Introduction to Python

Questions

What are the basic blocks of Python language?
How are functions and classes defined in Python?

Objectives

Get a very short introduction to Python types and syntax
Be able to follow the rest of the examples in the course, even if you don’t understand everything perfectly.

We expect everyone to be able to know the following basic material to follow the course (though it is not everything you need to know about Python).

If you are not familiar with Python, here is a very short introduction. It will not be enough to do everything in this course, but you will be able to follow along a bit more than you would otherwise.

Scalars

Scalar types, that is, single elements of various types:

i = 42       # integer
i = 2**77    # Integers have arbitrary precision
g = 3.14     # floating point number
c = 2 - 3j   # Complex number
b = True     # boolean
s = "Hello!" # String (Unicode)
q = b'Hello' # bytes (8-bit values)

Read more: int, float, complex, bool, str, bytes.

Collections

Collections are data structures capable of storing multiple values.

l = [1, 2, 3]                      # list
l[1]                               # lists are indexed by int
l[1] = True                        # list elements can be any type
d = {"Janne": 123, "Richard": 456} # dictionary
d["Janne"]
s = set(("apple", "cherry", "banana", "apple")) # Set of unique values
s

Read more: list, tuple, dict, set.

Control structures

Python has the usual control structures, that is conditional statements and loops. For example, the The if statement statement:

x = 2
if x == 3:
    print('x is 3')
elif x == 2:
    print('x is 2')
else:
    print('x is something else')

While loops loop until some condition is met:

x = 0
while x < 42:
    print('x is ', x)
    x += 0.2

For loops loop over some collection of values:

xs = [1, 2, 3, 4]
for x in xs:
    print(x)

Often you want to loop over a sequence of integers, in that case the range function is useful:

for x in range(9):
    print(x)

Another common need is to iterate over a collection, but at the same time also have an index number. For this there is the enumerate() function:

xs = [1, 'hello', 'world']
for ii, x in enumerate(xs):
    print(ii, x)

Functions and classes

Python functions are defined by the Function definitions keyword. They take a number of arguments, and return a number of return values.

def hello(name):
    """Say hello to the person given by the argument"""
    print('Hello', name)
    return 'Hello ' + name

hello("Anne")

Classes are defined by the Class definitions keyword:

class Hello:
    def __init__(self, name):
        self._name = name
    def say(self):
        print('Hello', self._name)

h = Hello("Richard")
h.say()

Python type system

Python is strongly and dynamically typed.

Strong here means, roughly, that it’s not possible to circumvent the type system (at least, not easily, and not without invoking undefined behavior).

x = 42
type(x)
x + "hello"

Dynamic typing means that types are determined at runtime, and a variable can be redefined to refer to an instance of another type:

x = 42
x = "hello"

Jargon: Types are associated with rvalues, not lvalues. In statically typed language, types are associated with lvalues, and are (typically) reified during compilation.

??? (lesson here)

Keypoints

Python offers a nice set of basic types as many other programming languages
Python is strongly typed and dynamically typed