lesje-python/syntax.py

# Lines starting with a '#' are seen as comments, you can 
# write whatever you want after them, python won't care and
# just discard it as white space

# Variables
# They are the building blocks of programming and maths,
# luckily for us the difference between python variables and
# math variables is minimal so you already know what they are.

# Variables have a name and a value. The name can be whatever
# you want it to be, it is not limited to one character.

i_am_a_variable = 5

# Here we define a variable with the name "i_am_a_variable" and 
# value 5

# Python looks a lot like english and math mixed.

variable_1 = 3
variable_2 = 5
variable_3 = variable_1 + variable_2

# The print function can be used to output the value of something
# to the screen, you will learn what functions are later

print(variable_3) # 8

# Types

# Every variable has a type, in the previous example that type
# was "integer" (shortened to "int") because we were using whole
# numbers
# If we were to assign a string of text to a variable like so

stringy_string = "hello"

# Now the type of the variable "stringy_string" is a string
# (shortened to "str")

# We can request the type of something with the type function
print(type(stringy_string)) # <class 'str'>


# For floating point values it is a float

floaty_float = 3.14
print(type(floaty_float)) # <class 'float'>


some_boolean_value = False
print(type(some_boolean_value)) # <class 'bool'>


# Lists
# Lists are essentially yet another type.
# Their value is simply a list of values

listy_list = [1, 2, 3]

# Here we create a variable with name listy_list of type list
# that contains the values 1, 2, 3

# You can manipulate these lists to your liking
# For example you can insert a value like so
listy_list.append(4)
print(listy_list) # [1, 2, 3, 4]

# Or change a certain value at a given index in the list
# note that indexes start at 0.
listy_list[2] = 5
print(listy_list) # [1, 2, 5, 4]

# Tuples are very similar in that they can store a list
# of values. The big difference is that they cannot be
# manipulated

t = (1, 2, 3)
# t.append(4)        would give an error
# t[2] = 5            would give an error

# Functions

def foo(x, y):
    return x + y

z = foo(1, 2)

print(z) # 3

# variable_3 is now equal to 8

# As you can see the foo function is called in the same way
# we use "print". This is because print is a function too!
# Functions most of the time have a return value, this means
# that we can even chain our print and our foo function 
# inside each other. This way we don't need the "z" variable.

print(foo(1, 3)) # 3

# Conditionals

# When you want to do something only IF a certain
# predicate is true, you can do that with the "if"
# keyword, surprising right

bar = 8

if bar == 8:
    print("bar is equal to 8")
else:
    print("bar is equal to something other than 8")


# Other comparators are

bar != 8    # bar not equal to 8
bar < 8     # bar smaller than 8
bar > 8     # bar larger than 8
bar <= 8    # bar smaller or equal to 8
bar >= 8    # bar larger or equal to 8

# and / or / not operations

# You can even chain these comparators

baz = 1

bar != 8 and baz < 4
bar < 1 or baz > 1
not bar < 1             # same as bar >= 1



# Loops

# Loops use the same conditionals as we just used above
# There are two types of loops. For loops and while loops.

# For loops are used to loop for each element of something
# (eg. a list).
# While loops are used to loop while a conditional is true

x = 0
while x < 4:
    print(x)
    x = x + 1

# This loop will loop 4 types and print the value of x each 
# time. This means the output will look like:

# 0
# 1
# 2
# 3

sentence = ["hello", "world", "it", "is", "cold", "outside"]

for word in sentence:
    print(word)

# hello
# world

# it
# is 
# cold
# outside


# Classes

# Classes can be seen as a bundle of functions and variables.
# They can contain both! They can be used to structure your code.

# A class can be defined with the class keyword 

class MyCoolClass:
    pass            # This "pass" keyword just means do nothing, ignore

# As you can see classes have a name too
# As I said they can contain functions and code as so

class CoolClass:
    cool_var = 3

    # This self parameter has to be added to all members functions 
    # of a class (unless they are static, but you can ignore that for now)
    # This self reference makes it possible to change values inside the
    # instance of the class that called this function.
    def change_cool_var(self, new_cool_var):
        self.cool_var = new_cool_var


# Now to use these classes you need to create an instance of this
# class. This is easy, just act as if it is a function call

my_class = CoolClass()

# Now the variable my_class contains an instance of CoolClass
# Now you can use the functions in this class

print(my_class.cool_var) # 3

my_class.change_cool_var(5)

print(my_class.cool_var) # 5