Builtin functions

Python 3.11 has 75 builtin functions. To start writing useful programs, knowing about 20 of them is sufficient. Many of them are shortcuts that make your everyday programming a lot easier.

These 20 functions are your basic vocabulary, knowing these is a must to write Python efficiently!

type conversion	I/O	math	iterables
int	print	abs	range
float	input	round	len
str	open	sum	sorted
bool		min	reversed
tuple		max	enumerate
list			zip
dict
set

Type Conversions

The type conversion functions convert one data type into another. Examples for them are in an earlier section:

int(x)        str(x)        dict(x)        bool(x)
float(x)      list(x)       tuple(x)       set(x)

Input and output

Basic reading and writing data requires just three functions:

print(s)       input(s)     open(filename, mode)

Mathematical functions

With abs() you can calculate the absolute value of a number:

>>> abs(-42)
42

With round() you can round numbers to a given number of digits:

>>> round(3.14159, 2)
3.14

The divmod() function calculates a division and a modulo at the same time:

>>> divmod(23, 5)
(4, 3)

The pow function does the same as the ** operator:

>>> pow(3, 3)
27

Working with sequences

There are tons of functions that help with Python sequences (lists, dictionaries, tuples and iterators). The most common ones are:

len(x)        min(x)        sorted(x)        enumerate(x)
sum(x)        max(x)        reversed(x)      zip(x)
range(x)

I will explain them one by one

Determining the length of sequences

The len() function returns an integer with the length of an argument. It works with strings, lists, tuples, and dictionaries.

>>> data = [0, 1, 2, 3]
>>> len(data)
4

Summing up numbers

The sum of a list of integer or float numbers can be calculated by the sum() function.

>>> data = [1, 2, 3, 4]
>>> sum(data)
10

Smallest and largest value

The functions min() and max() determine the smallest and largest value of a list:

>>> data = [3, 5, 1, 7]
>>> min(data)
1
>>> max(data)
7

Creating lists of integer numbers

The range() function allows to create lists of numbers on-the-fly. There are two optional parameters for the start value and the step size.

>>> list(range(4))
[0, 1, 2, 3]
>>> list(range(1, 5))
[1, 2, 3, 4]
>>> list(range(2, 9, 2))
[2, 4, 6, 8]
>>> list(range(5, 0, -1))
[5, 4, 3, 2, 1]

Note that because range() returns an iterator (a kind of lazy on-demand list), you need to convert it to a list to see the data.

Enumerating list elements

The enumerate() function helps with counting elements. It creates tuples consisting of an integer number starting from zero and the elements of the list.

>>> fruits = ['apple', 'banana', 'orange']
>>> list(enumerate(fruits))
[(0, 'apple'), (1, 'banana'), (2, 'orange')]

Note that enumerate() produces an iterator. To obtain a list, you need to convert it.

enumerate() is a great shortcut to loops with counter variables:

i = 0
for elem in data:
    print(i, elem)
    i += 1

becomes simply:

for i, elem in enumerate(data):
    print(i, elem)

Sorting data

The sorted() function sorts a list or the keys of a dictionary, but does not change the original data.

>>> sorted(data)
[1, 3, 5, 7]

Reversing data

The reversed() function reverses the order of list elements, but does not change the original data. It returns an iterator.

>>> data = [3, 5, 1, 7]
>>> list(reversed(data))
[7, 1, 5, 3]

Merging two lists

The zip() function associates the elements of two lists to a single list or tuple. Excess elements are ignored.

fruits = ['apple','banana','orange']
prices = [0.55, 0.75, 0.80, 1.23]
for fruit, price in zip(fruits, prices):
    print(fruit, price)