates toward zero regardless of how close a value is to the next whole number.

Measuring things: len()

len('hello')         # 5    how many characters in a string
len([10, 20, 30])    # 3    how many items in a list

len() is one of the most used functions in Python — it works on strings, lists, and many other collection types.

Generating sequences of numbers: range()

range(5)          # 0, 1, 2, 3, 4
range(1, 6)       # 1, 2, 3, 4, 5
range(0, 10, 2)   # 0, 2, 4, 6, 8  (step of 2)

range() gives you a sequence of numbers. You will use this constantly in loops — for example, for i in range(5) repeats something five times. The sequence always stops before the last number you give it — range(5) never includes 5 itself.

Common mistakes

• Comparing a boolean result with == True instead of using it directly
• Expecting int() to round like round() does — it truncates instead
• Assuming range(5) includes 5 — it stops one before the end value
• Calling len() on something that has no length, like an integer, and hitting a TypeError
• Confusing type(), which tells you the exact type, with isinstance(), which answers a yes/no question