Getting started with Python

The Python interpreter

Type python or python3 in a terminal window:

$ python3
Python 3.7.3 (default, Jun 19 2019, 07:38:49)
[Clang 10.0.1 (clang-1001.0.46.4)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>

As long as the response starts with "Python 3...", that's OK. If you don't get it by typing python try doing python3 instead.

You're now in the Python interpreter. To exit, hit control d.

In the interpreter, you type commands and press return:

>>> 2 + 2
4
>>> 23 * 4.3
98.89999999999999
>>> hello computer
  File "<stdin>", line 1
    hello computer
                 ^
SyntaxError: invalid syntax
>>>

Simple maths

We're using Python operators.

>>> 6 - 7
-1
>>> 8 / 2
4.0
>>> 8 / 3
2.6666666666666665

Some other useful operators:

>>> # 3 to the power of 2
>>> 3 ** 2
9
>>> # division, but only into whole numbers
>>> 19 // 4 
4
>>> # and the remainder 
>>> 19 % 4
3

https://docs.python.org/3/tutorial/introduction.html#using-python-as-a-calculator

Variables

>>> a = 3
>>> b = 27
>>> b / a
9.0
>>> c = a + b
>>> c
30

# errors!
>>> d
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'd' is not defined

Strings

>>> "hello" + "world"
'helloworld'
>>> a = "hello"
>>> b = "world"
>>> a + b
'helloworld'

Comparisons, and True and False

Python comparison operators

>>> # are they equal?
>>> 1 == 2
False
>>> 1 == 1
True
>>> a = 10
>>> b = 10
>>> a == b
True
# are they not equal?
>>> 1 != 2
True
# greater and less than
>>> 2 > 1
True
>>> 1 < 2
True
# also <= and >=

Tuples, lists and sets

Tuples, lists and sets are all examples of Python collections.

Tuples

>>> elements = ("hydrogen", "helium", "lithium", "beryllium", "boron")
>>> type(elements)
<type 'tuple'>

# slicing a tuple
>>> elements[0]
'hydrogen'
>>> elements[3]
'beryllium'
>>> elements[1:4]
('helium', 'lithium', 'beryllium')
>>> elements[-1]
'boron'

Lists

>>> elements = list(elements)
>>> type(elements)
<type 'list'>

Lists can be sliced in the same way as tuples. Unlike tuples, lists can be maniupulated once created:

>>> elements.sort()
>>> elements
['beryllium', 'boron', 'helium', 'hydrogen', 'lithium']

sort() is a method of the list class. Any list is a member of this class - it's a list object - and will have all the abilities that lists can have.

# the items in collection don't even have to be of the same type
>>> elements.append(37)
>>> elements
['beryllium', 'boron', 'helium', 'hydrogen', 'lithium', 37]

# and an item can itself be a collection
>>> elements.append(["pancakes", "bread"])
>>> elements
['beryllium', 'boron', 'helium', 'hydrogen', 'lithium', 37, ['pancakes',
'bread']]

http://docs.python.org/3/tutorial/introduction.html#lists

Sets

A set is an unordered collection with no duplicate elements.

>>> life = ["fun", "fun", "fun", "boring", "fun"]
>>> set(life)
set(['fun', 'boring'])

Dictionaries

>>> legs = {"spider": 6, "dog": 4, "bird": 2, "ant": 6}
>>> legs["bird"]
2

# we don't have humans
>>> legs["human"]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'human'
>>>

# a safer way if we're not sure if the key's present
>>> legs.get("human")
# or even
>>> legs.get("human", "no data available")

# better add human though anyway
>>> legs["human"] = 2

# and we'd better correct the entry for spiders
>>> legs["spider"] = 8

https://docs.python.org/3/tutorial/datastructures.html#dictionaries

Loops

>>> for item in range(100):
...     item
...
0
1
[etc]for item in elements

>>> for element in elements:
...     element
...
'beryllium'
'boron'
'helium'
'hydrogen'
'lithium'
37
['pancakes', 'bread']


# list comprehensions are an excellent way to build lists
>>> squares = [item * item for item in range(10)]
>>> squares
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

# you can add an if clause to filter the results
# let's get squares of even numbers only
>>> squares = [item * item for item in range(10) if item % 2 == 0]
>>> squares
[0, 4, 16, 36, 64]

Functions

>>> def squares():
...     return [item * item for item in range(10)]
...
>>> squares()
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

This function only does one thing, so it's not that useful. So:

# define squares() with a required argument
>>> def squares(up_to):
...     return [item * item for item in range(up_to)]
...
>>> squares()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: squares() takes exactly 1 argument (0 given)

# we have to provide the argument
>>> squares(15)
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196]

# or we could have defined it with a default argument of 10
>>> def squares(up_to=10):
...     return [item * item for item in range(up_to)]
...

We can have multiple arguments:

>>> def multiples(up_to=10, multiply_by=2):
...     return [item * multiply_by for item in range(up_to)]
...
>>> multiples()
[0, 2, 4, 6, 8, 10, 12, 14, 16, 18]
>>> multiples(10, 5)
[0, 5, 10, 15, 20, 25, 30, 35, 40, 45]

# using named arguments when calling a function allows you to use
# them in a different order
>>> multiples(multiply_by=10, up_to=5)
[0, 10, 20, 30, 40]

Let's play a game. For this we need to import the random module, and use the choice() function.

>>> import random

choice() takes an argument, which needs to be a sequence of some sort, and chooses between them at random:

>>> random.choice(("black", "white", "red"))

# strings are sequences too!
>>> random.choice("Refer to the documentation for details")

>>> def challenge(player_choice=None):
...     if player_choice is None:
...         print("you have to choose something!")
...     elif player_choice is random.choice([True, False]):
...         print("You win!")
...     else:
...         print("You lose!")
...

It's not a very interesting:

>>> challenge()
you have to choose something!
>>> challenge(True)
You win!
>>> challenge(True)
You lose!
>>> challenge(True)
You lose!

Let's make the computer play the game against itself:

>>> for r in range(1000):
...     challenge(random.choice([True, False]))

Scripts

Put all this in a file called game.py:

import random

# define the challenge function
def challenge(player_choice=None):
    if player_choice is None:
        print("You have to choose something!")
    elif player_choice is random.choice([True, False]):
        print("You win!")
    else:
        print("You lose!")

for r in range(1000):
    challenge(random.choice([True, False]))

Exit the Python interpreter (control d) and run the command:

python3 game.py

This tells Python to run the script - the program - game.py.

Classes

Things in Python are instances of classes. Some are already defined, with their own methods (methods are functions that belong to a class), such as lists and dictionaries and so on, but you can also create your own.

>>> class Animal(object):
...     def identify(self):
...         print("I am an animal")
...
>>> dog = Animal()
>>> dog.identify()
I am an animal
>>> cat = Animal()
>>> cat.identify()
I am an animal

We can make this a little more interesting:

>>> class Animal(object):
...     def __init__(self, noise=None):
...         self.noise = noise
...     def identify(self):
...         print("I am an animal, and I go", self.noise)
...

# create an Animal instance, and provide the string "woof" to its
# initialiser
>>> dog = Animal("woof")
>>> dog.identify()
I am an animal, and I go woof

# we can modify an object's attribute once it has been created
>>> dog.noise = "bow wow"
>>> dog.identify()
I am an animal, and I go bow wow